Chemistry 9th Chap 9 : Topic 3 - Maryam Publications

Chemistry Chapter 9 : Topic 3

Dynamic Chemical Equilibrium

1 / 20

In a chemical system, when reactants are converted into products, and at the same time products are also converting back into reactants under the same conditions, the system is said to involve a reversible reaction. If such a system is placed in a sealed container and allowed to proceed, it may eventually reach a state where no further net change is observed. Which statement correctly describes the condition at this stage?

A) Only forward reaction continues while backward reaction stops

B) Both reactions stop completely after some time

C) Reactants are completely used up

D) Forward and backward reactions occur at equal rates

2 / 20

A student observes a reversible reaction taking place in a closed flask. After some time, the concentration of reactants and products appears constant, but molecular activity continues at the microscopic level. Which term best describes this state?

A) Chemical completion

B) Irreversible state

C) Static equilibrium

D) Dynamic equilibrium

3 / 20

A reversible reaction is carried out in an open beaker, and gases are allowed to escape into the atmosphere. The system fails to reach equilibrium. What is the most appropriate reason?

A) System is not closed

B) Reaction is too fast

C) Catalyst is absent

D) Temperature is too high

4 / 20

At equilibrium in a reversible reaction, which of the following is always TRUE?

A) Concentration of reactants equals concentration of products

B) Reaction has stopped completely

C) Rate of forward reaction equals rate of backward reaction

D) Only products remain in the system

5 / 20

A teacher explains that even at equilibrium, molecules of reactants and products are continuously interchanging. This implies that equilibrium is:

A) A state where only products move

B) A static condition with no movement

C) A condition where reactions stop permanently

D) A dynamic process at molecular level

6 / 20

In a sealed container, nitrogen and hydrogen gases react to form ammonia. After some time, no visible change is observed in composition, although reaction continues at molecular level. What can be concluded?

A) Only forward reaction is occurring

B) System is at equilibrium

C) Reaction has stopped

D) Reaction is irreversible

7 / 20

Which statement best distinguishes equilibrium from completion of a reaction?

A) At equilibrium, reactants are fully consumed

B) At completion, reaction is reversible

C) At equilibrium, reaction occurs in both directions

D) At completion, products revert to reactants

8 / 20

A student incorrectly states that at equilibrium, the amount of reactants and products must be equal. Which correction is most accurate?

A) They are not necessarily equal, only rates are equal

B) They are always equal in all reactions

C) They are always zero

D) They depend only on temperature

9 / 20

Which condition is essential for establishing chemical equilibrium?

A) Closed system allowing no loss or gain of substances

B) Presence of catalyst only

C) High pressure only

D) Open container with continuous gas exchange

10 / 20

During equilibrium in a reversible reaction, which of the following is observed at the macroscopic level?

A) Only products are visible

B) Reaction stops completely

C) Continuous change in color or pressure

D) No observable change in properties

11 / 20

Which statement best explains why equilibrium is called “dynamic”?

A) Because reaction speeds are high.

B) Because forward and backward reactions continue simultaneously.

C) Because only products are moving.

D) Because reaction stops and restarts repeatedly.

12 / 20

If a reversible reaction reaches equilibrium in a closed system, what happens to the rate of forward reaction compared to the backward reaction?

A) Both become zero

B) Both become equal

C) Backward becomes zero

D) Forward becomes greater than backward

13 / 20

A reaction mixture is allowed to reach equilibrium. A catalyst is then added. What is the effect on equilibrium?

A) It stops the reaction.

B) It has no effect on position of equilibrium.

C) It shifts equilibrium toward reactants.

D) It shifts equilibrium toward products.

14 / 20

Which of the following best describes a closed system in chemical equilibrium?

A) A system where substances can enter but not leave.

B) A system where no exchange of matter occurs with surroundings.

C) A system where gases can escape freely.

D) A system open to atmosphere.

15 / 20

Why is equilibrium not considered a state of no reaction?

A) Because temperature becomes zero.

B) Because catalysts stop working.

C) Because reactions continue in both directions.

D) Because only reactants are present.

16 / 20

In a reversible reaction at equilibrium, what can be said about the system?

A) It is continuously changing macroscopically.

B) It is completely static.

C) It contains only products.

D) It shows no net change despite ongoing reactions.

17 / 20

Which situation best represents chemical equilibrium?

A) Water evaporating in an open container

B) Burning of paper in air

C) Liquid water and vapor in a closed container exchanging molecules

D) Ice melting completely in a warm room

18 / 20

What is a key feature of reversible reactions that can reach equilibrium?

A) Both forward and reverse reactions are possible.

B) They proceed only in one direction.

C) Products cannot reform reactants.

D) They always go to completion.

19 / 20

At equilibrium, if a small disturbance is introduced, what initially happens?

A) System remains unchanged forever

B) Only forward reaction continues

C) Forward and backward rates adjust until a new equilibrium is reached

D) Reaction stops permanently

20 / 20

Which statement correctly describes equilibrium in terms of molecular activity?

A) Molecules stop moving

B) Only product molecules are active

C) Molecules continuously react in both directions

D) Molecules become stable and inactive

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