1. Which statement best describes the main focus of chemical kinetics?
ⓐ. It studies the speed of a reaction and how that speed changes under different conditions.
ⓑ. It studies only the heat change associated with a reaction.
ⓒ. It studies only the equilibrium composition of a reaction mixture.
ⓓ. It studies whether products are more stable than reactants, without considering time.
Correct Answer: It studies the speed of a reaction and how that speed changes under different conditions.
Explanation: Chemical kinetics is concerned with reaction rate, that is, how fast reactants are converted into products. It also examines how factors such as concentration, temperature, and catalysts influence that speed. Questions about heat evolved or final equilibrium composition belong to other aspects of chemistry, not the central purpose of kinetics.
2. Two reactions are both thermodynamically feasible. Which statement can still be true?
ⓐ. Both must occur at the same speed.
ⓑ. One may be very fast while the other may be extremely slow.
ⓒ. Neither can occur unless a catalyst is present.
ⓓ. Both must produce the same amount of heat.
Correct Answer: One may be very fast while the other may be extremely slow.
Explanation: Thermodynamic feasibility tells whether a reaction can occur, not how rapidly it proceeds. Chemical kinetics deals with the time aspect of the process. Therefore, two feasible reactions can differ greatly in rate because speed depends on kinetic factors, not feasibility alone.
3. Which question is answered primarily by chemical kinetics?
ⓐ. What is the value of the equilibrium constant?
ⓑ. Is the reaction exothermic or endothermic?
ⓒ. How rapidly does the concentration of a reactant decrease with time?
ⓓ. Is the product more stable than the reactant?
Correct Answer: How rapidly does the concentration of a reactant decrease with time?
Explanation: Chemical kinetics deals with the rate of reaction and with concentration changes as time passes. Questions about heat change are thermodynamic, and questions about stability or equilibrium constants are not direct rate questions. A time-based concentration change is a kinetic quantity.
4. A reaction is highly exothermic, but it proceeds very slowly at room temperature. This observation shows that
ⓐ. exothermic reactions must be non-spontaneous.
ⓑ. the reaction cannot form products at all.
ⓒ. heat released by a reaction directly fixes its speed.
ⓓ. reaction speed and energetic favourability are not the same idea.
Correct Answer: reaction speed and energetic favourability are not the same idea.
Explanation: A reaction may release a large amount of energy and still proceed slowly if the kinetic barrier is significant. Chemical kinetics describes how fast a reaction occurs, whereas energetic favourability concerns whether the process is thermodynamically allowed. The two ideas are related to different questions.
5. Which pair is matched correctly?
ⓐ. Thermodynamics — studies rate of reaction; Kinetics — studies heat of reaction
ⓑ. Thermodynamics — studies catalyst action only; Kinetics — studies equilibrium only
ⓒ. Thermodynamics — studies feasibility; Kinetics — studies speed
ⓓ. Thermodynamics — studies molecularity; Kinetics — studies spontaneity
Correct Answer: Thermodynamics — studies feasibility; Kinetics — studies speed
Explanation: Thermodynamics tells whether a process is energetically possible or favourable. Chemical kinetics tells how fast that process takes place. This distinction is fundamental because a feasible reaction may still be very slow.
6. Rusting of iron often takes place over a long period, whereas ionic precipitation can occur almost immediately. The most appropriate conclusion is that
ⓐ. both processes must have the same reaction rate because both form products.
ⓑ. only slow reactions can occur naturally.
ⓒ. visible product formation always means the reaction is fast.
ⓓ. chemical reactions can differ widely in the time they take to proceed.
Correct Answer: chemical reactions can differ widely in the time they take to proceed.
Explanation: Some reactions are nearly instantaneous, while others require hours, days, or longer. This wide variation in time of occurrence is one of the central reasons for studying chemical kinetics. The presence of products does not imply equal reaction speeds.
7. Which statement is correct for the study of reaction progress in chemical kinetics?
ⓐ. It considers how reactants are converted into products as time passes.
ⓑ. It ignores time and compares only initial and final states.
ⓒ. It applies only after equilibrium has been reached.
ⓓ. It applies only to reactions involving gases.
Correct Answer: It considers how reactants are converted into products as time passes.
Explanation: Chemical kinetics follows the progress of a reaction with time. The core idea is that the composition of the reacting system changes as reactants are consumed and products are formed. Time is therefore an essential variable in kinetic study.
8. Which statement best distinguishes a kinetic description from a thermodynamic description?
ⓐ. A kinetic description states whether the reaction mixture contains solids or liquids.
ⓑ. A kinetic description tells whether the reaction equation is balanced.
ⓒ. A kinetic description tells how fast the system moves toward products.
ⓓ. A kinetic description tells only the total mass of products formed.
Correct Answer: A kinetic description tells how fast the system moves toward products.
Explanation: Kinetics is the time-dependent description of a reaction. It focuses on the speed of conversion of reactants into products and how that speed changes. A thermodynamic description, in contrast, addresses feasibility, stability, or energy aspects rather than reaction speed.
9. Two samples of different reactions are both capable of forming products under suitable conditions. One shows a visible change in seconds, while the other changes only after several hours. This difference is mainly a matter of
ⓐ. equilibrium position.
ⓑ. reaction rate.
ⓒ. molecular formula.
ⓓ. stoichiometric ratio.
Correct Answer: reaction rate.
Explanation: The key difference here is the time taken for observable chemical change. Chemical kinetics deals with how fast a reaction proceeds, so reactions that occur in seconds and reactions that take hours differ mainly in rate.
10. Which observation is least useful when the aim is to study how fast a reaction proceeds?
ⓐ. Time taken for turbidity to appear
ⓑ. Concentration of a reactant at regular intervals
ⓒ. Volume of gas evolved with time
ⓓ. Standard enthalpy change of the reaction
Correct Answer: Standard enthalpy change of the reaction
Explanation: Rate study requires a measurable change with time, such as concentration, gas volume, or appearance of turbidity. Standard enthalpy change describes the heat effect of the reaction, not the speed at which the reaction occurs.
11. Which statement is not necessarily true for a chemically feasible reaction?
ⓐ. It must occur rapidly at ordinary conditions.
ⓑ. It may proceed slowly because of a kinetic barrier.
ⓒ. It can take a long time to show noticeable change.
ⓓ. Its speed must be considered separately from feasibility.
Correct Answer: It must occur rapidly at ordinary conditions.
Explanation: Feasibility and speed are different ideas. A reaction may be capable of occurring and yet proceed very slowly under ordinary conditions. That is why reaction rate has to be studied separately.
12. Which quantity is commonly used to express how fast a chemical reaction is taking place?
ⓐ. Change in colour of the vessel with time
ⓑ. Change in molar mass of the catalyst with time
ⓒ. Change in concentration of a reactant or product with time
ⓓ. Change in atomic number of an element with time
Correct Answer: Change in concentration of a reactant or product with time
Explanation: Reaction rate is measured through how the concentration of reactants decreases or how the concentration of products increases as time passes. Concentration-time data therefore provide a direct way to describe reaction speed.
13. Why must a time interval be specified when describing how fast a reaction proceeds?
ⓐ. Because time fixes the stoichiometric coefficients of the reaction
ⓑ. Because time determines the equilibrium constant directly
ⓒ. Because time changes the chemical formula of the reactants
ⓓ. Because rate is based on concentration change over a stated time interval
Correct Answer: Because rate is based on concentration change over a stated time interval
Explanation: A reaction rate has meaning only when the concentration change is connected to a particular interval of time. Without the time interval, the amount of change alone does not tell how fast the reaction occurred.
14. For the same reaction, measured values of rate may differ when different observation intervals are chosen because
ⓐ. concentration has no role in deciding rate.
ⓑ. the concentration change is judged over the chosen interval.
ⓒ. the balanced equation changes with time.
ⓓ. the product stops obeying stoichiometry.
Correct Answer: the concentration change is judged over the chosen interval.
Explanation: The observed rate depends on how much concentration changes during the interval being considered. If the interval changes, the measured concentration change per unit time can also change, especially as the reaction progresses.
15. Which set of data is most suitable for following the progress of a reaction with time?
ⓐ. Initial colour of the mixture only
ⓑ. Final temperature of the mixture only
ⓒ. Concentration of a reactant measured at several known times
ⓓ. Melting point of the isolated product only
Correct Answer: Concentration of a reactant measured at several known times
Explanation: To track reaction progress, one needs repeated measurements linked to time. A series of concentration values taken at known times shows how the reacting system changes as the reaction proceeds.
16. For the reaction $A \rightarrow B$, which observation clearly indicates forward reaction progress?
ⓐ. $[A]$ decreases while $[B]$ increases with time
ⓑ. $[A]$ and $[B]$ remain unchanged from the start
ⓒ. $[A]$ increases while $[B]$ decreases in a closed system
ⓓ. The mass number of $A$ decreases with time
Correct Answer: $[A]$ decreases while $[B]$ increases with time
Explanation: In forward progress, reactant particles are consumed and product particles are formed. Therefore the concentration of $A$ falls while the concentration of $B$ rises as time passes.
17. A student says, “The reaction rate is 0.20.” Which additional information is essential before this statement becomes meaningful?
ⓐ. The colour of the reaction mixture
ⓑ. The concentration change and the time interval used
ⓒ. The balanced equation written in ionic form
ⓓ. The molar mass of each reactant
Correct Answer: The concentration change and the time interval used
Explanation: A reaction rate is defined through how much concentration changes in a given time. A number alone is incomplete unless it is tied to a concentration change per unit time. That is why both concentration information and the corresponding time interval are necessary.
18. Which quantity can directly serve as a measurable basis for reaction rate?
ⓐ. Change in concentration with time
ⓑ. Number of atoms in one molecule
ⓒ. Valency of the product element
ⓓ. Standard atomic mass of the reactant
Correct Answer: Change in concentration with time
Explanation: Reaction rate is measured from how quickly the concentration of a reactant decreases or that of a product increases. Atomic mass, valency, and atom count do not by themselves tell how fast a reaction is proceeding.
19. In a reaction mixture, the concentration of a reactant falls rapidly during the first few seconds and then decreases more slowly later. This shows that
ⓐ. the reaction rate must remain constant throughout.
ⓑ. the reaction has stopped after the first few seconds.
ⓒ. only products determine the rate of reaction.
ⓓ. the rate can change as the reaction proceeds.
Correct Answer: the rate can change as the reaction proceeds.
Explanation: Rate is often larger at the beginning and smaller later because the concentrations of reactants usually decrease with time. As the reaction progresses, the concentration-time pattern can change, so the rate need not stay constant.
20. Which statement correctly explains why concentration is commonly used to express reaction rate?
ⓐ. Concentration can show how composition changes with time.
ⓑ. Concentration remains the same throughout a reaction.
ⓒ. Concentration gives the activation energy directly.
ⓓ. Concentration is independent of the amount of substance present.
Correct Answer: Concentration can show how composition changes with time.
Explanation: The progress of a reaction is reflected in changing amounts of reactants and products. Expressing these changes through concentration makes it possible to connect composition with time, which is exactly what rate measurement requires.
21. For a reactant $R$, its concentration changes from $0.80\,\text{mol L}^{-1}$ to $0.50\,\text{mol L}^{-1}$ in $10\,\text{s}$. What is the average rate of disappearance of $R$?
ⓐ. $0.02\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $-0.03\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.05\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Initial concentration of $R$, $[R]_1 = 0.80\,\text{mol L}^{-1}$
Final concentration of $R$, $[R]_2 = 0.50\,\text{mol L}^{-1}$
Time interval, $\Delta t = 10\,\text{s}$
Required:
Average rate of disappearance of $R$
Relevant formula:
$\text{Average rate} = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
For a reactant, concentration decreases with time, so a negative sign is used to keep the rate positive.
Identify the concentration change:
$\Delta [R] = [R]_2 – [R]_1 = 0.50 – 0.80 = -0.30\,\text{mol L}^{-1}$
Substitution:
$\text{Average rate} = -\frac{-0.30}{10}$
Intermediate simplification:
$\text{Average rate} = \frac{0.30}{10}$
Final simplification:
$\text{Average rate} = 0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Unit check:
Concentration divided by time gives $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
$0.03\,\text{mol L}^{-1}\text{s}^{-1}$
22. The concentration of a product $P$ increases from $0.12\,\text{mol L}^{-1}$ to $0.30\,\text{mol L}^{-1}$ in $6\,\text{s}$. What is the average rate of formation of $P$?
ⓐ. $0.02\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $-0.03\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.05\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Initial concentration of $P$, $[P]_1 = 0.12\,\text{mol L}^{-1}$
Final concentration of $P$, $[P]_2 = 0.30\,\text{mol L}^{-1}$
Time interval, $\Delta t = 6\,\text{s}$
Required:
Average rate of formation of $P$
Relevant formula:
$\text{Average rate} = \frac{\Delta [P]}{\Delta t}$
Why this formula applies:
For a product, concentration increases with time, so no negative sign is needed.
