Chemical Kinetics Mock Test – NEET Chemistry

1. The overall reaction

is experimentally observed to be second order overall.
Two candidate mechanisms are proposed.
Mechanism X:

as one elementary step.
Mechanism Y:


An investigator measures the following:
- Both mechanisms predict a rate proportional to under the stated conditions.
- The reaction has an experimentally observed second-order rate law.
- A transient species I is independently detected during the reaction.
- I is subsequently observed to disappear while P continues forming.
Assume the detection of I is reliable and that no unrelated process produces the detected species.
Assertion (A): The experimental rate law alone is insufficient to distinguish the two mechanisms, but the detected transient intermediate supplies additional evidence favouring Mechanism Y.
Reason (R): An intermediate may be formed and consumed in different elementary steps without appearing in the overall balanced equation, whereas an elementary one-step mechanism has no such intermediate stage.
Choose the correct option.

2. A gaseous reactant A undergoes the first-order decomposition

in a closed movable-piston vessel at constant temperature and total pressure.
Initially, the vessel contains only A.
The experimentally established local disappearance law is

where is the instantaneous vessel volume.
Assume ideal-gas behaviour, no leakage and an unchanged positive rate constant.
Consider the following statements.
Statement I: The amount of unreacted A follows an exponential first-order decay law.
Statement II: After the first half-life of A in terms of amount remaining, its concentration is one-third of its initial concentration.
Statement III: Because the amount of A follows first-order kinetics, a plot of against time must be a straight line with slope despite expansion of the vessel.
Statement IV: At the first half-life in terms of amount remaining, the vessel volume is times its initial volume.
Which combination is correct?

3. At a fixed temperature, a reaction involving reactants A and B follows the experimentally determined form

The initial-rate measurements are:

A fourth experiment is performed under otherwise identical conditions, using and .
The predicted initial reaction rate in Experiment 4 is:

4. A first-order reaction

is carried out in a well-mixed, constant-temperature solution.
Initially, the vessel contains of solution with

and

The first-order half-life is 10 min.
After exactly 10 min, of the well-mixed reaction solution is withdrawn instantaneously and discarded. The remaining solution is allowed to react for another 10 min.
Assume that no reaction occurs during withdrawal, that the concentrations of all species in the remaining solution are unchanged at the instant of withdrawal, and that the intrinsic first-order rate constant remains unchanged.
Which pair gives the final concentration of P in the vessel and the total number of moles of P chemically formed during the entire 20-minute experiment, including P formed in the subsequently discarded sample?

5. Assertion (A): For the reaction , the instantaneous rate of appearance of B is times the instantaneous rate of disappearance of A.
Reason (R): For this reaction, the quantities and represent the same normalized reaction rate.
Choose the correct answer.

6. A hypothetical reaction follows the experimentally established rate law

The reaction is studied at fixed temperature using the following rapid concentration-change procedure.
Initially,

The measured normalized rate is

First, additional B is introduced instantaneously so that B concentration becomes , while A concentration remains unchanged.
Next, additional A is introduced instantaneously so that A concentration becomes , while B remains at .
No reaction occurs during either instantaneous addition.
After both additions, the reaction is allowed to proceed according to

When the concentration of P has increased by from its value immediately after the additions, what is the normalized reaction rate?

7. A reaction follows

at constant temperature. The available experiments have always changed A and B concentrations together.
A student finds that doubling both concentrations doubles the initial rate. No additional kinetic information is available.
Which statement correctly describes the extent of the conclusion and the most useful next experiment?

8. Consider the following reaction statements:
Statement I: For an elementary reaction, the sum of the reactant stoichiometric coefficients gives the molecularity of that elementary event.
Statement II: Molecularity can be assigned to the overall balanced equation of every complex reaction if its stoichiometric coefficients are known.
Statement III: The rate law for an elementary reaction follows the ordinary mass-action powers of the reacting species under the stated elementary-step description.
Statement IV: A complex reaction may exhibit fractional overall order even when every elementary step has an integral molecularity.
Statement V: If a complex reaction exhibits second-order kinetics, it must contain at least one bimolecular elementary step.
Which combination is correct?

