AP Chemistry Practice Quiz: Reaction Quotient and Equilibrium Constant

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

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

For the reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which expression correctly represents the reaction quotient, Qc?

A)[SO₃]² / ([SO₂]²[O₂])B)([SO₂]²[O₂]) / [SO₃]²C)[SO₃] / ([SO₂][O₂])D)(2[SO₃]) / (2[SO₂] + [O₂])

All Questions (9)

For the reversible reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which expression correctly represents the reaction quotient, Qc?

A) [SO₃]² / ([SO₂]²[O₂])

B) ([SO₂]²[O₂]) / [SO₃]²

C) [SO₃] / ([SO₂][O₂])

D) (2[SO₃]) / (2[SO₂] + [O₂])

Correct Answer: A

According to the law of mass action, the reaction quotient Qc is the ratio of the product concentrations raised to their stoichiometric coefficients to the reactant concentrations raised to their stoichiometric coefficients. For this reaction, it is [SO₃]² / ([SO₂]²[O₂]).

Consider the reaction: C(s) + H₂O(g) ⇌ CO(g) + H₂(g). What is the correct expression for the equilibrium constant, Kc?

A) ([CO][H₂]) / ([C][H₂O])

B) [CO][H₂]

C) ([CO][H₂]) / [H₂O]

D) [C][H₂O] / ([CO][H₂])

Correct Answer: C

The equilibrium expression does not include substances whose concentrations are independent of their amount, such as pure solids. Therefore, C(s) is omitted from the expression, resulting in Kc = ([CO][H₂]) / [H₂O].

A chemical system is at equilibrium. Which of the following statements accurately describes the relationship between the reaction quotient (Q) and the equilibrium constant (K)?

A) Q > K

B) Q < K

C) Q = K

D) Q = 0

Correct Answer: C

The reaction quotient (Q) describes the relative concentrations of species at any time. By definition, when a reaction has reached equilibrium, the reaction quotient is equal to the equilibrium constant (Q = K).

For the decomposition of hydrogen peroxide, 2H₂O₂(l) ⇌ 2H₂O(l) + O₂(g), what is the correct expression for the reaction quotient, Qp?

A) P_O₂

B) (P_H₂O)²(P_O₂) / (P_H₂O₂)²

C) 1 / P_O₂

D) P_O₂ / (P_H₂O₂)²

Correct Answer: A

The reaction quotient expression does not include pure liquids because their partial pressures (or concentrations) are considered constant. In this reaction, both H₂O₂(l) and H₂O(l) are pure liquids and are excluded. Therefore, Qp is simply the partial pressure of the only gaseous product, P_O₂.

The expression Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ for the reaction aA + bB ⇌ cC + dD is defined by which principle?

A) Le Châtelier's Principle

B) The Law of Mass Action

C) The Law of Conservation of Mass

D) The First Law of Thermodynamics

Correct Answer: B

The law of mass action defines the equilibrium expression, stating that the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient, is constant at equilibrium.

For the gas-phase reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), which expression represents the reaction quotient, Qp?

A) P_PCl₅ / ((P_PCl₃)(P_Cl₂))

B) (P_PCl₃)(P_Cl₂) / P_PCl₅

C) P_PCl₃ + P_Cl₂ - P_PCl₅

D) P_PCl₅ / (P_PCl₃ + P_Cl₂)

Correct Answer: B

The reaction quotient for partial pressures, Qp, is calculated by taking the partial pressures of the products and dividing by the partial pressures of the reactants, each raised to the power of their stoichiometric coefficient (which are all 1 in this case).

Which of the following substances would be included in the equilibrium constant expression, Kc, for the reaction: Zn(s) + 2H⁺(aq) ⇌ Zn²⁺(aq) + H₂(g)?

A) Only Zn(s)

B) Zn(s) and H₂(g)

C) H⁺(aq), Zn²⁺(aq), and H₂(g)

D) All four substances

Correct Answer: C

The equilibrium expression excludes pure solids and liquids. In this reaction, Zn(s) is a solid and is excluded. Aqueous ions (H⁺ and Zn²⁺) and gases (H₂) are included in the expression.

The reaction quotient (Qc) is calculated for a reaction at a time when it has not yet reached equilibrium. What information does the value of Qc provide?

A) The final concentrations of all species once equilibrium is reached.

B) The rate of the forward reaction.

C) The relative amounts of products and reactants present at that specific moment.

D) The temperature at which the reaction is at equilibrium.

Correct Answer: C

The reaction quotient, Qc, is a measure of the relative concentrations of products and reactants at any given point in time, not just at equilibrium. It provides a snapshot of the reaction's progress.

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium constant is Kp. At a certain time, the partial pressures are measured, and the reaction quotient Qp is calculated. If Qp = Kp, what can be concluded?

A) The reaction will proceed to the right to form more products.

B) The reaction will proceed to the left to form more reactants.

C) The system is at equilibrium, and the net rate of reaction is zero.

D) The partial pressure of NH₃ is equal to the partial pressures of N₂ and H₂.

Correct Answer: C

The condition for equilibrium is that the reaction quotient (Q) is equal to the equilibrium constant (K). When Qp = Kp, the system is at equilibrium, meaning the rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of reactants or products.