Identify the concentration change:
$\Delta [P] = [P]_2 – [P]_1 = 0.30 – 0.12 = 0.18\,\text{mol L}^{-1}$
Substitution:
$\text{Average rate} = \frac{0.18}{6}$
Intermediate simplification:
$\text{Average rate} = 0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Unit check:
Concentration per unit time has unit $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
$0.03\,\text{mol L}^{-1}\text{s}^{-1}$
23. Which expression correctly represents the average rate of disappearance of a reactant $R$ over a finite time interval?
ⓐ. $\frac{\Delta [R]}{\Delta t}$
ⓑ. $-\frac{\Delta t}{\Delta [R]}$
ⓒ. $-\frac{\Delta [R]}{\Delta t}$
ⓓ. $\frac{[R]_0}{t}$
Correct Answer: $-\frac{\Delta [R]}{\Delta t}$
Explanation: For a reactant, concentration decreases with time, so $\Delta [R]$ is negative over the interval. The negative sign in $-\frac{\Delta [R]}{\Delta t}$ makes the rate a positive quantity for disappearance. This is the standard average-rate expression for a reactant.
24. A reactant concentration changes from $1.00\,\text{mol L}^{-1}$ to $0.76\,\text{mol L}^{-1}$ in $8\,\text{s}$. Which value gives the average rate of disappearance of the reactant?
ⓐ. $0.24\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $-0.03\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.04\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Initial concentration, $[R]_1 = 1.00\,\text{mol L}^{-1}$
Final concentration, $[R]_2 = 0.76\,\text{mol L}^{-1}$
Time interval, $\Delta t = 8\,\text{s}$
Required:
Average rate of disappearance
Relevant formula:
$\text{Average rate} = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
The concentration of a reactant decreases, so the negative sign is required to report a positive disappearance rate.
Identify the concentration change:
$\Delta [R] = [R]_2 – [R]_1 = 0.76 – 1.00 = -0.24\,\text{mol L}^{-1}$
Substitution:
$\text{Average rate} = -\frac{-0.24}{8}$
Intermediate simplification:
$\text{Average rate} = \frac{0.24}{8}$
Final simplification:
$\text{Average rate} = 0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Unit / notation check:
The unit is concentration per time, so $\text{mol L}^{-1}\text{s}^{-1}$ is correct.
Final Answer:
$0.03\,\text{mol L}^{-1}\text{s}^{-1}$
25. Which statement correctly describes the average rate of a chemical reaction?
ⓐ. It is the change in concentration measured over a finite time interval.
ⓑ. It is the concentration of products at the end of the reaction only.
ⓒ. It is the rate calculated exactly at a single instant of time.
ⓓ. It is the total amount of reactant taken, without using time.
Correct Answer: It is the change in concentration measured over a finite time interval.
Explanation: Average rate is based on a measurable concentration change during a specified interval of time. It does not refer to a single instant, and it cannot be described without including time. That is why it is a finite-interval quantity.
26. The average rate of disappearance of a reactant is $0.04\,\text{mol L}^{-1}\text{s}^{-1}$ for $5\,\text{s}$. What is the decrease in concentration of the reactant during this interval?
ⓐ. $0.01\,\text{mol L}^{-1}$
ⓑ. $0.09\,\text{mol L}^{-1}$
ⓒ. $0.20\,\text{mol L}^{-1}$
ⓓ. $0.80\,\text{mol L}^{-1}$
Correct Answer: $0.20\,\text{mol L}^{-1}$
Explanation: Given:
Average rate of disappearance, $r = 0.04\,\text{mol L}^{-1}\text{s}^{-1}$
Time interval, $\Delta t = 5\,\text{s}$
Required:
Decrease in concentration of the reactant
Relevant formula:
$r = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
For disappearance of a reactant, the concentration decreases with time, so the negative sign is used in the rate expression.
Identify the magnitude of concentration change:
$r = \frac{\text{decrease in concentration}}{\Delta t}$
Substitution:
$\text{decrease in concentration} = r \times \Delta t$
$\text{decrease in concentration} = 0.04 \times 5$
Intermediate simplification:
$\text{decrease in concentration} = 0.20$
Unit / notation check:
Rate unit is $\text{mol L}^{-1}\text{s}^{-1}$ and time is in $\text{s}$, so the result is in $\text{mol L}^{-1}$
Final Answer:
Decrease in concentration $= 0.20\,\text{mol L}^{-1}$
27. For a reaction, the concentration of a reactant changes from $1.20\,\text{mol L}^{-1}$ to $0.90\,\text{mol L}^{-1}$ in the first $10\,\text{s}$ and from $0.90\,\text{mol L}^{-1}$ to $0.75\,\text{mol L}^{-1}$ in the next $10\,\text{s}$. Which statement is correct?
ⓐ. The average rate is the same in both intervals.
ⓑ. The average rate is greater in the first interval.
ⓒ. The average rate is greater in the second interval.
ⓓ. The average rate cannot be compared without product data.
Correct Answer: The average rate is greater in the first interval.
Explanation: Given:
First interval: $1.20 \to 0.90\,\text{mol L}^{-1}$ in $10\,\text{s}$
Second interval: $0.90 \to 0.75\,\text{mol L}^{-1}$ in $10\,\text{s}$
Required:
Compare the average rates in the two intervals
Relevant formula:
$\text{Average rate of disappearance} = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
The reactant concentration is decreasing in each interval.
First interval:
$\Delta [R] = 0.90 – 1.20 = -0.30\,\text{mol L}^{-1}$
$r_1 = -\frac{-0.30}{10} = 0.03\,\text{mol L}^{-1}\text{s}^{-1}$
Second interval:
$\Delta [R] = 0.75 – 0.90 = -0.15\,\text{mol L}^{-1}$
$r_2 = -\frac{-0.15}{10} = 0.015\,\text{mol L}^{-1}\text{s}^{-1}$
Comparison:
$0.03 > 0.015$
Final Answer:
The average rate is greater in the first interval.
28. Which unit cannot represent the rate of a chemical reaction?
ⓐ. $\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $\text{mol L}^{-1}\text{min}^{-1}$
ⓒ. $\text{mmol L}^{-1}\text{s}^{-1}$
ⓓ. $\text{mol}^2\text{L}^{-1}\text{s}^{-1}$
Correct Answer: $\text{mol}^2\text{L}^{-1}\text{s}^{-1}$
Explanation: Reaction rate has the dimension of concentration divided by time. So valid units must look like amount-per-volume per unit time, such as $\text{mol L}^{-1}\text{s}^{-1}$ or $\text{mol L}^{-1}\text{min}^{-1}$. The unit $\text{mol}^2\text{L}^{-1}\text{s}^{-1}$ does not match concentration per time.
29. Why is a negative sign used in the average rate expression for a reactant?
ⓐ. To convert the decreasing concentration change into a positive rate value
ⓑ. To show that products are not formed during the reaction
ⓒ. To make the rate numerically smaller than the product rate
ⓓ. To indicate that the reaction is always exothermic
Correct Answer: To convert the decreasing concentration change into a positive rate value
Explanation: For a reactant, concentration falls with time, so $\Delta [R]$ is negative. The minus sign in $-\frac{\Delta [R]}{\Delta t}$ ensures that the rate of disappearance is reported as a positive quantity. It is a sign convention linked to concentration decrease.
30. The average rate of formation of a product is $0.015\,\text{mol L}^{-1}\text{s}^{-1}$ for $20\,\text{s}$. If the initial concentration of the product was $0.10\,\text{mol L}^{-1}$, what is its concentration after $20\,\text{s}$?
ⓐ. $0.25\,\text{mol L}^{-1}$
ⓑ. $0.30\,\text{mol L}^{-1}$
ⓒ. $0.40\,\text{mol L}^{-1}$
ⓓ. $0.50\,\text{mol L}^{-1}$
Correct Answer: $0.40\,\text{mol L}^{-1}$
Explanation: Given:
Average rate of formation, $r = 0.015\,\text{mol L}^{-1}\text{s}^{-1}$
Time interval, $\Delta t = 20\,\text{s}$
Initial concentration of product, $[P]_1 = 0.10\,\text{mol L}^{-1}$
Required:
Final concentration of the product
Relevant formula:
$r = \frac{\Delta [P]}{\Delta t}$
Why this formula applies:
For a product, concentration increases with time.
Substitution:
$\Delta [P] = r \times \Delta t = 0.015 \times 20$
Intermediate simplification:
$\Delta [P] = 0.30\,\text{mol L}^{-1}$
Now add this increase to the initial concentration:
$[P]_2 = 0.10 + 0.30$
Final simplification:
$[P]_2 = 0.40\,\text{mol L}^{-1}$
Unit check:
Product concentration remains in $\text{mol L}^{-1}$
Final Answer:
$0.40\,\text{mol L}^{-1}$
31. Which statement is correct about the average rate measured over a time interval?
ⓐ. It always gives the fastest possible rate during that interval.
ⓑ. It is always equal to the rate at the midpoint of the interval.
ⓒ. It depends only on the initial concentration, not on time.
ⓓ. It represents the overall concentration change per unit time for that interval.
Correct Answer: It represents the overall concentration change per unit time for that interval.
Explanation: Average rate summarizes how much concentration changes over the chosen interval as a whole. It does not necessarily match the rate at any particular instant within that interval. Its value depends on the finite interval selected.
32. For a product $P$, the average rate of formation is $\frac{\Delta [P]}{\Delta t} = 2.5 \times 10^{-3}\,\text{mol L}^{-1}\text{s}^{-1}$. What amount of product is formed in $200\,\text{s}$ per litre of solution?
ⓐ. $1.25 \times 10^{-5}\,\text{mol L}^{-1}$
ⓑ. $0.50\,\text{mol L}^{-1}$
ⓒ. $80\,\text{mol L}^{-1}$
ⓓ. $5.0 \times 10^{-1}\,\text{mol L}^{-1}$
Correct Answer: $5.0 \times 10^{-1}\,\text{mol L}^{-1}$
Explanation: Given:
Average rate of formation, $r = 2.5 \times 10^{-3}\,\text{mol L}^{-1}\text{s}^{-1}$
Time interval, $\Delta t = 200\,\text{s}$
Required:
Amount of product formed per litre in $200\,\text{s}$
Relevant formula:
$r = \frac{\Delta [P]}{\Delta t}$
Why this formula applies:
The product concentration increases with time, so the change in concentration is found by multiplying rate and time.
Substitution:
$\Delta [P] = r \times \Delta t$
$\Delta [P] = \left(2.5 \times 10^{-3}\right)(200)$
Intermediate simplification:
$\Delta [P] = 2.5 \times 2 \times 10^{-3} \times 10^2$
$\Delta [P] = 5.0 \times 10^{-1}$
Unit / notation check:
The unit is $\text{mol L}^{-1}$ because rate was in $\text{mol L}^{-1}\text{s}^{-1}$ and time in $\text{s}$
Final Answer:
$\Delta [P] = 5.0 \times 10^{-1}\,\text{mol L}^{-1}$
33. The average rate of disappearance of a reactant is $0.025\,\text{mol L}^{-1}\text{s}^{-1}$ for $12\,\text{s}$. If its initial concentration is $0.90\,\text{mol L}^{-1}$, what is its concentration after $12\,\text{s}$?
ⓐ. $0.60\,\text{mol L}^{-1}$
ⓑ. $1.20\,\text{mol L}^{-1}$
ⓒ. $0.30\,\text{mol L}^{-1}$
ⓓ. $0.875\,\text{mol L}^{-1}$
Correct Answer: $0.60\,\text{mol L}^{-1}$
Explanation: Given:
Average rate of disappearance, $r = 0.025\,\text{mol L}^{-1}\text{s}^{-1}$
Time interval, $\Delta t = 12\,\text{s}$
Initial concentration, $[R]_1 = 0.90\,\text{mol L}^{-1}$
Required:
Final concentration after $12\,\text{s}$
Relevant formula:
$r = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
For a reactant, concentration decreases with time.
First find the decrease in concentration:
$\text{decrease} = r \times \Delta t$
Substitution:
$\text{decrease} = 0.025 \times 12 = 0.30\,\text{mol L}^{-1}$
Now subtract this decrease from the initial concentration:
$[R]_2 = 0.90 – 0.30$
Final simplification:
$[R]_2 = 0.60\,\text{mol L}^{-1}$
Unit / notation check:
Concentration remains in $\text{mol L}^{-1}$
Final Answer:
$0.60\,\text{mol L}^{-1}$
34. The concentration of a product increases from $0.18\,\text{mol L}^{-1}$ to $0.42\,\text{mol L}^{-1}$ in $8\,\text{s}$. What is the average rate of formation of the product?