9. Consider the reaction , occurring at constant volume with no competing reactions.
Statement I: The instantaneous disappearance rate of A is twice the normalized reaction rate.
Statement II: The instantaneous appearance rate of C is three times the normalized reaction rate.
Statement III: If B decreases by , A decreases by over the same interval.
Statement IV: If A decreases by , C increases by over the same interval.
Which combination is correct?

10. Reactant A undergoes two independent parallel first-order reactions:

and

Both rate constants obey Arrhenius behaviour with temperature-independent pre-exponential factors and activation energies over the investigated range.
At ,

The activation energy of the Q-forming pathway exceeds that of the P-forming pathway by

The temperature is increased to .
Assume that the two reactions remain independent and that each product is stable.
Use

Which pair gives the fraction of instantaneous A consumption forming P at and at , respectively?

11. A pure gaseous reactant A undergoes the first-order decomposition

in a rigid vessel maintained at constant temperature.
Initially, the partial pressure of A is , and an inert gas has a partial pressure of . No B is initially present.
After 10 min, the total pressure is .
Immediately afterward, additional inert gas is introduced without changing the vessel volume or temperature. The added inert gas contributes an additional partial pressure of , and negligible reaction occurs during its introduction.
The decomposition then continues for another 10 min.
Assume ideal-gas behaviour, unchanged first-order kinetics and no leakage.
Which pair gives the first-order rate constant and the total pressure after the full 20 min?
Use .

12. A reaction involving A and B obeys

at constant temperature.
Several reaction mixtures are prepared so that the total initial concentration of A and B is always .
Initially, a mixture contains and .
The researcher can redistribute the fixed total concentration between A and B without changing the rate constant or introducing any other reacting species.
Which pair gives the concentrations that maximize the initial rate and the maximum-rate-to-original-rate ratio?

13. A first-order reaction follows the Arrhenius equation with constant activation energy and pre-exponential factor.

Use

Which pair gives the activation energy and the first-order half-life at ?

14. Reactant A can form product P through two independent parallel elementary pathways:
Pathway I:

Pathway II:

At a fixed temperature, their rate laws are

and

where and .
Initially, and .
A second mixture at the same temperature contains twice the initial concentrations of both A and B.
Assuming the two pathways remain independent, what is the ratio of the total initial rate of appearance of P in the second mixture to that in the first?

15. The rate constant of a first-order reaction is measured at four temperatures.
The reaction is believed to obey one Arrhenius equation with constant activation energy and pre-exponential factor over the complete temperature range.

An independent laboratory confirms that the first three measurements are accurate.
Only one of the original four measurements is therefore suspected to contain a reporting error.
Use

Which pair gives the corrected rate constant at the remaining temperature and the activation energy consistent with the confirmed measurements?

16. A reaction occurs through the following elementary mechanism in a constant-volume vessel:
Step 1:

Step 2:

At a particular instant, the concentrations are and . The rate constants are and .
A suitable catalyst is introduced instantaneously and selectively doubles the rate constant of Step 2 without changing Step 1, the concentrations or the temperature. Assume negligible reaction during catalyst introduction.
Which pair correctly gives the net rate of change of I immediately before and immediately after catalyst addition?

17. A reaction occurs at constant temperature and volume. Three concentration measurements are made:
No rows
Assume that all measurements are exact and that the experimental conditions remain unchanged.
A student proposes that the reaction follows either a single ideal zero-order law or a single ideal first-order law throughout the interval.
Which conclusion is justified?

18. A reaction involving A and B follows the experimentally established rate law

Two initial-rate experiments are performed at temperature . A third experiment is performed at temperature , where an independent measurement establishes that

The observations are:

Assume the partial orders remain unchanged between the two temperatures.
What is the predicted initial rate at when

19. Two normalized molecular-energy distributions are obtained for the same gaseous substance at temperatures and , with .
Two fixed activation-energy thresholds satisfy

Which pair gives the increase in the fraction of molecules having energies between and , and the fraction of molecules already above at that also exceed ?

20. A reaction

is known to follow first-order kinetics at constant temperature and volume.
An instrument measures the average positive disappearance rate of A over successive non-overlapping 10-minute intervals.
The measurements are:

The initial concentration of A was not recorded.
Assume both measured average rates are exact and the first-order rate constant remains unchanged.
Which pair gives the initial concentration of A and the first-order rate constant?
Use .

