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AP Chemistry Practice Quiz: Introduction to Equilibrium

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

Test your understanding with short quizzes. This quiz has 11 questions to check your progress.

Question 1 of 11

Which of the following statements best describes a chemical system that has reached equilibrium?

All Questions (11)

Which of the following statements best describes a chemical system that has reached equilibrium?

A) The concentrations of reactants and products are equal.

B) The forward and reverse reactions have completely stopped.

C) The concentrations of reactants and products remain constant over time.

D) The amount of reactants has been completely converted to products.

Correct Answer: C

According to the provided content, at equilibrium, no observable changes occur, and the concentrations or partial pressures of reactants and products remain constant. The concentrations do not have to be equal, and the reactions do not stop; they proceed at equal rates.

The state of equilibrium is described as dynamic because, at the molecular level:

A) the molecules stop moving and reacting.

B) the forward and reverse processes continue to occur at equal rates.

C) the concentrations of the substances are constantly changing.

D) only the forward reaction is occurring.

Correct Answer: B

The content explicitly states that the equilibrium state is dynamic, meaning the forward and reverse processes continue at equal rates. This results in no net observable change, but the reactions have not stopped.

A graph plots the concentration of a reactant and a product versus time for a reversible reaction. Equilibrium is first established at the point where:

A) the concentration of the reactant is zero.

B) the concentration curves for the reactant and product cross.

C) the slopes of both concentration curves become zero.

D) the concentration of the product is at its maximum possible value.

Correct Answer: C

The content mentions that graphs of concentration versus time can illustrate the establishment of equilibrium. Equilibrium is reached when concentrations stop changing, which is represented on a graph by the point where the concentration curves become horizontal (i.e., their slopes become zero).

Which of the following is an example of a reversible physical process that can establish equilibrium?

A) Burning a piece of paper.

B) Dissolving a limited amount of salt in a fixed amount of water.

C) An iron nail rusting in the open air.

D) Baking a cake.

Correct Answer: B

The provided content lists salt dissolution as an example of an observable, reversible process. When a salt dissolves, an equilibrium can be established between the dissolved ions and the undissolved solid, especially in a saturated solution. The other options are examples of irreversible chemical changes.

A student observes a sealed flask containing a saturated solution of a colored salt with excess solid salt at the bottom. The student notes that the color of the solution and the amount of solid do not change over several days. This observation indicates that:

A) the process of dissolving has completely stopped.

B) all the water has evaporated.

C) the system has reached a state of dynamic equilibrium.

D) the salt is no longer soluble in the water.

Correct Answer: C

The constant observable properties (color of the solution, amount of solid) indicate that the system is at equilibrium. Because salt dissolution is a reversible process, this is a dynamic equilibrium where the rate of dissolving equals the rate of crystallization, resulting in no net change.

For a reversible reaction, what is the relationship between the rates of the forward and reverse reactions once equilibrium is established?

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

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

C) Both the forward and reverse reaction rates are zero.

D) The rate of the forward reaction is equal to the rate of the reverse reaction.

Correct Answer: D

The definition of a dynamic equilibrium, as stated in the content, is that the forward and reverse processes continue at equal rates. This equality of rates is what leads to the constant concentrations and lack of observable change.

Which statement correctly distinguishes the macroscopic properties from the microscopic processes in a system at chemical equilibrium?

A) Macroscopically, the system is static and unchanging, while microscopically, reactions have stopped.

B) Macroscopically, the system is static and unchanging, while microscopically, forward and reverse reactions occur at equal rates.

C) Macroscopically, concentrations are changing, while microscopically, only the forward reaction is occurring.

D) Macroscopically, the reaction is complete, while microscopically, only the reverse reaction is occurring.

Correct Answer: B

The content explains that at equilibrium, there are no observable (macroscopic) changes, and concentrations are constant. However, it is also a dynamic (microscopic) state where forward and reverse processes continue at equal rates. Option B correctly captures this dual nature.

A graph shows the rate of a forward reaction starting high and decreasing over time, while the rate of the reverse reaction starts at zero and increases over time. At the moment the two rate curves intersect and become a single horizontal line:

A) the reaction has stopped.

B) the concentrations of reactants and products are equal.

C) the system has reached equilibrium.

D) the reaction has gone to completion.

Correct Answer: C

The content states that graphs of reaction rate versus time can illustrate equilibrium. Equilibrium is achieved when the forward and reverse rates become equal. The intersection of the two rate curves, which then continue as a constant, non-zero horizontal line, visually represents this condition.

The establishment of equilibrium in a chemical or physical process is only possible if the process is:

A) irreversible.

B) spontaneous.

C) reversible.

D) exothermic.

Correct Answer: C

The first point of the provided content explicitly links the establishment of equilibrium to the occurrence of a reversible chemical or physical process. An irreversible process proceeds in one direction only and cannot reach equilibrium.

In a sealed container, liquid bromine is in equilibrium with its vapor (Br₂(l) ⇌ Br₂(g)). Which of the following is NOT true about this system at equilibrium?

A) The partial pressure of Br₂(g) is constant.

B) The concentration of Br₂(l) is constant.

C) The processes of evaporation and condensation have both stopped.

D) Both liquid and gaseous bromine are simultaneously present.

Correct Answer: C

This is an example of a phase change equilibrium. At equilibrium, the system is dynamic. Evaporation (forward process) and condensation (reverse process) are still occurring, but at equal rates. Therefore, stating that the processes have stopped is incorrect. The other statements are all characteristics of this system at equilibrium.

A key feature of any system at chemical equilibrium is that:

A) the amounts of reactants and products are equal.

B) the reaction is a simple, one-step process.

C) reactants and products are simultaneously present at constant concentrations.

D) the temperature of the system must be high.

Correct Answer: C

The content states that at equilibrium, reactants and products are simultaneously present, and their concentrations or partial pressures remain constant. A common misconception is that their amounts must be equal, but this is not required for equilibrium.