ⓐ. $0.015\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.030\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $-0.030\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.075\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.030\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Initial concentration, $[P]_1 = 0.18\,\text{mol L}^{-1}$
Final concentration, $[P]_2 = 0.42\,\text{mol L}^{-1}$
Time interval, $\Delta t = 8\,\text{s}$
Required:
Average rate of formation of the product
Relevant formula:
$\text{Average rate} = \frac{\Delta [P]}{\Delta t}$
Why this formula applies:
For a product, concentration increases with time, so no negative sign is used.
Identify the concentration change:
$\Delta [P] = 0.42 – 0.18 = 0.24\,\text{mol L}^{-1}$
Substitution:
$\text{Average rate} = \frac{0.24}{8}$
Final simplification:
$\text{Average rate} = 0.030\,\text{mol L}^{-1}\text{s}^{-1}$
Unit check:
The unit is concentration per unit time, so $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
$0.030\,\text{mol L}^{-1}\text{s}^{-1}$
35. For the disappearance of a reactant $R$, the average rate over a certain interval is written as $-\frac{\Delta [R]}{\Delta t}$. This negative sign is needed because
ⓐ. time is always taken as a negative quantity in kinetics.
ⓑ. reactants always have smaller stoichiometric coefficients than products.
ⓒ. the concentration of a reactant decreases, making $\Delta [R]$ negative.
ⓓ. the rate of a reaction must always be numerically less than unity.
Correct Answer: the concentration of a reactant decreases, making $\Delta [R]$ negative.
Explanation: During reaction progress, the concentration of a reactant falls with time, so the value of $\Delta [R]$ is negative over a finite interval. The negative sign converts this into a positive rate of disappearance. It is therefore a sign convention linked to decreasing reactant concentration.
36. Which statement best defines the instantaneous rate of a reaction?
ⓐ. It is the total concentration change from start to completion.
ⓑ. It is the rate obtained over any large time interval.
ⓒ. It is the concentration of reactant remaining at the end.
ⓓ. It is the rate at a particular moment, found using an extremely small time interval.
Correct Answer: It is the rate at a particular moment, found using an extremely small time interval.
Explanation: Instantaneous rate refers to the rate at one specific moment during the reaction. It is obtained by considering the concentration change over a very small interval of time. This distinguishes it from average rate, which is based on a finite interval.
37. Which statement correctly compares average rate and instantaneous rate?
ⓐ. Average rate is measured over a finite interval, whereas instantaneous rate refers to a specific moment.
ⓑ. Average rate is always greater than instantaneous rate for every reaction.
ⓒ. Instantaneous rate is measured only when the reaction is complete.
ⓓ. Average rate and instantaneous rate are always numerically identical.
Correct Answer: Average rate is measured over a finite interval, whereas instantaneous rate refers to a specific moment.
Explanation: Average rate is based on an overall concentration change during a chosen interval of time. Instantaneous rate describes the rate at one particular moment. They may be close for a very small interval, but they are not defined in the same way.
38. In a concentration-time description for a reactant, one portion shows a steeper downward change than another. What does the steeper portion indicate?
ⓐ. A smaller instantaneous rate
ⓑ. Zero concentration of the reactant
ⓒ. A larger magnitude of instantaneous rate
ⓓ. Completion of the reaction in that interval
Correct Answer: A larger magnitude of instantaneous rate
Explanation: A steeper fall in reactant concentration with time means the concentration is decreasing more rapidly. That corresponds to a larger magnitude of rate at that moment. A less steep fall indicates a smaller instantaneous rate.
39. If the rate of a reaction changes continuously during an experiment, which statement is most appropriate?
ⓐ. The average rate over a long interval gives the exact rate at every moment.
ⓑ. The instantaneous rate can vary from moment to moment during the reaction.
ⓒ. The instantaneous rate must remain constant until all reactant is used.
ⓓ. The average rate becomes undefined if the concentration changes nonlinearly.
Correct Answer: The instantaneous rate can vary from moment to moment during the reaction.
Explanation: When the reaction does not proceed at a constant speed, the rate at one moment need not match the rate at another moment. Instantaneous rate captures this moment-by-moment variation. Average rate over a longer interval only gives an overall summary.
40. Which situation gives the closest numerical estimate of the instantaneous rate at time $t$?
ⓐ. Using the concentration change from $t=0$ to the end of the reaction
ⓑ. Using the concentration change over a very small interval around time $t$
ⓒ. Using the initial concentration only
ⓓ. Using the final concentration only
Correct Answer: Using the concentration change over a very small interval around time $t$
Explanation: Instantaneous rate is approached by taking the concentration change over an interval that is extremely small and centered around the chosen time. The smaller the interval, the closer the value comes to the rate at that exact moment. Large intervals instead give average rate.
41. Under which condition does the average rate give the closest value to the instantaneous rate at a chosen moment?
ⓐ. When the time interval taken is extremely small
ⓑ. When the concentrations of all species are integers
ⓒ. When the reaction has already gone to completion
ⓓ. When only the final concentration is known
Correct Answer: When the time interval taken is extremely small
Explanation: Instantaneous rate refers to the rate at a particular moment. A very small time interval around that moment makes the average rate over that interval nearly equal to the instantaneous rate. Larger intervals smooth out the rate variation and therefore give only an overall average.
42. If the concentration of a reactant decreases linearly with time with a constant negative slope, which statement is correct?
ⓐ. The instantaneous rate must increase with time.
ⓑ. The average rate must become zero after some time.
ⓒ. The instantaneous rate and the average rate cannot be compared.
ⓓ. The instantaneous rate has the same magnitude at all times.
Correct Answer: The instantaneous rate has the same magnitude at all times.
Explanation: A constant negative slope means the concentration changes by equal amounts in equal time intervals. That indicates a constant rate. When the slope does not change, the instantaneous rate remains the same at every moment.
43. For a reaction, the average rate during $0$ to $5\,\text{s}$ is not equal to the average rate during $5$ to $10\,\text{s}$. What does this show most directly?
ⓐ. The reaction is impossible at room temperature.
ⓑ. The reaction rate is changing with time.
ⓒ. The concentration unit must be incorrect.
ⓓ. The stoichiometric equation is unbalanced.
Correct Answer: The reaction rate is changing with time.
Explanation: If two equal time intervals give different average rates, the speed of the reaction is not constant throughout the experiment. This means the instantaneous rate changes as the reaction proceeds.
44. At a certain moment, the concentration-time description of a product shows no change in concentration for a very small interval around that moment. What is the instantaneous rate of formation at that moment?
ⓐ. Maximum
ⓑ. Negative
ⓒ. Zero
ⓓ. Infinite
Correct Answer: Zero
Explanation: Instantaneous rate depends on how rapidly concentration changes at that moment. If there is no concentration change in an extremely small interval, the slope is zero, so the instantaneous rate is zero.
45. For the reaction $2A + B \rightarrow 3C$, which expression correctly represents the reaction rate?
ⓐ. $-\frac{1}{2}\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{1}{3}\frac{d[C]}{dt}$
ⓑ. $-\frac{d[A]}{dt} = -\frac{1}{2}\frac{d[B]}{dt} = \frac{1}{3}\frac{d[C]}{dt}$
ⓒ. $-\frac{1}{3}\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{1}{2}\frac{d[C]}{dt}$
ⓓ. $-\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{d[C]}{dt}$
Correct Answer: $-\frac{1}{2}\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{1}{3}\frac{d[C]}{dt}$
Explanation: In a general reaction, concentration changes are normalized by stoichiometric coefficients so that one single reaction rate is obtained. For $2A + B \rightarrow 3C$, reactant terms carry negative signs and each term is divided by its coefficient. That gives $-\frac{1}{2}\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{1}{3}\frac{d[C]}{dt}$.
46. For the reaction $2A + B \rightarrow 3C$, the rate of disappearance of $A$ is $0.24\,\text{mol L}^{-1}\text{s}^{-1}$. What is the rate of the reaction?
ⓐ. $0.06\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.24\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.36\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.12\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.12\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Reaction: $2A + B \rightarrow 3C$
Rate of disappearance of $A = -\frac{d[A]}{dt} = 0.24\,\text{mol L}^{-1}\text{s}^{-1}$
Required:
Rate of the reaction
Relevant formula:
$\text{Rate} = -\frac{1}{2}\frac{d[A]}{dt}$
Why this formula applies:
The reactant $A$ has stoichiometric coefficient $2$, so its concentration-change term must be divided by $2$ to obtain the common reaction rate.
Identify the known value:
$-\frac{d[A]}{dt} = 0.24\,\text{mol L}^{-1}\text{s}^{-1}$
Substitution:
$\text{Rate} = \frac{1}{2}\times 0.24$
Intermediate simplification:
$\text{Rate} = 0.12$
Unit / notation check:
Rate unit remains $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
$\text{Rate} = 0.12\,\text{mol L}^{-1}\text{s}^{-1}$
47. In the reaction $aA + bB \rightarrow cC + dD$, why are concentration-change terms divided by stoichiometric coefficients in the rate expression?
ⓐ. To remove the effect of temperature from the reaction
ⓑ. To obtain one consistent value for the reaction rate from all species
ⓒ. To ensure that all reactants have the same initial concentration
ⓓ. To make the rate expression independent of the balanced equation
Correct Answer: To obtain one consistent value for the reaction rate from all species
Explanation: Different species in a reaction do not generally change concentration at the same numerical rate because stoichiometric coefficients differ. Dividing by the respective coefficients normalizes these changes and gives a single consistent reaction rate.
48. For the reaction $2A \rightarrow 3B$, the rate of formation of $B$ is $0.30\,\text{mol L}^{-1}\text{s}^{-1}$. What is the rate of disappearance of $A$?
ⓐ. $0.20\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.30\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.20\,\text{mol L}^{-1}\text{s}^{-1}$ in magnitude
ⓓ. $0.45\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.20\,\text{mol L}^{-1}\text{s}^{-1}$ in magnitude
Explanation: Given:
Reaction: $2A \rightarrow 3B$
Rate of formation of $B$, $\frac{d[B]}{dt} = 0.30\,\text{mol L}^{-1}\text{s}^{-1}$
Required:
Rate of disappearance of $A$
Relevant formula:
$-\frac{1}{2}\frac{d[A]}{dt} = \frac{1}{3}\frac{d[B]}{dt}$
Why this formula applies:
The concentration changes must be normalized by stoichiometric coefficients to represent the same reaction rate.
First find the reaction rate from product data:
$\text{Rate} = \frac{1}{3}\frac{d[B]}{dt} = \frac{1}{3}\times 0.30 = 0.10\,\text{mol L}^{-1}\text{s}^{-1}$
Now relate this to reactant $A$:
$-\frac{1}{2}\frac{d[A]}{dt} = 0.10$
Multiply both sides by $2$:
$-\frac{d[A]}{dt} = 0.20\,\text{mol L}^{-1}\text{s}^{-1}$
Unit / notation check:
The rate of disappearance is reported as a positive magnitude in $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
Rate of disappearance of $A = 0.20\,\text{mol L}^{-1}\text{s}^{-1}$
49. For the reaction $aA + bB \rightarrow cC + dD$, which expression correctly gives a single consistent reaction rate?
ⓐ. $-\frac{d[A]}{dt} = -\frac{d[B]}{dt} = \frac{d[C]}{dt} = \frac{d[D]}{dt}$
ⓑ. $-\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt}$
ⓒ. $\frac{1}{a}\frac{d[A]}{dt} = \frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt}$
ⓓ. $-\frac{a\,d[A]}{dt} = -\frac{b\,d[B]}{dt} = \frac{c\,d[C]}{dt} = \frac{d\,d[D]}{dt}$
Correct Answer: $-\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} = \frac{1}{d}\frac{d[D]}{dt}$
Explanation: A single reaction rate is obtained by normalizing the concentration-change terms with stoichiometric coefficients. Reactant terms carry negative signs because their concentrations decrease, while product terms are positive because their concentrations increase. This gives one common value for the rate, regardless of which species is used.
50. For the reaction $3A + 2B \rightarrow 4C$, the rate of disappearance of $B$ is $0.18\,\text{mol L}^{-1}\text{s}^{-1}$. What is the rate of formation of $C$?
ⓐ. $0.09\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.18\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.27\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.36\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.36\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Reaction: $3A + 2B \rightarrow 4C$
Rate of disappearance of $B$, $-\frac{d[B]}{dt} = 0.18\,\text{mol L}^{-1}\text{s}^{-1}$
Required:
Rate of formation of $C$
Relevant formula:
$-\frac{1}{2}\frac{d[B]}{dt} = \frac{1}{4}\frac{d[C]}{dt}$
Why this formula applies:
The concentration changes must be divided by stoichiometric coefficients to represent the same reaction rate.