21. A reaction-coordinate diagram is used to compare the uncatalysed and catalysed pathways of the same reaction.

Which option correctly gives the activation energy for the reverse catalysed reaction, the reaction energy change for the forward reaction, and the reduction in the forward activation energy caused by the catalyst?

22. A reaction proceeds according to

in a closed constant-volume vessel at fixed temperature.
Its experimentally established normalized rate law is

Initially,

and

The initial normalized reaction rate is

At a later instant, the normalized rate becomes

Assume the rate law remains valid and no other reactions occur.
Which pair gives the concentration of P at the later instant and the rate constant?

23. The concentration dependence of a reaction is examined through two logarithmic plots.

Assuming a single power-law expression applies throughout, which conclusion is correct?

24. Reactant A undergoes two competing irreversible first-order reactions:


Both products are stable.
Initially,

During the first 10 min,

and the total disappearance of A has a half-life of 10 min.
At exactly 10 min, a selective catalyst is introduced instantaneously.
It triples but leaves unchanged.
The solution volume and temperature remain unchanged, and no reaction occurs during catalyst addition.
The mixture then reacts for another 5.0 min.
Which pair gives the final concentration of P and the ratio of the instantaneous total disappearance rate immediately after catalyst addition to that immediately before addition?

25. A reactant A undergoes two independent parallel elementary reactions:
Pathway I:

Pathway II:

The corresponding elementary reaction rates are

and

At constant temperature, the measured total initial rates of disappearance of A are:

Assume both pathways follow the stated elementary rate laws.
At , what percentage of the total instantaneous disappearance of A occurs through Pathway II?

26. An experimenter studies the influence of temperature on a chemical reaction and makes the following observations:
- Increasing temperature by approximately doubles the rate constant over one limited temperature interval.
- At a different temperature, a increase produces a smaller proportional change.
- A catalyst increases the rate constant at fixed temperature.
Consider the following statements.
Statement I: The approximate doubling of a rate constant for a rise is a useful empirical observation for many reactions, not an exact universal law.
Statement II: The Arrhenius equation predicts that equal temperature increments in kelvin must produce identical values of for constant positive activation energy.
Statement III: At fixed Arrhenius parameters, the temperature sensitivity depends on reciprocal absolute temperatures.
Statement IV: An increase in the rate constant caused by heating necessarily proves that the activation energy itself has decreased.
Statement V: A catalyst can change the kinetic pathway, whereas heating can increase the fraction of molecules with sufficient energy without necessarily changing the pathway barrier.
Which combination is correct?

27. A first-order reaction obeys

with constant A and between and .
At , the measured rate constant is

The pre-exponential factor is independently measured as

Use

Which pair gives the activation energy and the rate constant at ?

28. The reaction obeys the experimentally determined rate law

Initially, and . The reaction occurs at constant temperature and volume.
Consider three stages: Stage I is the initial state; Stage II is reached after of each reactant has been consumed; Stage III is reached after of each reactant has been consumed.
Assuming that the given rate law remains valid throughout these stages, which order of the instantaneous reaction rates is correct?

29. A reaction proceeds through the following proposed mechanism:
Step 1 — Slow:

Step 2 — Fast:

Here, C is a species supplied in small amount and regenerated during the reaction, while I is short-lived.
Assume the first elementary step controls the rate and the proposed mechanism remains applicable.
Consider the following statements.
Statement I: The overall chemical reaction is .
Statement II: I is an intermediate because it is formed in one step, consumed in another and absent from the overall equation.
Statement III: C acts as a catalyst because it is consumed in one elementary step and regenerated in another.
Statement IV: The controlling elementary-step rate law is proportional to , since B appears in the overall equation.
Statement V: The molecularity of the slow elementary step is two, whereas the overall complex reaction is not assigned a single elementary molecularity.
Which combination is correct?

30. A dissolved compound A undergoes first-order decomposition at constant temperature.
Its concentration is monitored by withdrawing small aliquots at different times and titrating each aliquot with the same standard reagent.
The volume of standard reagent required is proportional to the number of moles of unreacted A present in the aliquot.
Different aliquot volumes are deliberately used.
The following observations are obtained:

Assume negligible change in the bulk solution volume due to sampling, exact titration measurements, and unchanged first-order kinetics.
Which pair gives the first-order rate constant and the titrant volume required for a aliquot withdrawn at 20 min?
Use

31. A reaction follows

at constant temperature and volume. The disappearance of A obeys first-order kinetics throughout the experiment.
Initially,

After 20 min, the concentration of P is .
At what total elapsed time will the concentrations of A and P first become equal?

