PrepGo

AP Chemistry Practice Quiz: Representations of Equilibrium

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 7 questions to check your progress.

Question 1 of 7

A particulate diagram shows a closed container with a mixture of N₂ molecules, H₂ molecules, and NH₃ molecules. An observer notes that the total number of each type of molecule remains constant over time. Which statement best describes this system?

All Questions (7)

A particulate diagram shows a closed container with a mixture of N₂ molecules, H₂ molecules, and NH₃ molecules. An observer notes that the total number of each type of molecule remains constant over time. Which statement best describes this system?

A) The reaction has stopped completely.

B) The system has reached a state of dynamic equilibrium.

C) Only the forward reaction, N₂ + 3H₂ → 2NH₃, is occurring.

D) Only the reverse reaction, 2NH₃ → N₂ + 3H₂, is occurring.

Correct Answer: B

A system at equilibrium is characterized by constant macroscopic properties (like the number of particles of each species) because the forward and reverse reaction rates are equal. This is a dynamic process, meaning both reactions are still occurring, but at the same rate.

Consider the reversible reaction: 2X(g) ⇌ Y(g). A particulate model of the system at equilibrium in a sealed container shows 10 particles of reactant X and 2 particles of product Y. Based on this representation, what can be concluded about the equilibrium constant, K?

A) K > 1

B) K < 1

C) K = 1

D) K = 0

Correct Answer: B

At equilibrium, the number of reactant particles (10 of X) is significantly greater than the number of product particles (2 of Y). This indicates that the equilibrium lies to the left, favoring the reactants. When reactants are favored over products at equilibrium, the value of the equilibrium constant, K, is less than 1.

A particulate diagram represents the initial state of a reversible reaction, showing 6 molecules of A₂. A second diagram represents the same system after it has reached equilibrium, showing 2 molecules of A₂ and 8 molecules of A. Which balanced chemical equation is consistent with these particulate representations?

A) A₂ ⇌ 2A

B) 2A₂ ⇌ A

C) A₂ ⇌ A

D) 2A₂ ⇌ 4A

Correct Answer: A

The system started with 6 A₂ molecules and ended with 2 A₂ molecules, which means 4 molecules of A₂ reacted. In the process, 8 molecules of A were formed. The ratio of A₂ reacted to A produced is 4:8, which simplifies to a stoichiometric ratio of 1:2. This corresponds to the balanced equation A₂ ⇌ 2A.

The following reaction reaches equilibrium in a closed container: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). A particulate model of the equilibrium mixture contains 3 particles of PCl₅, 4 particles of PCl₃, and 4 particles of Cl₂. If the reaction started with only PCl₅, how many particles of PCl₅ were present initially?

A) 3

B) 4

C) 7

D) 11

Correct Answer: C

According to the 1:1:1 stoichiometry, for every 1 particle of PCl₃ and 1 particle of Cl₂ formed, 1 particle of PCl₅ must have reacted. Since there are 4 particles of PCl₃ and 4 particles of Cl₂ at equilibrium, 4 particles of PCl₅ must have decomposed. The initial number of PCl₅ particles is the sum of the amount that reacted and the amount remaining at equilibrium: 4 (reacted) + 3 (remaining) = 7 particles.

For the reaction A(g) + B(g) ⇌ C(g), the equilibrium constant is very large (K >> 1). An experiment is started with an equal number of A and B particles and no C particles in a sealed container. Which particulate representation best describes the system once it has reached equilibrium?

A) A mixture containing mostly A and B particles, with very few C particles.

B) A mixture containing roughly equal numbers of A, B, and C particles.

C) A mixture containing mostly C particles, with very few A and B particles.

D) A container with only C particles, as all A and B particles have been consumed.

Correct Answer: C

A large equilibrium constant (K >> 1) signifies that the equilibrium position lies far to the right, strongly favoring the formation of products. Therefore, at equilibrium, most of the initial reactants (A and B) will have been converted to the product (C). Option D is incorrect because equilibrium is dynamic; a small number of reactant particles will always be present in a reversible reaction.

A particulate model of a reversible chemical reaction at equilibrium shows that the number of reactant particles and product particles are both constant. What does this constant composition imply about the reaction rates?

A) The rates of the forward and reverse reactions are both zero.

B) The rate of the forward reaction is greater than the rate of the reverse reaction.

C) The rate of the reverse reaction is greater than the rate of the forward reaction.

D) The rates of the forward and reverse reactions are equal and non-zero.

Correct Answer: D

Chemical equilibrium is a dynamic state where reactions continue to occur in both directions. The reason the number of reactant and product particles remains constant is not because the reactions have ceased, but because the rate at which reactants form products (forward rate) is exactly balanced by the rate at which products form reactants (reverse rate).

Consider the reaction 2A(g) ⇌ B(g). A particulate diagram of the system at equilibrium shows 4 particles of A and 6 particles of B. A second experiment is performed under identical temperature and volume conditions, but the initial state consists of 8 particles of B and 0 particles of A. Which of the following particulate representations best describes the new equilibrium state?

A) 0 particles of A and 8 particles of B.

B) 2 particles of A and 7 particles of B.

C) 8 particles of A and 4 particles of B.

D) 4 particles of A and 6 particles of B.

Correct Answer: D

The composition of an equilibrium mixture is independent of the direction from which equilibrium is approached, provided the temperature, volume, and total number of atoms of each element are the same. In the first equilibrium state (4 A, 6 B), the total number of 'A' atoms is 4 + (6*2) = 16. In the second initial state (8 B), the total number of 'A' atoms is 8*2 = 16. Since the atom count and conditions are identical, the system will reach the exact same equilibrium composition. Therefore, the new equilibrium state will also contain 4 particles of A and 6 particles of B.