First find the reaction rate from $B$:
$\text{Rate} = \frac{1}{2}\times 0.18 = 0.09\,\text{mol L}^{-1}\text{s}^{-1}$
Now relate this to $C$:
$\frac{1}{4}\frac{d[C]}{dt} = 0.09$
Multiply both sides by $4$:
$\frac{d[C]}{dt} = 0.36\,\text{mol L}^{-1}\text{s}^{-1}$
Unit / notation check:
The unit remains $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
Rate of formation of $C = 0.36\,\text{mol L}^{-1}\text{s}^{-1}$
51. In the reaction $2A + B \rightarrow 3C$, the numerical values of $-\frac{d[A]}{dt}$, $-\frac{d[B]}{dt}$, and $\frac{d[C]}{dt}$ are generally different. Why?
ⓐ. Their raw concentration changes reflect different stoichiometric consumption and formation ratios.
ⓑ. Reactants and products are measured in different concentration units.
ⓒ. Product concentrations are always measured more accurately than reactant concentrations.
ⓓ. Only one species actually changes concentration during the reaction.
Correct Answer: Their raw concentration changes reflect different stoichiometric consumption and formation ratios.
Explanation: Species in a reaction are consumed and formed according to stoichiometric coefficients. Because of that, their direct concentration changes per unit time are not usually equal in magnitude. Dividing by the respective coefficients converts them into one common reaction rate.
52. For the reaction $A + 2B \rightarrow C$, the rate of formation of $C$ is $0.08\,\text{mol L}^{-1}\text{s}^{-1}$. What is the rate of disappearance of $B$?
ⓐ. $0.04\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $0.08\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $0.16\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.24\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.16\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Reaction: $A + 2B \rightarrow C$
Rate of formation of $C$, $\frac{d[C]}{dt} = 0.08\,\text{mol L}^{-1}\text{s}^{-1}$
Required:
Rate of disappearance of $B$
Relevant formula:
$-\frac{1}{2}\frac{d[B]}{dt} = \frac{d[C]}{dt}$
Why this formula applies:
The common rate is obtained after dividing each concentration-change term by its stoichiometric coefficient.
Substitution:
$-\frac{1}{2}\frac{d[B]}{dt} = 0.08$
Multiply both sides by $2$:
$-\frac{d[B]}{dt} = 0.16\,\text{mol L}^{-1}\text{s}^{-1}$
Unit / notation check:
The disappearance rate is expressed in $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
Rate of disappearance of $B = 0.16\,\text{mol L}^{-1}\text{s}^{-1}$
53. Which statement correctly describes the unit of reaction rate?
ⓐ. It depends only on the balanced equation, not on the chosen time unit.
ⓑ. It must always be written as $\text{s}^{-1}$.
ⓒ. It is always the same as the unit of the rate constant.
ⓓ. It is concentration per unit time.
Correct Answer: It is concentration per unit time.
Explanation: Reaction rate tells how much concentration changes in a given time. Therefore its unit must be a concentration unit divided by a time unit. Examples include $\text{mol L}^{-1}\text{s}^{-1}$ and $\text{mol L}^{-1}\text{min}^{-1}$.
54. A reaction rate is calculated using concentration in $\text{mol L}^{-1}$ and time in minutes. Which unit is correct for the rate?
ⓐ. $\text{mol}^2\text{L}^{-1}\text{min}^{-1}$
ⓑ. $\text{mol L}^{-1}\text{min}^{-1}$
ⓒ. $\text{L mol}^{-1}\text{min}^{-1}$
ⓓ. $\text{min mol}^{-1}\text{L}^{-1}$
Correct Answer: $\text{mol L}^{-1}\text{min}^{-1}$
Explanation: Since rate is concentration divided by time, the unit must combine the chosen concentration unit with the chosen time unit. If concentration is in $\text{mol L}^{-1}$ and time in minutes, the rate unit becomes $\text{mol L}^{-1}\text{min}^{-1}$.
55. A reactant concentration decreases from $0.90\,\text{mol L}^{-1}$ to $0.60\,\text{mol L}^{-1}$ in $30\,\text{s}$. What is the average rate of disappearance?
ⓐ. $0.010\,\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $-0.010\,\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $9.00\,\text{mol L}^{-1}\text{s}^{-1}$
ⓓ. $0.300\,\text{mol L}^{-1}\text{s}^{-1}$
Correct Answer: $0.010\,\text{mol L}^{-1}\text{s}^{-1}$
Explanation: Given:
Initial concentration, $[R]_1 = 0.90\,\text{mol L}^{-1}$
Final concentration, $[R]_2 = 0.60\,\text{mol L}^{-1}$
Time interval, $\Delta t = 30\,\text{s}$
Required:
Average rate of disappearance
Relevant formula:
$\text{Average rate} = -\frac{\Delta [R]}{\Delta t}$
Why this formula applies:
The reactant concentration decreases, so the negative sign is used to report a positive disappearance rate.
Identify the concentration change:
$\Delta [R] = 0.60 – 0.90 = -0.30\,\text{mol L}^{-1}$
Substitution:
$\text{Average rate} = -\frac{-0.30}{30}$
Final simplification:
$\text{Average rate} = 0.010\,\text{mol L}^{-1}\text{s}^{-1}$
Unit / notation check:
Concentration per time gives $\text{mol L}^{-1}\text{s}^{-1}$
Final Answer:
$0.010\,\text{mol L}^{-1}\text{s}^{-1}$
56. Which change will alter the numerical unit used for reaction rate without changing the meaning of rate itself?
ⓐ. Writing the balanced equation in ionic form instead of molecular form
ⓑ. Replacing one reactant by a catalyst
ⓒ. Measuring time in minutes instead of seconds
ⓓ. Using a different stoichiometric coefficient normalization
Correct Answer: Measuring time in minutes instead of seconds
Explanation: The meaning of reaction rate remains concentration change per unit time, but the written unit depends on how concentration and time are measured. If time is taken in minutes rather than seconds, the unit changes from a per-second form to a per-minute form.
57. A reaction rate is reported as $0.50\,\text{mol L}^{-1}\text{min}^{-1}$. What does this unit indicate?
ⓐ. The concentration changes by $0.50\,\text{mol L}^{-1}$ every second.
ⓑ. The concentration of the reactant is fixed at $0.50\,\text{mol L}^{-1}$.
ⓒ. The concentration changes by $0.50\,\text{mol L}^{-1}$ in one minute.
ⓓ. The rate constant has the value $0.50\,\text{min}^{-1}$.
Correct Answer: The concentration changes by $0.50\,\text{mol L}^{-1}$ in one minute.
Explanation: A reaction-rate unit combines concentration and time. The unit $\text{mol L}^{-1}\text{min}^{-1}$ means concentration change per minute. So a value of $0.50\,\text{mol L}^{-1}\text{min}^{-1}$ states that the concentration changes by $0.50\,\text{mol L}^{-1}$ in one minute, not in one second.
58. Which of the following can validly represent the unit of reaction rate if concentration is measured in $\text{mol m}^{-3}$ and time in seconds?
ⓐ. $\text{mol m}^{-3}\text{s}^{-1}$
ⓑ. $\text{m}^3\text{mol}^{-1}\text{s}^{-1}$
ⓒ. $\text{mol m}^{-3}\text{s}$
ⓓ. $\text{mol}^2\text{m}^{-3}\text{s}^{-1}$
Correct Answer: $\text{mol m}^{-3}\text{s}^{-1}$
Explanation: Reaction rate is concentration per unit time. If concentration is expressed in $\text{mol m}^{-3}$ and time in $\text{s}$, the correct unit becomes $\text{mol m}^{-3}\text{s}^{-1}$. The other choices either invert the concentration unit or introduce the wrong power of amount.
59. The concentration of a reactant decreases by $0.24\,\text{mol L}^{-1}$ in $2\,\text{min}$. What is the average rate of disappearance in $\text{mol L}^{-1}\text{min}^{-1}$?
ⓐ. $0.48\,\text{mol L}^{-1}\text{min}^{-1}$
ⓑ. $-0.12\,\text{mol L}^{-1}\text{min}^{-1}$
ⓒ. $2.12\,\text{mol L}^{-1}\text{min}^{-1}$
ⓓ. $0.12\,\text{mol L}^{-1}\text{min}^{-1}$
Correct Answer: $0.12\,\text{mol L}^{-1}\text{min}^{-1}$
Explanation: Given:
Decrease in concentration $= 0.24\,\text{mol L}^{-1}$
Time interval $= 2\,\text{min}$
Required:
Average rate of disappearance
Relevant formula:
$\text{Average rate} = \frac{\text{decrease in concentration}}{\Delta t}$
Why this formula applies:
The magnitude of disappearance rate is found by dividing the concentration decrease by the time taken.
Substitution:
$\text{Average rate} = \frac{0.24}{2}$
Intermediate simplification:
$\text{Average rate} = 0.12$
Unit / notation check:
Concentration per time gives $\text{mol L}^{-1}\text{min}^{-1}$
Final Answer:
$0.12\,\text{mol L}^{-1}\text{min}^{-1}$
60. A reaction rate has the unit $\text{mmol L}^{-1}\text{s}^{-1}$. Which statement is correct?
ⓐ. This unit is invalid because rate must always be in moles per litre per second.
ⓑ. This unit is valid because any concentration unit per time can represent rate.
ⓒ. This unit can be used only for gaseous reactions.
ⓓ. This unit represents rate constant rather than reaction rate.
Correct Answer: This unit is valid because any concentration unit per time can represent rate.
Explanation: The unit of reaction rate depends on how concentration and time are measured. If concentration is taken in millimoles per litre and time in seconds, then $\text{mmol L}^{-1}\text{s}^{-1}$ is a valid rate unit. The idea remains concentration change per unit time.
61. Why does the rate of many reactions decrease as the reaction proceeds?
ⓐ. Reactant concentrations usually fall, so effective collision frequency tends to decrease.
ⓑ. Product molecules always destroy the catalyst after a short time.
ⓒ. The stoichiometric coefficients become smaller as time passes.
ⓓ. The unit of rate changes automatically during the reaction.
Correct Answer: Reactant concentrations usually fall, so effective collision frequency tends to decrease.
Explanation: As reactants are consumed, fewer reacting particles are present per unit volume. This generally lowers the frequency of collisions that can lead to reaction. Because of that, the rate commonly decreases with time.
62. In a reaction mixture, the concentration of reactants keeps decreasing with time. What is the most likely effect on the reaction rate, if other conditions remain unchanged?
ⓐ. The rate must become negative.
ⓑ. The rate remains exactly constant for all time.
ⓒ. The rate becomes independent of collisions.
ⓓ. The rate generally decreases with time.
Correct Answer: The rate generally decreases with time.
Explanation: For many reactions, rate depends on how often reactant particles collide effectively. When reactant concentrations fall, such collisions usually become less frequent. Therefore the reaction rate commonly decreases as the reaction proceeds.
63. Equal time intervals are used to follow a reaction. The concentration of a reactant falls by $0.40\,\text{mol L}^{-1}$ in the first interval and by $0.10\,\text{mol L}^{-1}$ in a later equal interval. Which conclusion is most reasonable?
ⓐ. The reaction must have changed into a different reaction.
ⓑ. The reaction was faster in the earlier interval.
ⓒ. The reaction rate was zero in the earlier interval.
ⓓ. The unit of concentration must be incorrect.
Correct Answer: The reaction was faster in the earlier interval.
Explanation: When equal time intervals are compared, a larger concentration decrease in one interval means a larger average rate in that interval. Since the reactant concentration dropped more in the first interval, the reaction was proceeding faster then than later.
64. Which statement best explains why a reaction mixture often shows a higher rate near the beginning than near the end?
ⓐ. Products are absent at the start, so the reaction has no stoichiometry initially.
ⓑ. Time itself acts as a catalyst in the first stage.
ⓒ. More reactant particles are available initially, so collision frequency is usually higher.
ⓓ. Activation energy decreases continuously as the reaction proceeds.
Correct Answer: More reactant particles are available initially, so collision frequency is usually higher.
Explanation: At the beginning of a reaction, reactant concentrations are usually highest. That means more particles are available to collide, so the frequency of effective collisions is often greater. As reactants are consumed, the rate commonly becomes smaller.
65. In many reactions, the magnitude of concentration change observed in successive equal time intervals becomes smaller as the reaction proceeds. What does this indicate?
ⓐ. The reaction rate is increasing with time.
ⓑ. The reaction has already reached equilibrium from the start.
ⓒ. The reaction rate is generally decreasing with time.
ⓓ. The stoichiometric coefficients are changing during the reaction.
Correct Answer: The reaction rate is generally decreasing with time.
Explanation: When equal time intervals show progressively smaller concentration changes, the reaction is proceeding more slowly as time passes. This usually happens because reactant concentrations decrease during the course of the reaction, so effective collisions become less frequent.