32. A gaseous reaction follows

at constant temperature. All gases behave ideally, and the following initial-rate experiments are performed:

The remaining mole fraction in each mixture belongs to an inert gas. Assume the rate law remains valid for all mixtures.
What initial rate is predicted for a mixture having total pressure , mole fraction of A equal to , and mole fraction of B equal to ?

33. The same overall reaction is studied at a fixed temperature in the absence and presence of a catalyst. Initial rates are measured at two initial concentrations of A.

Assume that each set of measurements obeys a power-law rate expression over the investigated concentration range and that all other kinetic conditions are identical within each set.
Which conclusion is supported by the data?

34. A reaction is represented by the overall equation

Two candidate mechanisms are proposed.
Mechanism I:
Step 1 (slow):

Step 2 (fast):

Mechanism II:
Step 1 (slow):

Step 2 (fast):

The initial-rate experiments at fixed temperature establish that doubling A concentration at unchanged B doubles the rate, while doubling B concentration at unchanged A also doubles the rate.
Assume the slow-step approximation is valid for both candidate mechanisms and each intermediate is promptly consumed in its following step.
Consider the following statements.
Statement I: Both mechanisms reproduce the same overall balanced equation.
Statement II: The measured concentration dependence is compatible with Mechanism I under the stated slow-step approximation.
Statement III: The measured concentration dependence is compatible with Mechanism II under the stated slow-step approximation.
Statement IV: The overall reaction is necessarily elementary bimolecular because its measured overall order is two.
Statement V: The molecularities of the slow steps in Mechanisms I and II are both two, although their predicted partial orders differ.
Which combination is correct?

35. A hypothetical elementary reaction has a forward activation energy of

and an overall forward reaction energy change of

A catalyst provides an alternative pathway that lowers the forward activation energy by

The catalyst does not change the energies of the reactants or products.
Assume each pathway has one relevant transition-state maximum and that the reverse process follows the corresponding reverse pathway.
Which option gives the uncatalysed reverse activation energy and the catalysed reverse activation energy?

36. A gaseous reaction obeys the rate law . Two gas mixtures at the same temperature have the following total pressures and mole fractions:

The remaining gas in each mixture is inert. Assuming ideal-gas behaviour and identical kinetic conditions, the ratio is:

37. A reaction represented by occurs at constant volume, with no other reaction taking place. During a particular time interval, the concentration of X decreases by , while those of Y and Z increase by and , respectively. At a later instant, the rate of appearance of Y is . The instantaneous rate of appearance of Z at that instant is:

38. A reaction can proceed either through a direct uncatalysed pathway or through a two-step catalysed pathway involving an intermediate I.

Which pair gives the activation energy of the forward elementary step and the activation energy of the reverse elementary step , respectively?

39. A pure gaseous reactant A undergoes the reaction

in a rigid vessel maintained at constant temperature. Initially, the vessel contains only A at a total pressure of .
The normalized reaction rate follows

After 10 min, the total pressure is .
Assume ideal-gas behaviour and that the stated first-order law remains valid.
Use .
Which pair gives the normalized rate constant and the total pressure after 20 min?

40. A first-order reaction occurs at constant temperature and volume. Initially,

Assuming first-order kinetics throughout, which pair gives and the concentration of P at that instant?
Use .

41. Two independent first-order decompositions occur in separate constant-volume vessels at the same temperature:

and

Their initial concentrations are

and

The half-lives are

and

Both reactions begin simultaneously.
At a particular elapsed time, their positive instantaneous disappearance rates become equal.
Which pair gives that elapsed time and the ratio of the remaining concentrations at that instant?

42. A researcher applies a simple bimolecular collision model to the gaseous reaction

At constant temperature, the following initial measurements are obtained:

The researcher proposes that the energetic fraction and orientation factor are identical in all three experiments, so product-formation rate must remain directly proportional to total collision frequency.
Assume the listed measurements are accurate and that every successful collision produces one unit of product.
Which conclusion is best supported?

