1. In chemistry, an equilibrium state is best described as a condition in which
ⓐ. all particles stop moving
ⓑ. only the forward change continues
ⓒ. dynamic balance with no net observable change
ⓓ. reactants and products are always present in equal amounts
Correct Answer: dynamic balance with no net observable change
Explanation: Equilibrium means a balance between two opposite tendencies. The system may look unchanged from outside because the net change is zero. This does not mean that particles have stopped moving or reacting. At the microscopic level, opposite processes can still continue. The balance is about equal opposing effects, not necessarily equal amounts of substances.
2. A closed bottle contains some water and water vapour above it at a fixed temperature. The water level becomes constant after some time because
ⓐ. water vapour has changed into a new chemical substance
ⓑ. evaporation and condensation occur at equal rates
ⓒ. condensation has completely stopped
ⓓ. evaporation has completely stopped
Correct Answer: evaporation and condensation occur at equal rates
Explanation: In a closed bottle, water molecules can escape from the liquid surface into the vapour phase. Vapour molecules can also return to the liquid by condensation. When these two rates become equal, the amount of liquid and vapour remains constant. The system appears unchanged, but molecular exchange continues. This is a physical example of dynamic equilibrium.
3. A saturated sugar solution is left with some undissolved sugar at the bottom. At equilibrium, the best particle-level description is that
ⓐ. dissolution and crystallization balance equally
ⓑ. no sugar particles dissolve from the solid
ⓒ. dissolved sugar particles never return to the solid
ⓓ. the solution has become chemically inactive
Correct Answer: dissolution and crystallization balance equally
Explanation: In a saturated solution, the maximum amount of solute is dissolved at that temperature. Extra solid solute remains in contact with the solution. Sugar particles may still leave the solid and enter the solution, while dissolved particles may return to the solid surface. At equilibrium, these two processes balance each other. The constant concentration of the solution is a macroscopic result of continuing microscopic movement.
4. The idea “equilibrium is static because no visible change occurs” is weak because
ⓐ. equilibrium can occur only in gases
ⓑ. equilibrium always produces equal masses of reactants and products
ⓒ. equilibrium requires the temperature to become \(0\,\text{K}\)
ⓓ. visible constancy can hide opposite processes
Correct Answer: visible constancy can hide opposite processes
Explanation: A system at equilibrium may look unchanged because measurable properties remain constant. However, this constancy does not prove that all motion or reaction has stopped. Molecules may still evaporate and condense, dissolve and crystallize, or react in opposite directions. The key idea is dynamic balance at the microscopic level. Static appearance should not be confused with absence of molecular change.
5. A simple reversible physical change is represented by
ⓐ. a forward-and-reverse physical change
ⓑ. only one possible direction of change
ⓒ. a process in which temperature has no role
ⓓ. a change in which products disappear permanently
Correct Answer: a forward-and-reverse physical change
Explanation: A reversible change can proceed in two opposite directions. For example, evaporation and condensation are opposite changes between liquid and vapour. Melting and freezing are also opposite changes between solid and liquid. Equilibrium becomes possible when both directions can occur in the same system. A one-way change cannot form a dynamic balance with its reverse.
6. The quantity that most directly tells how much solute is present in a given volume of solution is
ⓐ. temperature
ⓑ. concentration
ⓒ. pressure
ⓓ. reaction rate
Correct Answer: concentration
Explanation: Concentration expresses the amount of solute present in a specified amount of solution, often per unit volume. It is important in equilibrium because concentrations of reacting species are used to describe the composition of a mixture. Pressure is more directly linked with gases, while temperature measures thermal condition. Reaction rate describes how fast a change occurs. These quantities may all appear in equilibrium, but they do not mean the same thing.
7. Pressure is especially important in equilibrium discussions when the system contains
ⓐ. only a pure solid sample with no gas phase
ⓑ. a lump of metal at room temperature
ⓒ. a dry salt sample with no vapour phase
ⓓ. gases with variable partial pressures
Correct Answer: gases with variable partial pressures
Explanation: Pressure is a major variable for gases because gas particles spread through available space and collide with container walls. In gaseous equilibria, changes in pressure can alter the conditions under which the system balances. For a pure solid alone, pressure usually does not describe composition in the same direct way. Gas pressure is also useful in physical equilibria such as vapour above a liquid. This is why closed containers often matter in equilibrium examples involving gases or vapours.