66. A gas-evolving reaction gives $36\,\text{cm}^3$ of gas in the first minute, $21\,\text{cm}^3$ in the second minute, and $10\,\text{cm}^3$ in the third minute under the same conditions. Which conclusion is most reasonable?
ⓐ. The reaction is fastest at the beginning.
ⓑ. The reaction rate is constant throughout.
ⓒ. The reaction becomes fastest in the third minute.
ⓓ. The gas volume has no relation to reaction progress.
Correct Answer: The reaction is fastest at the beginning.
Explanation: Equal time intervals can be compared directly. Since the largest amount of gas is formed in the first minute, the rate is greatest then. The smaller amounts formed later show that the rate decreases as the reaction proceeds.
67. Which statement is most accurate for many ordinary reactions carried out at fixed temperature and without adding more reactant during the process?
ⓐ. The rate must remain constant from start to finish.
ⓑ. The rate becomes independent of reactant concentration.
ⓒ. The rate becomes zero immediately after mixing.
ⓓ. The rate is often higher at the start and lower later.
Correct Answer: The rate is often higher at the start and lower later.
Explanation: At the beginning, reactant concentrations are usually highest, so collisions that can lead to reaction are more frequent. As the reactants are consumed, the reaction commonly slows down. This is why many reactions show a falling rate with time.
68. Two equal time intervals are chosen during the same reaction. The average rate in the first interval is greater than that in the second. Which explanation best fits this observation?
ⓐ. The reaction equation becomes unbalanced in the second interval.
ⓑ. Reactant concentration is usually lower in the later interval.
ⓒ. Product concentration can never increase in the first interval.
ⓓ. Time changes the chemical identity of the reactants.
Correct Answer: Reactant concentration is usually lower in the later interval.
Explanation: A later interval often begins with a smaller concentration of reactants because some of them have already been consumed. Lower reactant concentration generally means fewer effective collisions per unit time, so the average rate is commonly smaller in the later interval.
69. Which statement correctly explains why the rate of many reactions decreases as the reaction proceeds?
ⓐ. The number of reacting particles per unit volume usually decreases.
ⓑ. The activation energy automatically increases with time.
ⓒ. The rate constant changes continuously during the reaction.
ⓓ. Products always prevent any further collision between reactants.
Correct Answer: The number of reacting particles per unit volume usually decreases.
Explanation: As reactants are used up, there are usually fewer reacting particles present in a given volume. This lowers the frequency of collisions that can lead to product formation. As a result, the reaction rate commonly decreases with time.
70. Which set contains only factors that commonly affect the rate of a chemical reaction?
ⓐ. Colour, odour, atomic number, melting point
ⓑ. Concentration, temperature, catalyst, nature of reactants
ⓒ. Density, refractive index, taste, humidity
ⓓ. pH paper colour, flame colour, atomic mass, conductivity
Correct Answer: Concentration, temperature, catalyst, nature of reactants
Explanation: The rate of a reaction is commonly influenced by factors such as the chemical nature of the reactants, their concentration, temperature, and the presence of a catalyst. In suitable cases, surface area and pressure for gaseous systems also affect the rate. The other sets do not represent the standard rate-controlling factors as a group.
71. Which factor is especially important when a solid reactant is involved in a heterogeneous reaction?
ⓐ. Colour of the solid
ⓑ. Surface area of the solid
ⓒ. Magnetic property of the solid
ⓓ. Number of neutrons in the solid atoms
Correct Answer: Surface area of the solid
Explanation: In a heterogeneous reaction, reacting particles meet at the interface. A larger surface area exposes more particles of the solid for possible collision, which can increase the rate. That is why powdered solids often react faster than large lumps.
72. Which statement about factors affecting reaction rate is correct?
ⓐ. Temperature affects only endothermic reactions.
ⓑ. A catalyst changes the equilibrium position by increasing reactant concentration.
ⓒ. Pressure has no relation to the rate of gaseous reactions.
ⓓ. Different reactions can proceed at different rates because the nature of reactants matters.
Correct Answer: Different reactions can proceed at different rates because the nature of reactants matters.
Explanation: The chemical nature of reactants strongly influences how easily bonds break and form during a reaction. Some reactions, such as ionic reactions in solution, are often very fast, while others involving extensive bond rearrangement can be much slower. Rate therefore depends not only on conditions but also on the reactants themselves.
73. Which type of reaction is generally faster in aqueous solution under comparable conditions?
ⓐ. An ionic reaction involving oppositely charged ions
ⓑ. A reaction requiring extensive covalent bond breaking
ⓒ. A reaction involving a large solid crystal only
ⓓ. A reaction proceeding through several slow molecular rearrangements
Correct Answer: An ionic reaction involving oppositely charged ions
Explanation: Ionic reactions in aqueous solution are often fast because the reacting species are already present as ions and can combine directly on collision. Reactions that require breaking and rearranging covalent bonds usually involve more complex steps and are often slower.
74. Why are reactions involving extensive bond rearrangement often slower than simple ionic reactions?
ⓐ. They always require a catalyst.
ⓑ. They always occur in the solid state only.
ⓒ. They usually involve more complicated bond breaking and bond making steps.
ⓓ. They release less heat than ionic reactions.
Correct Answer: They usually involve more complicated bond breaking and bond making steps.
Explanation: When a reaction requires major rearrangement of covalent bonds, several atomic connections may need to break and reform. That usually makes the process kinetically more difficult than a direct ionic combination. As a result, such reactions are often slower.
75. Which observation best supports the statement that the nature of reactants affects reaction rate?
ⓐ. The same reaction gives the same products in two experiments.
ⓑ. A catalyst increases the speed of a reaction.
ⓒ. Higher temperature increases the speed of many reactions.
ⓓ. An ionic precipitation occurs almost immediately, while a covalent reaction may be much slower.
Correct Answer: An ionic precipitation occurs almost immediately, while a covalent reaction may be much slower.
Explanation: The chemical nature of reactants influences how easily they can react. Ionic species in solution may react very quickly, while reactions involving covalent molecules can be slower because more bond reorganization is needed. This contrast directly shows the effect of reactant nature on rate.
76. Two reactions are carried out under the same temperature and concentration conditions. Reaction I is an ionic reaction in solution, and Reaction II requires covalent bond cleavage before products form. Which statement is most reasonable?
ⓐ. Reaction II must be faster because covalent compounds are more stable.
ⓑ. Reaction I is likely to be faster.
ⓒ. Both must proceed at exactly the same rate.
ⓓ. Their rates cannot differ because the temperature is the same.
Correct Answer: Reaction I is likely to be faster.
Explanation: Ionic reactions in solution often proceed rapidly because ions can react directly. A reaction that first requires covalent bond cleavage generally involves a more difficult pathway. Therefore Reaction I is more likely to be faster.
77. In a heterogeneous reaction involving a solid and a gas, where does the reaction mainly occur?
ⓐ. At the exposed surface of the solid
ⓑ. Uniformly throughout the full interior of the solid at once
ⓒ. Only at the bottom of the reaction vessel
ⓓ. Only after the solid has completely melted
Correct Answer: At the exposed surface of the solid
Explanation: In a heterogeneous reaction, the reacting species are in different phases. A gas can react only where it comes into contact with the solid, so the reaction mainly occurs at the exposed solid surface. This is why surface area becomes important.
78. A student crushes a lump of calcium carbonate into a fine powder before reacting it with dilute acid. Why does the reaction usually become faster?
ⓐ. Powdering increases the molar mass of the solid.
ⓑ. Powdering changes the chemical formula of the solid.
ⓒ. Powdering lowers the temperature needed for reaction to start.
ⓓ. Powdering increases the surface area available for contact.
Correct Answer: Powdering increases the surface area available for contact.
Explanation: Breaking a solid into smaller particles exposes more surface to the reacting fluid. With more surface available, collisions between reactant particles become more frequent at the interface. That commonly increases the reaction rate.
79. Two equal masses of the same solid react separately with the same acid under identical conditions. One sample is in large chips and the other is in fine powder. Which sample usually reacts faster?
ⓐ. The chip sample, because larger pieces are heavier individually
ⓑ. Both react at the same rate because the masses are equal
ⓒ. The powdered sample, because it exposes more surface area
ⓓ. Neither reacts unless heat is supplied
Correct Answer: The powdered sample, because it exposes more surface area
Explanation: Even when the total mass is the same, the powdered sample presents much more surface area than large chips. More exposed surface allows more effective collisions with acid particles per unit time. Therefore the powdered sample usually reacts faster.
80. Which statement about surface area and reaction rate is correct?
ⓐ. Surface area matters only for homogeneous liquid reactions.
ⓑ. Increasing the exposed surface of a solid can increase the rate of a heterogeneous reaction.
ⓒ. Surface area changes the stoichiometric coefficients of the reaction.
ⓓ. Surface area affects only the equilibrium composition, not the speed.
Correct Answer: Increasing the exposed surface of a solid can increase the rate of a heterogeneous reaction.
Explanation: When a solid participates in a heterogeneous reaction, only the exposed surface is available for interaction with the other reactant. Increasing that surface allows more collisions at the interface and can therefore increase the rate. This effect is especially clear when comparing lumps with powdered solids.
81. In many reactions, why does increasing the concentration of a reactant increase the reaction rate?
ⓐ. More reactant particles are present per unit volume, so collisions become more frequent.
ⓑ. The activation energy of the reaction becomes zero.
ⓒ. The stoichiometric coefficients of the reaction increase automatically.
ⓓ. The products become unstable at higher concentration.
Correct Answer: More reactant particles are present per unit volume, so collisions become more frequent.
Explanation: When concentration increases, more reacting particles are present in a given volume. This raises the frequency of collisions among them. As a result, the number of effective collisions per unit time usually increases, so the rate often becomes higher.
82. Two experiments are performed at the same temperature using the same reactants. In Experiment I, the reactants are more concentrated than in Experiment II. Which statement is generally correct?
ⓐ. Experiment II must be faster because dilution helps particles move.
ⓑ. Both experiments must proceed at the same rate because temperature is unchanged.
ⓒ. Experiment I is likely to be faster because the frequency of collisions is usually greater.
ⓓ. Experiment I must have a smaller rate because the same substances are used.
Correct Answer: Experiment I is likely to be faster because the frequency of collisions is usually greater.
Explanation: At the same temperature, a higher concentration usually means more particles per unit volume. That leads to more frequent collisions and often a greater number of effective collisions. Therefore the more concentrated system is generally faster.
83. Which change is most likely to increase the rate of a reaction between two dissolved reactants, provided all other conditions remain unchanged?
ⓐ. Lowering the concentration of both reactants
ⓑ. Increasing the concentration of at least one reactant
ⓒ. Decreasing the surface area of one solid container wall
ⓓ. Reducing the total volume after the reaction is complete
Correct Answer: Increasing the concentration of at least one reactant
Explanation: A higher concentration means a larger number of reacting particles in a given volume. This increases the chance of collisions between them. Therefore the reaction rate usually rises when the concentration of reactants is increased.
84. A student says that concentration affects reaction rate because it changes the number of particles available in a given volume. Which statement best matches this idea?
ⓐ. Higher concentration always lowers the rate because particles block one another.
ⓑ. Higher concentration changes the balanced chemical equation.
ⓒ. Higher concentration removes the need for proper orientation.
ⓓ. Higher concentration usually increases the collision frequency.
Correct Answer: Higher concentration usually increases the collision frequency.
Explanation: Concentration and collision frequency are directly connected in kinetic reasoning. When more reactant particles occupy the same volume, they encounter one another more often. That increased collision frequency commonly leads to a higher reaction rate.
85. Which statement about concentration and reaction rate is correct?
ⓐ. Increasing concentration can increase reaction rate by increasing the number of possible collisions per unit time.
ⓑ. Concentration affects only the final amount of product, never the speed.
ⓒ. Concentration changes the nature of reactants into different substances.
ⓓ. Concentration can affect only reactions involving catalysts.
Correct Answer: Increasing concentration can increase reaction rate by increasing the number of possible collisions per unit time.
Explanation: The main kinetic effect of concentration is on how often reacting particles meet. More particles per unit volume create more opportunities for collision. Because rate depends on successful collisions, the reaction often becomes faster.
86. In a gaseous reaction, pressure is increased while temperature is kept constant. What is the usual effect on reaction rate?
ⓐ. The rate decreases because gas particles lose kinetic energy.
ⓑ. The rate becomes zero because pressure prevents collisions.
ⓒ. The rate is unaffected because only temperature matters.
ⓓ. The rate usually increases because the effective concentration of the gas increases.
Correct Answer: The rate usually increases because the effective concentration of the gas increases.
Explanation: For gases, increasing pressure at constant temperature compresses the gas into a smaller volume. This raises the number of particles per unit volume, which is equivalent to increasing concentration. More frequent collisions usually lead to a higher reaction rate.