43. Two independent decomposition reactions are studied at constant temperature.
Reaction X follows zero-order disappearance:

Reaction Y follows first-order disappearance:

For each reaction, two experiments begin with initial reactant concentrations and , respectively.
The intrinsic rate constant of each reaction remains unchanged between its two experiments.
All measurements are restricted to concentrations for which the stated rate laws remain valid.
Consider the following statements.
Statement I: For Reaction X, doubling the initial concentration doubles the time required to reach conversion.
Statement II: For Reaction Y, the fraction of initial reactant remaining after a specified elapsed time is independent of its initial concentration.
Statement III: For Reaction X, doubling the initial concentration doubles the initial instantaneous disappearance rate.
Statement IV: For Reaction Y, doubling the initial concentration doubles the initial instantaneous disappearance rate.
Statement V: If zero-order and first-order reactions begin at the same concentration and have equal initial instantaneous disappearance rates, their first half-lives must also be equal.
Which combination is correct?

44. A surface-catalysed reaction exhibits two experimentally established kinetic regimes at constant temperature and volume.
At sufficiently high reactant concentration, the surface is saturated and the disappearance of A is zero order:

At lower reactant concentration, the disappearance is first order:

Assume that the two rate expressions describe a continuous transition and that the transition occurs where their predicted instantaneous rates are equal.
Initially,

What total time is required for the concentration of A to decrease to ?
Use .

45. Two independent reactions occur simultaneously in a well-mixed solution at constant temperature and volume:


Initially,


During a 10.0-minute interval, the total concentration decreases are measured as

and

Assume that both reactions proceed only in the forward direction and that P and Q are stable.
Which pair gives the ratio of the average normalized rates of Reaction I to Reaction II and the final concentration of Q?

46. For the reaction , a concentration–time curve of R is obtained.

The ratio of the average disappearance rate of R during to the instantaneous appearance rate of P at is:

47. Reactant R can form two different products through competing reactions:


At temperature , the instantaneous rates of formation of P and Q are in the ratio .
When the temperature is increased to , the rate constant for the formation of P becomes three times its value at , while that for Q becomes eight times its original value.
Assuming that both pathways are first order in R, the concentration of R is identical in the two comparisons, and the rate-law forms remain unchanged, the percentage of total instantaneous consumption of R occurring through the Q-forming pathway at is:

48. A gaseous bimolecular reaction

is investigated at constant temperature.
The reacting gases are compressed instantaneously from volume V to .
No reaction occurs during compression, the numbers of A and B molecules remain unchanged, and their molecular identities are unchanged.
Use the simplified collision-frequency model

under otherwise identical conditions.
Assertion (A): Immediately after compression, the A–B collision frequency per unit volume becomes four times its original value, while the total A–B collision frequency in the vessel becomes twice its original value.
Reason (R): Halving the volume doubles both concentrations, and total collision frequency equals collision frequency per unit volume multiplied by vessel volume.
Choose the correct option.

49. A reaction occurs in solution through the following simplified mechanism:
Fast equilibrium:

Slow elementary reaction:

Here, AI is an unreactive complex, and only free A participates in the slow product-forming reaction.
The equilibrium relationship is

The total analytical concentration of A is

The free concentration of inhibitor I is externally maintained at its stated value in each experiment.
The elementary rate constant for product formation is

Initially,

In a second experiment, only the maintained free inhibitor concentration is increased to .
Assume the same total A concentration, B concentration, temperature and equilibrium model.
Which pair gives the initial product-formation rates in the first and second experiments?

50. Two hypothetical bimolecular gaseous reactions, X and Y, are studied at the same temperature.
Reaction X involves relatively simple molecules whose reactive regions are accessible from several approach directions.
Reaction Y involves more structurally complicated molecules whose reactive regions are accessible only through a restricted range of approach directions.
Independent measurements show that the total collision frequency of Y is larger than that of X.
However, Y has the smaller experimentally observed product-formation rate.
The activation-energy thresholds and energetic fractions have not been determined independently.
Consider the following statements.
Statement I: The larger total collision frequency of Y does not guarantee a larger effective-collision frequency.
Statement II: Restricted molecular orientation in Y is one possible explanation for its smaller product-formation rate.
Statement III: The observations prove that the activation energy of Y must be greater than that of X.
Statement IV: The simplest hard-sphere collision model may be insufficient to describe all the geometrical requirements of structurally complicated molecules.
Statement V: The smaller rate of Y proves that its molecules undergo fewer total encounters than those of X.
Which combination is correct?