8. The word “rate” in an equilibrium discussion refers to
ⓐ. the final colour of the mixture
ⓑ. the temperature unit used in the experiment
ⓒ. the speed at which a process occurs
ⓓ. the total mass of the system
Correct Answer: the speed at which a process occurs
Explanation: Rate tells how fast a process takes place. In equilibrium, the forward and reverse rates are compared. When the two opposing rates become equal, there is no net change in the measured composition. This does not require the amounts of the two sides to be equal. Equal rate is a condition of dynamic balance, while amount is a composition measure.
9. A beaker of water is left open in a room. After a long time, much of the water disappears. This situation is not a good example of liquid-vapour equilibrium because
ⓐ. evaporation is always a chemical reaction
ⓑ. water cannot evaporate at room temperature
ⓒ. vapour escapes instead of returning to condense
ⓓ. condensation is impossible below \(100\,^\circ\text{C}\)
Correct Answer: vapour escapes instead of returning to condense
Explanation: Liquid-vapour equilibrium needs both evaporation and condensation to occur in opposition. In an open beaker, many water vapour molecules escape into the surroundings. Because they do not remain confined above the liquid, condensation back into the beaker is not sufficiently balanced with evaporation. The water level can keep decreasing. A closed system is usually needed when matter can escape.
10. At equilibrium, “no net change” means that
ⓐ. balanced opposite changes with no net change
ⓑ. the amount of every substance must become zero
ⓒ. no particles are moving
ⓓ. all forward changes are forbidden
Correct Answer: balanced opposite changes with no net change
Explanation: No net change describes the overall measurable result of an equilibrium state. A property such as concentration, pressure, or amount of liquid may remain constant. This constancy comes from two opposite processes occurring at equal rates. It does not imply that individual molecules have stopped moving. Equilibrium is best understood as balanced activity, not inactivity.
11. In the notation \(\mathrm{A\rightleftharpoons B}\), the arrow \(\rightleftharpoons\) shows that
ⓐ. \(\mathrm{A}\) changes to \(\mathrm{B}\) only once
ⓑ. the process is reversible in both directions
ⓒ. \(\mathrm{B}\) cannot change back to \(\mathrm{A}\)
ⓓ. \(\mathrm{A}\) and \(\mathrm{B}\) must have equal concentrations
Correct Answer: the process is reversible in both directions
Explanation: The symbol \(\rightleftharpoons\) is used for a reversible process or reaction. It shows that the forward change \(\mathrm{A\to B}\) and the reverse change \(\mathrm{B\to A}\) are both possible. At equilibrium, both directions continue at equal rates. The symbol does not say that the concentrations of \(\mathrm{A}\) and \(\mathrm{B}\) are equal. Equal rates and equal concentrations are different ideas.
12. For a reversible change \(\mathrm{Reactants\rightleftharpoons Products}\), the forward direction refers to
ⓐ. products changing into reactants
ⓑ. the system cooling to stop reaction
ⓒ. both sides disappearing together
ⓓ. reactants changing into products
Correct Answer: reactants changing into products
Explanation: The forward direction is the direction in which reactants form products. The reverse direction is the opposite process, where products form reactants. In a reversible reaction, both directions can occur under suitable conditions. At equilibrium, the forward and reverse rates become equal. Naming the direction does not decide which side has the larger amount.
13. The blank in the statement is best filled by: At dynamic equilibrium, the forward rate is ______ the reverse rate.
ⓐ. unrelated to
ⓑ. greater than
ⓒ. smaller than
ⓓ. equal to
Correct Answer: equal to
Explanation: Dynamic equilibrium is reached when the two opposing rates become equal. If the forward rate were greater, the amount of products would still be increasing overall. If the reverse rate were greater, the amount of reactants would still be increasing overall. Equal rates explain why the composition remains constant with time. The equality applies to rates, not necessarily to concentrations or masses.
14. A mixture at equilibrium has \([\mathrm{A}]=0.20\,\text{mol L}^{-1}\) and \([\mathrm{B}]=0.80\,\text{mol L}^{-1}\) for \(\mathrm{A\rightleftharpoons B}\). This data shows that
ⓐ. the reverse reaction has stopped completely
ⓑ. equilibrium is possible even when concentrations are not equal
ⓒ. the forward reaction has stopped completely
ⓓ. the mixture cannot be at equilibrium because \([\mathrm{A}]\neq[\mathrm{B}]\)
Correct Answer: equilibrium is possible even when concentrations are not equal
Explanation: Equilibrium requires equal forward and reverse rates, not equal concentrations. The concentrations of different species depend on the nature of the process and the conditions. A system may have more \(\mathrm{B}\) than \(\mathrm{A}\) and still be at equilibrium. What remains constant at equilibrium is the composition with time, not equality of all amounts. This distinction prevents confusing rate balance with concentration equality.