87. Why is pressure mainly discussed for gaseous reactions when considering reaction rate?
ⓐ. Because solids and liquids cannot react
ⓑ. Because pressure changes only the colour of gases
ⓒ. Because pressure strongly affects the concentration of gases
ⓓ. Because pressure changes the stoichiometric coefficients of gases
Correct Answer: Because pressure strongly affects the concentration of gases
Explanation: In gases, pressure and concentration are closely related. When pressure changes, the number of gas particles per unit volume changes significantly, which affects collision frequency. This is why pressure is an important rate factor mainly for gaseous systems.
88. A gaseous reaction is carried out in a smaller vessel at the same temperature using the same amount of reactants. Which statement is most appropriate?
ⓐ. The reaction must become slower because the gases have less space to move.
ⓑ. The reaction is likely to become faster because the gas concentration becomes higher.
ⓒ. The reaction rate becomes independent of collisions in the smaller vessel.
ⓓ. The reaction cannot occur because the volume has changed.
Correct Answer: The reaction is likely to become faster because the gas concentration becomes higher.
Explanation: Putting the same amount of gas into a smaller volume increases its concentration. With more particles present per unit volume, collisions occur more frequently. This usually increases the rate of a gaseous reaction.
89. In many reactions, raising the temperature increases the reaction rate mainly because
ⓐ. the balanced equation changes at higher temperature.
ⓑ. a larger fraction of molecules can take part in effective collisions.
ⓒ. the concentration of every reactant automatically doubles.
ⓓ. the reaction becomes independent of activation energy.
Correct Answer: a larger fraction of molecules can take part in effective collisions.
Explanation: Increasing temperature raises the kinetic energy of reacting particles. As a result, a greater fraction of collisions occur with enough energy to lead to reaction. This usually increases the number of effective collisions per unit time, so the rate becomes higher.
90. Two experiments use the same reactants at the same concentration, but one is carried out at a higher temperature. Which statement is generally correct?
ⓐ. The higher-temperature experiment is likely to be faster.
ⓑ. Both experiments must have the same rate because concentration is unchanged.
ⓒ. The higher-temperature experiment must give a lower yield.
ⓓ. Temperature affects only equilibrium, not reaction speed.
Correct Answer: The higher-temperature experiment is likely to be faster.
Explanation: Even when concentration is unchanged, higher temperature usually makes reacting particles move more energetically. This increases the probability of effective collisions. Therefore the reaction commonly proceeds faster at the higher temperature.
91. Which statement best describes the effect of temperature on reaction rate?
ⓐ. Temperature changes only the colour of the reaction mixture.
ⓑ. Temperature affects only ionic reactions, not covalent ones.
ⓒ. Higher temperature often increases the rate because more collisions become effective.
ⓓ. Higher temperature always changes the reaction into a different one.
Correct Answer: Higher temperature often increases the rate because more collisions become effective.
Explanation: A rise in temperature does more than simply increase collision frequency slightly. Its major kinetic effect is that more particles have enough energy to react when they collide. That is why reaction rate often increases noticeably with temperature.
92. A reaction is slow at room temperature but becomes much faster on heating. Which explanation is the most appropriate?
ⓐ. Heating changes the stoichiometric coefficients of the reaction.
ⓑ. Heating reduces the concentration of the reactants to zero.
ⓒ. Heating makes the reaction independent of molecular collisions.
ⓓ. Heating increases the fraction of particles that can overcome the energy barrier.
Correct Answer: Heating increases the fraction of particles that can overcome the energy barrier.
Explanation: Reactions often speed up on heating because more molecules gain sufficient energy to cross the required energy barrier for reaction. This leads to more successful collisions in a given time. The stoichiometry remains unchanged.
93. Which statement is correct about the role of a catalyst in a chemical reaction?
ⓐ. It increases the rate by providing an alternative pathway with lower activation energy.
ⓑ. It increases the rate by increasing the concentration of products at the start.
ⓒ. It changes the overall stoichiometric equation of the reaction.
ⓓ. It is consumed completely during the reaction.
Correct Answer: It increases the rate by providing an alternative pathway with lower activation energy.
Explanation: A catalyst increases reaction rate by allowing the reaction to proceed through a different path that requires less activation energy. Because the barrier is lower, a greater fraction of collisions becomes effective. The catalyst itself is not consumed overall.
94. Which statement about a catalyst is not correct?
ⓐ. It can increase the reaction rate.
ⓑ. It provides a different reaction pathway.
ⓒ. It remains chemically unchanged overall after the reaction.
ⓓ. It shifts the equilibrium position by increasing the amount of reactant present.
Correct Answer: It shifts the equilibrium position by increasing the amount of reactant present.
Explanation: A catalyst affects how quickly equilibrium is reached, not the equilibrium position itself in the usual school-level treatment. It does not increase the amount of reactant present. Its role is kinetic, not stoichiometric.
95. A catalyst is added to a reaction mixture. Which change is most directly responsible for the increased rate?
ⓐ. The concentration terms in the balanced equation become larger.
ⓑ. The reaction begins to release more heat per mole.
ⓒ. The activation-energy requirement for the pathway is lowered.
ⓓ. The reactant molecules stop colliding randomly.
Correct Answer: The activation-energy requirement for the pathway is lowered.
Explanation: The catalyst offers an alternative route with a lower activation-energy barrier. Since more molecules can now react successfully at the same temperature, the rate increases. The overall heat change of the reaction is not the direct reason for the faster rate.
96. Which comparison between temperature increase and catalyst addition is correct?
ⓐ. Both necessarily increase reactant concentration.
ⓑ. Both can increase reaction rate, but they do so in different ways.
ⓒ. Both change the final stoichiometric coefficients.
ⓓ. Both must be consumed during the reaction.
Correct Answer: Both can increase reaction rate, but they do so in different ways.
Explanation: Raising temperature increases the fraction of molecules with enough energy for effective reaction, whereas a catalyst lowers the activation-energy barrier by providing another pathway. Both can speed up a reaction, but the mechanisms are different. Neither changes the stoichiometric coefficients.
97. Which statement about a catalyst is correct?
ⓐ. It changes the products formed by altering the balanced equation.
ⓑ. It increases the rate by offering an alternative pathway.
ⓒ. It is always used up completely by the end of the reaction.
ⓓ. It increases the rate only by raising the temperature of the mixture.
Correct Answer: It increases the rate by offering an alternative pathway.
Explanation: A catalyst speeds up a reaction by making available a different pathway that is kinetically easier to follow. This alternative pathway has a lower activation-energy barrier than the uncatalysed one. The balanced equation and the overall products remain unchanged.
98. Which statement best describes the effect of a catalyst on equilibrium in the usual school-level treatment?
ⓐ. It changes the equilibrium position by favouring products only.
ⓑ. It changes the equilibrium position by favouring reactants only.
ⓒ. It does not change the equilibrium position, but equilibrium may be reached faster.
ⓓ. It prevents the reverse reaction while speeding up the forward reaction.
Correct Answer: It does not change the equilibrium position, but equilibrium may be reached faster.
Explanation: A catalyst affects reaction speed, not the final equilibrium composition in the standard rate discussion. It speeds up both forward and reverse processes by lowering the barrier for each through an alternative route. As a result, equilibrium is attained sooner without shifting its position.
99. A reaction proceeds very slowly at a fixed temperature. After adding a small amount of catalyst, the reaction becomes fast, and the catalyst can still be recovered afterward. Which conclusion is most appropriate?
ⓐ. The catalyst increased the concentration of reactants permanently.
ⓑ. The catalyst changed the reaction into a different overall reaction.
ⓒ. The catalyst was consumed first and then regenerated from products only.
ⓓ. The catalyst participated in the process but remained unchanged overall.
Correct Answer: The catalyst participated in the process but remained unchanged overall.
Explanation: A catalyst may take part in intermediate steps, but it is regenerated by the end of the reaction sequence. That is why it can often be recovered unchanged overall after the reaction. Its role is to speed up the process without being consumed as a reactant.
100. At the same temperature, a catalysed reaction is faster than the uncatalysed reaction mainly because, in the catalysed path,
ⓐ. the activation-energy barrier is lower.
ⓑ. the reactant concentration is automatically doubled.
ⓒ. the stoichiometric coefficients become smaller.
ⓓ. the products are converted back into reactants more slowly.
Correct Answer: the activation-energy barrier is lower.
Explanation: The most direct reason for a higher rate in the presence of a catalyst is the lower activation-energy requirement of the catalysed pathway. Because of this, a larger fraction of collisions becomes successful at the same temperature. Concentration and stoichiometric coefficients do not automatically change.
101. Which statement about the rate law of a reaction is correct?
ⓐ. It is always obtained directly from the overall balanced equation.
ⓑ. It is fixed only by the coefficients of products in the reaction equation.
ⓒ. It is determined experimentally from how rate depends on concentration.
ⓓ. It can be written only after the reaction goes to completion.
Correct Answer: It is determined experimentally from how rate depends on concentration.
Explanation: The rate law connects reaction rate with reactant concentrations, and its exponents are found from experiment. For complex reactions, these exponents cannot in general be taken directly from the overall balanced equation. The law is therefore an experimental result.
102. Which expression represents a general rate law for a reaction involving reactants $A$ and $B$?
ⓐ. $r = k[A]^m[B]^n$
ⓑ. $r = \frac{[A] + [B]}{k}$
ⓒ. $r = k[A_m][B_n]$
ⓓ. $r = k[A+B]^{m+n}$
Correct Answer: $r = k[A]^m[B]^n$
Explanation: A general rate law expresses the rate $r$ in terms of concentrations raised to experimentally determined powers. Here $k$ is the rate constant, and $m$ and $n$ describe how strongly the rate depends on $[A]$ and $[B]$. This is the standard school-level form for a two-reactant rate law.
103. For a reaction, the experimentally found rate law is $r = k[A]^2[B]$. Which statement is correct?
ⓐ. The rate depends on the square of $B$ only.
ⓑ. The rate is independent of $A$.
ⓒ. Doubling $A$ makes the rate four times if $B$ is unchanged.
ⓓ. Doubling both $A$ and $B$ leaves the rate unchanged.
Correct Answer: Doubling $A$ makes the rate four times if $B$ is unchanged.
Explanation: From $r = k[A]^2[B]$, the rate is proportional to the square of $[A]$ and to the first power of $[B]$. If only $[A]$ is doubled, the factor becomes $2^2 = 4$. So the rate becomes four times.
104. For a reaction with rate law $r = k[A]^m[B]^n$, what does the symbol $k$ represent?
ⓐ. The equilibrium constant of the reaction
ⓑ. The stoichiometric coefficient of the slowest step
ⓒ. The total concentration of all reactants
ⓓ. The rate constant for that reaction at a fixed temperature
Correct Answer: The rate constant for that reaction at a fixed temperature
Explanation: In the rate law, $k$ is the proportionality constant that connects the rate with the concentration terms. Its value is characteristic of the reaction at a particular temperature. When temperature changes, the value of $k$ generally changes as well.
105. Which statement about the rate law of a complex reaction is correct?
ⓐ. Its exponents are always identical to the stoichiometric coefficients in the overall equation.
ⓑ. It can be written only after the products have been isolated in pure form.
ⓒ. It is found from experimental observation of how rate changes with concentration.
ⓓ. It depends only on the physical state symbols written in the balanced equation.
Correct Answer: It is found from experimental observation of how rate changes with concentration.
Explanation: The rate law is an experimental relation between reaction rate and reactant concentrations. For a complex reaction, the powers of concentration terms cannot generally be assigned directly from the overall balanced equation. They must be obtained from rate data.
106. For the reaction $2A + B \rightarrow \text{products}$, the experimentally observed rate law is $r = k[A][B]^2$. Which statement is correct?
ⓐ. The exponent of $A$ in the rate law is taken from its stoichiometric coefficient.
ⓑ. The rate is proportional to $[A][B]^2$, not to $[A]^2[B]$.
ⓒ. The balanced equation becomes invalid because the rate law is different.
ⓓ. The reaction cannot be studied by kinetic methods.
Correct Answer: The rate is proportional to $[A][B]^2$, not to $[A]^2[B]$.
Explanation: The overall balanced equation shows the stoichiometric relationship among species, but the rate law shows how the rate actually depends on concentrations. Since the experimentally found rate law is $r = k[A][B]^2$, the rate is first order in $A$ and second order in $B$, regardless of the overall stoichiometric coefficients.
107. For a reaction with rate law $r = k[A]^2[B]$, what happens to the rate if $[A]$ is doubled and $[B]$ is halved?
ⓐ. The rate becomes half.
ⓑ. The rate becomes twice.
ⓒ. The rate becomes four times.
ⓓ. The rate remains unchanged.
