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) Forward and backward reactions occur at equal rates

B) Both reactions stop completely after some time

C) Only forward reaction continues while backward reaction stops

D) Reactants are completely used up

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) Dynamic equilibrium

C) Static equilibrium

D) Irreversible state

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) Temperature is too high

B) System is not closed

C) Reaction is too fast

D) Catalyst is absent

4 / 20

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

A) Reaction has stopped completely

B) Rate of forward reaction equals rate of backward reaction

C) Concentration of reactants equals concentration of products

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) System is at equilibrium

B) Only forward reaction is occurring

C) Reaction is irreversible

D) Reaction has stopped

7 / 20

Which statement best distinguishes equilibrium from completion of a reaction?

A) At completion, products revert to reactants

B) At equilibrium, reaction occurs in both directions

C) At completion, reaction is reversible

D) At equilibrium, reactants are fully consumed

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 depend only on temperature

B) They are always equal in all reactions

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

D) They are always zero

9 / 20

Which condition is essential for establishing chemical equilibrium?

A) Open container with continuous gas exchange

B) High pressure only

C) Presence of catalyst only

D) Closed system allowing no loss or gain of substances

10 / 20

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

A) No observable change in properties

B) Reaction stops completely

C) Continuous change in color or pressure

D) Only products are visible

11 / 20

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

A) Because forward and backward reactions continue simultaneously.

B) Because only products are moving.

C) Because reaction stops and restarts repeatedly.

D) Because reaction speeds are high.

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) Forward becomes greater than backward

B) Backward becomes zero

C) Both become equal

D) Both become zero

13 / 20

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

A) It shifts equilibrium toward reactants.

B) It shifts equilibrium toward products.

C) It has no effect on position of equilibrium.

D) It stops the reaction.

14 / 20

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

A) A system where gases can escape freely.

B) A system open to atmosphere.

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

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

15 / 20

Why is equilibrium not considered a state of no reaction?

A) Because catalysts stop working.

B) Because reactions continue in both directions.

C) Because temperature becomes zero.

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 shows no net change despite ongoing reactions.

C) It is completely static.

D) It contains only products.

17 / 20

Which situation best represents chemical equilibrium?

A) Water evaporating in an open container

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

C) Burning of paper in air

D) Ice melting completely in a warm room

18 / 20

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

A) They proceed only in one direction.

B) Products cannot reform reactants.

C) They always go to completion.

D) Both forward and reverse reactions are possible.

19 / 20

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

A) Only forward reaction continues

B) System remains unchanged forever

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 continuously react in both directions

B) Molecules stop moving

C) Molecules become stable and inactive

D) Only product molecules are active

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