15. Use the table to identify the set that matches an equilibrium state.
| Set | Forward rate | Reverse rate | Observed composition with time |
|---|
| P | Greater than reverse rate | Smaller than forward rate | Changing |
| Q | Equal to reverse rate | Equal to forward rate | Constant |
| R | Zero | Zero | Constant because all motion stops |
| S | Smaller than reverse rate | Greater than forward rate | Changing |
ⓐ. Set P
ⓑ. Set R
ⓒ. Set Q
ⓓ. Set S
Correct Answer: Set Q
Explanation: Equilibrium is identified by equal forward and reverse rates. The observed composition remains constant because the two opposite changes balance each other. Set \(Q\) includes both of these requirements. Set \(R\) is not a dynamic equilibrium because it describes both rates as zero. A constant observation by itself is not enough unless the continuing opposite processes are also recognized.
16. A graph for a reversible reaction shows the concentration of a reactant decreasing at first and then becoming a horizontal line. The horizontal part most reasonably represents
ⓐ. constant concentration at equilibrium
ⓑ. equal concentrations of all substances
ⓒ. stopping of molecular collisions
ⓓ. complete disappearance of the reactant
Correct Answer: constant concentration at equilibrium
Explanation: A horizontal concentration-time line means the measured concentration is no longer changing with time. In a reversible reaction, this usually indicates that the system has reached equilibrium. The reactant may still be present and may still react in the forward direction. Its concentration remains constant because the reverse process replaces it at the same rate at which it is used. A plateau in concentration is evidence of no net change, not proof of no molecular activity.
17. Assertion: At equilibrium, the forward and reverse reactions continue.
Reason: At equilibrium, the forward and reverse rates are equal.
ⓐ. The assertion is true, but the reason is false
ⓑ. Both assertion and reason are true, and the reason explains the assertion
ⓒ. The assertion is false, but the reason is true
ⓓ. Both assertion and reason are true, but the reason does not explain the assertion
Correct Answer: Both assertion and reason are true, and the reason explains the assertion
Explanation: The assertion is true because equilibrium is dynamic. The reaction does not stop at the molecular level after equilibrium is reached. The reason is also true because equal forward and reverse rates are the condition for no net change. This equality explains how both reactions can continue while the observable composition remains constant. If both rates were zero, the state would not represent the usual dynamic meaning of chemical equilibrium.
18. Two statements about \(\mathrm{X\rightleftharpoons Y}\) are given:
I. At equilibrium, \(\mathrm{X}\) and \(\mathrm{Y}\) must have equal concentrations.
II. At equilibrium, the rate of formation of \(\mathrm{Y}\) equals the rate of formation of \(\mathrm{X}\).
The valid choice is
ⓐ. both I and II
ⓑ. neither I nor II
ⓒ. only II
ⓓ. only I
Correct Answer: only II
Explanation: Statement I is not generally true because equilibrium does not require equal concentrations. The amounts of \(\mathrm{X}\) and \(\mathrm{Y}\) depend on the particular reversible process and its conditions. Statement II expresses the rate condition for dynamic equilibrium. The rate of formation of \(\mathrm{Y}\) is the forward rate, while the rate of formation of \(\mathrm{X}\) is the reverse rate. Equal rates keep the composition constant even when the two concentrations are different.
19. A physical equilibrium is most likely to be established when
ⓐ. opposite physical processes balance in a closed system
ⓑ. one physical process occurs without any possible reverse process
ⓒ. temperature keeps changing while matter escapes
ⓓ. matter involved in the process can freely escape from the system
Correct Answer: opposite physical processes balance in a closed system
Explanation: Physical equilibrium involves a balance between opposite physical changes, such as evaporation and condensation or dissolution and crystallization. A closed system is often needed because the particles taking part in the reverse process must remain available. If vapour or gas escapes, the reverse process cannot balance the forward process properly. At equilibrium, measurable properties become constant, but particles continue to move between the two forms. The balance depends on equal opposing rates under fixed conditions.
20. In a closed vessel containing liquid water and its vapour, the measurable property that becomes constant at fixed temperature is
ⓐ. the colour of water
ⓑ. the chemical formula of water
ⓒ. the molar mass of water
ⓓ. the vapour pressure of water
Correct Answer: the vapour pressure of water
Explanation: In a closed vessel, water molecules evaporate from the liquid and vapour molecules condense back into the liquid. When the rates become equal, the pressure exerted by the vapour becomes constant. This pressure is a macroscopic sign of liquid-vapour equilibrium at a fixed temperature. The chemical formula and molar mass of water do not become equilibrium variables in this physical process. The constant vapour pressure still comes from continuing molecular exchange.