Correct Answer: The rate becomes twice.
Explanation: Given:
Rate law, $r = k[A]^2[B]$
Required:
Effect on rate when $[A]$ is doubled and $[B]$ is halved
Relevant principle:
Rate changes according to the powers of concentration terms in the rate law.
Why this principle applies:
The exponent on each concentration tells how strongly the rate depends on that concentration.
Let the initial rate be:
$r_1 = k[A]^2[B]$
After the change:
$[A] \to 2[A]$
$[B] \to \frac{[B]}{2}$
Substitution into the rate law:
$r_2 = k(2[A])^2\left(\frac{[B]}{2}\right)$
Intermediate simplification:
$r_2 = k(4[A]^2)\left(\frac{[B]}{2}\right)$
$r_2 = 2k[A]^2[B]$
So,
$r_2 = 2r_1$
Final Answer:
The rate becomes twice.
108. A reaction follows the rate law $r = k[A][B]$. If both $[A]$ and $[B]$ are doubled simultaneously, by what factor does the rate change?
ⓐ. 2
ⓑ. 4
ⓒ. 6
ⓓ. 8
Correct Answer: 4
Explanation: Given:
Rate law, $r = k[A][B]$
Required:
Factor by which rate changes when both concentrations are doubled
Relevant principle:
Each concentration affects the rate according to its exponent in the rate law.
Initial rate:
$r_1 = k[A][B]$
After doubling both concentrations:
$r_2 = k(2[A])(2[B])$
Intermediate simplification:
$r_2 = 4k[A][B]$
Therefore:
$r_2 = 4r_1$
Unit / notation check:
Only the multiplicative factor is asked, so no unit is needed.
Final Answer:
The rate becomes 4 times.
109. Which statement best defines the specific rate constant $k$ in a rate law?
ⓐ. It is the concentration of product formed in one second.
ⓑ. It is the total concentration of all reactants at equilibrium.
ⓒ. It is the numerical value of rate when all concentration terms are unity in the rate law.
ⓓ. It is the stoichiometric coefficient of the slowest reactant.
Correct Answer: It is the numerical value of rate when all concentration terms are unity in the rate law.
Explanation: In the rate law, $k$ is the proportionality constant connecting rate with the concentration terms. If each concentration term is taken as unity, the rate becomes numerically equal to $k$. This gives the usual school-level interpretation of the specific rate constant.
110. For a reaction obeying $r = k[A][B]$, what is the numerical value of the rate when $[A] = 1\,\text{mol L}^{-1}$ and $[B] = 1\,\text{mol L}^{-1}$?
ⓐ. $r = 2k$
ⓑ. $r = k^2$
ⓒ. $r = \frac{k}{2}$
ⓓ. $r = k$
Correct Answer: $r = k$
Explanation: Given:
Rate law, $r = k[A][B]$
$[A] = 1\,\text{mol L}^{-1}$
$[B] = 1\,\text{mol L}^{-1}$
Required:
Numerical value of the rate
Relevant formula:
$r = k[A][B]$
Why this formula applies:
The reaction is stated to obey this rate law.
Substitution:
$r = k(1)(1)$
Intermediate simplification:
$r = k$
Interpretation:
When the concentration terms are unity, the rate becomes numerically equal to the rate constant.
Final Answer:
$r = k$
111. Which statement correctly compares reaction rate and rate constant?
ⓐ. Both always have the same numerical value during a reaction.
ⓑ. Rate changes with concentration during the reaction, whereas the rate constant is fixed for a given reaction at a fixed temperature.
ⓒ. Rate constant changes continuously with concentration, whereas rate does not.
ⓓ. Rate and rate constant are two names for the same quantity.
Correct Answer: Rate changes with concentration during the reaction, whereas the rate constant is fixed for a given reaction at a fixed temperature.
Explanation: Reaction rate usually changes as reactant concentrations change with time. The rate constant, however, is characteristic of the reaction at a given temperature and does not vary just because concentrations change during the run. This is why rate and rate constant are distinct quantities.
112. Which statement about the rate constant $k$ is correct?
ⓐ. Its value depends only on the balanced coefficients in the reaction equation.
ⓑ. Its value remains the same even when temperature changes.
ⓒ. Its value can change with temperature for the same reaction.
ⓓ. Its value is always equal to the concentration of the fastest reactant.
Correct Answer: Its value can change with temperature for the same reaction.
Explanation: The rate constant is characteristic of a reaction only at a fixed temperature. When temperature changes, the value of $k$ generally changes, often quite noticeably. This is why temperature is one of the most important factors affecting reaction rate.
113. For a first-order reaction, which unit is correct for the rate constant $k$?
ⓐ. $\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $\text{L mol}^{-1}\text{s}^{-1}$
ⓒ. $\text{s}^{-1}$
ⓓ. $\text{mol}^{-1}\text{L s}^{-1}$
Correct Answer: $\text{s}^{-1}$
Explanation: For a first-order reaction, the rate law is $r = k[A]$. Since rate has unit $\text{mol L}^{-1}\text{s}^{-1}$ and concentration has unit $\text{mol L}^{-1}$, dividing rate by concentration leaves only $\text{s}^{-1}$. Therefore the unit of $k$ for a first-order reaction is $\text{s}^{-1}$.
114. For a zero-order reaction, which unit is correct for the rate constant $k$?
ⓐ. $\text{mol L}^{-1}\text{s}^{-1}$
ⓑ. $\text{s}^{-1}$
ⓒ. $\text{L mol}^{-1}\text{s}^{-1}$
ⓓ. $\text{L}^2\text{mol}^{-2}\text{s}^{-1}$
Correct Answer: $\text{mol L}^{-1}\text{s}^{-1}$
Explanation: In a zero-order reaction, the rate law is $r = k$. That means the rate constant has the same unit as the rate itself. Since reaction rate is expressed as concentration per unit time, the unit of $k$ is $\text{mol L}^{-1}\text{s}^{-1}$.
115. A reaction follows the rate law $r = k[A]^2$. What is the unit of $k$ if rate is expressed in $\text{mol L}^{-1}\text{s}^{-1}$ and concentration in $\text{mol L}^{-1}$?
ⓐ. $\text{s}^{-1}$
ⓑ. $\text{mol L}^{-1}\text{s}^{-1}$
ⓒ. $\text{L}^2\text{mol}^{-2}\text{s}^{-1}$
ⓓ. $\text{L mol}^{-1}\text{s}^{-1}$
Correct Answer: $\text{L mol}^{-1}\text{s}^{-1}$
Explanation: Given:
Rate law, $r = k[A]^2$
Required:
Unit of $k$
Relevant formula / principle:
$k = \frac{r}{[A]^2}$
Why this formula applies:
The unit of $k$ is obtained by dividing the unit of rate by the unit of the concentration term in the rate law.
Identify known units:
Unit of rate $= \text{mol L}^{-1}\text{s}^{-1}$
Unit of $[A]^2 = (\text{mol L}^{-1})^2 = \text{mol}^2\text{L}^{-2}$
Substitution:
\[
\text{Unit of }k = \frac{\text{mol L}^{-1}\text{s}^{-1}}{\text{mol}^2\text{L}^{-2}}
\]
Intermediate simplification:
\[
\text{Unit of }k = \text{mol}^{1-2}\text{L}^{-1-(-2)}\text{s}^{-1}
\]
\[
\text{Unit of }k = \text{mol}^{-1}\text{L}\text{s}^{-1}
\]
Final simplification:
$\text{Unit of }k = \text{L mol}^{-1}\text{s}^{-1}$
Final Answer:
$\text{L mol}^{-1}\text{s}^{-1}$
116. A reaction follows the rate law $r = k[A]^{1/2}[B]$. What is the overall order of the reaction?
ⓐ. $\frac{1}{2}$
ⓑ. $\frac{3}{2}$
ⓒ. $2$
ⓓ. $1$
Correct Answer: $\frac{3}{2}$
Explanation: Given:
Rate law, $r = k[A]^{1/2}[B]$
Required:
Overall order of the reaction
Relevant principle:
Overall order is the sum of the powers of concentration terms in the rate law.
Identify the partial orders:
Order with respect to $A = \frac{1}{2}$
Order with respect to $B = 1$
Substitution:
\[
\text{Overall order} = \frac{1}{2} + 1
\]
Final simplification:
\[
\text{Overall order} = \frac{3}{2}
\]
Final Answer:
Overall order $= \frac{3}{2}$
117. For the rate law $r = k[A]^2[B]^3$, which statement is correct?
ⓐ. The overall order is $5$.
ⓑ. The overall order is $6$.
ⓒ. The reaction is third order.
ⓓ. The order with respect to $A$ is $5$.
Correct Answer: The overall order is $5$.
Explanation: The overall order is the sum of the exponents of all concentration terms in the rate law. Here the powers are $2$ for $[A]$ and $3$ for $[B]$. Their sum is $2 + 3 = 5$, so the reaction is fifth order overall.
118. Which statement correctly distinguishes partial order from overall order?
ⓐ. Partial order is always equal to molecularity, while overall order is always twice the molecularity.
ⓑ. Partial order is the sum of all exponents, while overall order is the exponent of one reactant only.
ⓒ. Partial order is defined only for products, while overall order is defined only for reactants.
ⓓ. Partial order is the exponent of one reactant in the rate law, while overall order is the sum of all exponents.
Correct Answer: Partial order is the exponent of one reactant in the rate law, while overall order is the sum of all exponents.
Explanation: In a rate law such as $r = k[A]^m[B]^n$, the partial order with respect to $A$ is $m$ and with respect to $B$ is $n$. The overall order is $m+n$. So partial order refers to one concentration term, whereas overall order combines them all.
119. The unit of the rate constant for a reaction is $\text{s}^{-1}$. Which conclusion is correct?
ⓐ. The reaction must be zero order.
ⓑ. The reaction is first order.
ⓒ. The reaction is second order.
ⓓ. The reaction is third order.
Correct Answer: The reaction is first order.
Explanation: The unit $\text{s}^{-1}$ is characteristic of a first-order rate constant. In zero order, $k$ has the same unit as rate, and in second order the unit is typically $\text{L mol}^{-1}\text{s}^{-1}$. So $\text{s}^{-1}$ identifies first-order behavior.
120. Which statement about the order of a reaction is correct?
ⓐ. It must always be a whole number greater than zero.
ⓑ. It is always equal to the sum of stoichiometric coefficients in the balanced equation.
ⓒ. It can be zero, fractional, or integral because it is determined experimentally.
ⓓ. It is defined only for elementary gas-phase reactions.
Correct Answer: It can be zero, fractional, or integral because it is determined experimentally.
Explanation: Reaction order is obtained from the experimentally determined rate law, not directly from the balanced equation. Because of this, the order may be zero, fractional, or an integer depending on how the rate depends on concentration. This makes order a kinetic quantity rather than a simple stoichiometric count.
121. For a reaction with rate law $r = k[A]^2[B]^0$, which statement is correct?
ⓐ. It is first order in $A$, zero order in $B$, and first order overall.
ⓑ. It is second order in $A$, first order in $B$, and third order overall.
ⓒ. It is second order in $A$, zero order in $B$, and second order overall.
ⓓ. It is zero order in $A$, second order in $B$, and second order overall.
Correct Answer: It is second order in $A$, zero order in $B$, and second order overall.
Explanation: The exponent of $[A]$ is $2$, so the reaction is second order with respect to $A$. The exponent of $[B]$ is $0$, so it is zero order with respect to $B$. The overall order is the sum of the exponents, $2 + 0 = 2$.
122. A reaction follows the rate law $r = k[A]^0[B]$. If only $[A]$ is doubled while $[B]$ remains unchanged, what happens to the rate?
ⓐ. The rate remains unchanged.
ⓑ. The rate becomes twice.
ⓒ. The rate becomes four times.
ⓓ. The rate becomes half.
Correct Answer: The rate remains unchanged.
Explanation: Given:
Rate law, $r = k[A]^0[B]$
Required:
Effect on rate when only $[A]$ is doubled
Relevant principle:
Any non-zero concentration raised to the power $0$ equals $1$.
Why this principle applies:
The concentration term $[A]^0$ does not affect the numerical value of the rate.
Initial rate:
$r_1 = k[A]^0[B] = k(1)[B]$
After doubling $[A]$:
$r_2 = k(2[A])^0[B] = k(1)[B]$
Comparison:
$r_2 = r_1$
Final Answer:
The rate remains unchanged.
123. Which statement correctly defines the order of a reaction?
ⓐ. It is the sum of the stoichiometric coefficients of all reactants.
ⓑ. It is the sum of the powers of concentration terms in the experimentally determined rate law.
ⓒ. It is the number of products formed in one elementary step.
ⓓ. It is the value of the rate constant at unit concentration.
Correct Answer: It is the sum of the powers of concentration terms in the experimentally determined rate law.
Explanation: Reaction order is defined from the rate law, not from the balanced chemical equation. The exponents of the concentration terms are added to give the overall order. Since the rate law is found experimentally, order is also an experimental quantity.
124. For the rate law $r = k[A]^{1/2}[B]^{3/2}$, what is the overall order of the reaction?
ⓐ. $1$
ⓑ. $2$
ⓒ. $3$
ⓓ. $\frac{5}{2}$
Correct Answer: $\frac{5}{2}$
Explanation: Given:
Rate law, $r = k[A]^{1/2}[B]^{3/2}$
Required:
Overall order of the reaction
Relevant principle:
Overall order is the sum of all exponents in the rate law.
Identify the partial orders:
Order with respect to $A = \frac{1}{2}$
Order with respect to $B = \frac{3}{2}$
Substitution:
\[
\text{Overall order} = \frac{1}{2} + \frac{3}{2}
\]
Intermediate simplification:
\[
\text{Overall order} = \frac{4}{2}
\]
Final simplification:
\[
\text{Overall order} = 2
\]
Final Answer:
Overall order $= 2$
125. A reaction has the rate law $r = k[B]^2$. Which statement is correct?
ⓐ. The reaction is zero order with respect to $A$ if $A$ does not appear in the rate law.
ⓑ. The reaction must be second order with respect to every reactant present.
ⓒ. The overall order cannot be found unless the balanced equation is known.
ⓓ. The reaction is first order overall because only one concentration term appears.
Correct Answer: The reaction is zero order with respect to $A$ if $A$ does not appear in the rate law.
Explanation: If a reactant does not appear in the rate law, the rate does not depend on its concentration under the stated conditions. That means its partial order is zero. In $r = k[B]^2$, the reaction is second order in $B$ and zero order in any omitted reactant such as $A$.
126. For a reaction with rate law $r = k[A][B]^2$, what happens to the rate if $[A]$ is halved and $[B]$ is doubled?
ⓐ. The rate remains unchanged.
ⓑ. The rate becomes twice.
ⓒ. The rate becomes four times.
ⓓ. The rate becomes eight times.
Correct Answer: The rate becomes four times.
Explanation: Given:
Rate law, $r = k[A][B]^2$
Required:
Effect on rate when $[A]$ is halved and $[B]$ is doubled
Relevant formula:
$r \propto [A][B]^2$
Why this formula applies:
The rate changes according to the exponents of the concentration terms in the rate law.
Initial rate:
$r_1 = k[A][B]^2$
After the change:
$r_2 = k\left(\frac{[A]}{2}\right)(2[B])^2$
Intermediate simplification:
$r_2 = k\left(\frac{[A]}{2}\right)(4[B]^2)$
$r_2 = 2k[A][B]^2$
Comparison:
$r_2 = 2r_1$
Final Answer:
The rate becomes twice.
127. Which statement correctly compares partial order and overall order for the rate law $r = k[A]^2[B]$?
ⓐ. Partial order with respect to $A$ is $3$, and overall order is $2$.
ⓑ. Partial order with respect to $B$ is $2$, and overall order is $1$.
ⓒ. Partial order with respect to $A$ is $2$, and overall order is $3$.
ⓓ. Partial order with respect to $A$ is $1$, and overall order is $2$.
Correct Answer: Partial order with respect to $A$ is $2$, and overall order is $3$.
Explanation: In the rate law $r = k[A]^2[B]$, the exponent of $[A]$ is $2$, so the partial order with respect to $A$ is $2$. The overall order is the sum of the exponents, $2 + 1 = 3$. This distinguishes the order for one reactant from the total order.
128. Which statement about molecularity is correct?
ⓐ. It may be zero, fractional, or integral.
ⓑ. It is defined as the sum of exponents in the rate law.
ⓒ. It applies to the overall complex reaction in every case.
ⓓ. It is the number of reacting species colliding in an elementary step.
Correct Answer: It is the number of reacting species colliding in an elementary step.
Explanation: Molecularity refers to an elementary step and counts how many reacting species take part in that step. It is therefore a mechanistic count, not an experimentally determined exponent. That is why molecularity is associated with elementary processes only.
129. Which statement correctly describes the molecularity of an elementary reaction?
ⓐ. It is the number of reacting species taking part in a single elementary step.
ⓑ. It is the sum of the stoichiometric coefficients in the overall balanced equation.
ⓒ. It is the sum of powers in the experimentally obtained rate law.
ⓓ. It is the number of products formed per unit time.
Correct Answer: It is the number of reacting species taking part in a single elementary step.
Explanation: Molecularity is defined only for an elementary step. It counts how many reacting species participate in that single event. It is therefore a mechanistic count, not an experimentally fitted exponent.
130. Which elementary process is correctly classified as bimolecular?
ⓐ. One molecule decomposes into products in a single step.
ⓑ. Three species collide simultaneously in one step.
ⓒ. Two ions are produced from one compound in one step.
ⓓ. Two reacting species collide in a single elementary step.
Correct Answer: Two reacting species collide in a single elementary step.
Explanation: A bimolecular elementary step involves exactly two reacting species colliding or interacting in that step. One reacting species gives unimolecular behavior, while three reacting species correspond to a termolecular step.
131. Which value is not possible for molecularity in the standard treatment of elementary reactions?
ⓐ. $1$
ⓑ. $\frac{1}{2}$
ⓒ. $2$
ⓓ. $3$
Correct Answer: $\frac{1}{2}$
Explanation: Molecularity is a count of the number of reacting species involved in a single elementary step. Since it is a count, it must be a positive integer such as $1$, $2$, or $3$. Fractional values are not allowed.
132. Which statement about molecularity is correct?
ⓐ. It can be zero for a reaction whose rate is independent of concentration.
ⓑ. It may be fractional when the mechanism is complex.
ⓒ. It is always a positive integer for an elementary step.
ⓓ. It is obtained by adding the exponents in the rate law.
Correct Answer: It is always a positive integer for an elementary step.
Explanation: Molecularity refers to how many species participate in one elementary act. Because it is a simple count, it must be a positive integer. It is not zero, negative, or fractional.
133. Which statement correctly compares order and molecularity?
ⓐ. Both are always obtained directly from the overall balanced equation.
ⓑ. Order applies only to elementary reactions, while molecularity applies to all reactions.
ⓒ. Order and molecularity are always numerically equal.
ⓓ. Order is obtained experimentally, whereas molecularity refers to an elementary step.
Correct Answer: Order is obtained experimentally, whereas molecularity refers to an elementary step.
Explanation: Order comes from the experimentally determined rate law, so it is a kinetic result. Molecularity is a mechanistic idea that applies to one elementary step and counts the participating species. Because they arise differently, they need not be equal.
134. Which statement is correct for the reaction order but not for molecularity?
ⓐ. It may be zero or fractional.
ⓑ. It is always a positive integer.
ⓒ. It is defined only for an elementary step.
ⓓ. It is obtained by counting reacting species in one collision event.
Correct Answer: It may be zero or fractional.
Explanation: Reaction order is taken from the experimentally observed rate law, so it can be zero, fractional, or integral. Molecularity, being a count of species in an elementary step, cannot take such values.
135. Which statement about a complex reaction is correct?
ⓐ. Its overall stoichiometric coefficients always give the molecularity directly.
ⓑ. Its rate law cannot in general be inferred directly from the overall balanced equation.
ⓒ. Its order must always equal its molecularity.
ⓓ. Its elementary steps must all have the same molecularity.
Correct Answer: Its rate law cannot in general be inferred directly from the overall balanced equation.
Explanation: A complex reaction proceeds through more than one elementary step. Because the observed rate depends on the mechanism, the rate law usually cannot be written just by looking at the overall balanced equation. Experimental evidence is needed.
136. Which statement correctly distinguishes an elementary reaction from a complex reaction?
ⓐ. An elementary reaction has no reactants, whereas a complex reaction has reactants.
ⓑ. An elementary reaction always has first-order kinetics, whereas a complex reaction never does.
ⓒ. An elementary reaction occurs in one step, whereas a complex reaction occurs through multiple steps.
ⓓ. An elementary reaction gives no products, whereas a complex reaction gives products.
Correct Answer: An elementary reaction occurs in one step, whereas a complex reaction occurs through multiple steps.
Explanation: An elementary reaction represents a single step at the molecular level. A complex reaction proceeds through two or more elementary steps. This difference is why molecularity is defined for elementary steps, not for an overall complex reaction.
137. Which statement correctly compares order and molecularity?
ⓐ. Both are always obtained from the overall balanced equation.
ⓑ. Order is based on the rate law, whereas molecularity is based on an elementary step.
ⓒ. Order applies only to elementary reactions, whereas molecularity applies to all reactions.
ⓓ. Both must always have the same numerical value.
Correct Answer: Order is based on the rate law, whereas molecularity is based on an elementary step.
Explanation: Order is obtained from the experimentally determined rate law and shows how the rate depends on concentration. Molecularity, on the other hand, is a mechanistic idea that counts the number of reacting species in one elementary step. Since they come from different ideas, they need not be equal.
138. Which statement about molecularity in a complex reaction is correct?
ⓐ. It is equal to the sum of coefficients in the overall balanced equation.
ⓑ. It can be fractional if the mechanism has many steps.
ⓒ. It is obtained by adding the exponents in the observed rate law.
ⓓ. It is not assigned to the overall reaction as a whole, but to individual elementary steps.
Correct Answer: It is not assigned to the overall reaction as a whole, but to individual elementary steps.
Explanation: Molecularity is meaningful only for a single elementary step because it counts the number of species taking part in that one event. A complex reaction occurs through several steps, so molecularity is not assigned to the overall reaction directly.
139. For an elementary reaction, which statement is generally correct?
ⓐ. The stoichiometric coefficients can correspond to the powers in its rate expression.
ⓑ. The rate law can never be connected with the stoichiometric equation.
ⓒ. The molecularity may be zero if one reactant is in excess.
ⓓ. The order must always be fractional.
Correct Answer: The stoichiometric coefficients can correspond to the powers in its rate expression.
Explanation: In an elementary reaction, the process occurs in a single step, so the rate expression may directly reflect the stoichiometric participation of the reacting species. This direct correspondence is not generally valid for an overall complex reaction.
140. Which statement is true for order but not for molecularity?
ⓐ. It is defined only for a single elementary step.
ⓑ. It is always a positive integer.
ⓒ. It may be zero or fractional.
ⓓ. It counts the number of colliding species.
Correct Answer: It may be zero or fractional.
Explanation: Order comes from the experimentally observed rate law, so it can be zero, fractional, or an integer. Molecularity is a count of species in an elementary step, so it must be a positive integer.
141. A reaction follows the rate law $r = k[A]^m[B]^n$. In two experiments, $[B]$ is kept constant. When $[A]$ is doubled, the rate becomes four times. What is the value of $m$?
ⓐ. $2$
ⓑ. $1$
ⓒ. $3$
ⓓ. $\frac{1}{2}$
Correct Answer: $2$
Explanation: Given:
Rate law, $r = k[A]^m[B]^n$
Between the two experiments:
$[B]$ is constant
$[A]$ is doubled
Rate becomes $4$ times
Required:
Value of $m$
Relevant principle:
When only one concentration changes, the rate factor depends on the power of that concentration term.
Why this principle applies:
Since $[B]$ is constant, only the $[A]^m$ term changes the rate.
Initial rate:
$r_1 = k[A]^m[B]^n$
New rate after doubling $[A]$:
$r_2 = k(2[A])^m[B]^n = 2^m k[A]^m[B]^n$
Rate ratio:
\[
\frac{r_2}{r_1} = 2^m
\]
Given rate becomes four times:
\[
2^m = 4
\]
Now compare powers of 2:
\[
4 = 2^2
\]
So,
\[
m = 2
\]
Final Answer:
$m = 2$
142. In an initial-rate study, $[A]$ is kept constant. When $[B]$ is doubled, the rate also doubles. What is the order of the reaction with respect to $B$?
ⓐ. $0$
ⓑ. $1$
ⓒ. $2$
ⓓ. $\frac{3}{2}$
Correct Answer: $1$
Explanation: Given:
$[A]$ is constant
$[B]$ is doubled
Rate also doubles
Required:
Order with respect to $B$
Relevant principle:
If rate changes by the same factor as the concentration of one reactant, the exponent of that reactant is $1$.
Why this principle applies:
Only $[B]$ is changed, so its exponent alone controls the observed rate change.
Let the rate law contain $[B]^n$.
Rate ratio:
\[
\frac{r_2}{r_1} = \left(\frac{2[B]}{[B]}\right)^n = 2^n
\]
Given:
\[
\frac{r_2}{r_1} = 2
\]
So,
\[
2^n = 2
\]
Therefore:
\[
n = 1
\]
Final Answer:
Order with respect to $B = 1$
