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AP Chemistry Practice Quiz: Reaction Quotient and Le Châtelier’s Principle

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 a reversible reaction, if the calculated reaction quotient (Q) is less than the equilibrium constant (K), in which direction will the reaction proceed to reach equilibrium?

All Questions (9)

For a reversible reaction, if the calculated reaction quotient (Q) is less than the equilibrium constant (K), in which direction will the reaction proceed to reach equilibrium?

A) The reaction will proceed in the forward direction, toward products.

B) The reaction will proceed in the reverse direction, toward reactants.

C) The system is already at equilibrium and no net change will occur.

D) The value of K will decrease until it is equal to Q.

Correct Answer: A

Based on the provided content, the relationship between Q and K determines the direction of the reaction. When Q < K, the reaction will proceed in the forward direction to increase the concentration of products and decrease the concentration of reactants until Q equals K.

A chemical system is at equilibrium. A change is made that causes the reaction quotient (Q) to become greater than the equilibrium constant (K). How will the system respond?

A) The system will shift to favor the forward reaction to re-establish equilibrium.

B) The system will shift to favor the reverse reaction to re-establish equilibrium.

C) The system will not respond because it was initially at equilibrium.

D) The equilibrium constant (K) will increase to match the new value of Q.

Correct Answer: B

According to the provided content, if a disturbance causes Q to be greater than K, the system is no longer at equilibrium. To return to equilibrium, the reaction will proceed in the reverse direction, consuming products and forming reactants, which decreases the value of Q until it equals K again.

Which of the following disturbances applied to a chemical system at equilibrium will cause a change in the value of the equilibrium constant (K) itself, rather than only causing a temporary change in the reaction quotient (Q)?

A) An increase in the concentration of a reactant.

B) A decrease in the concentration of a product.

C) A change in temperature.

D) The removal of a reactant.

Correct Answer: C

The provided content explicitly states, 'A change in temperature causes a change in K.' It distinguishes this from other stresses, like changes in concentration, which 'cause a change in Q only.'

When a disturbance, such as a change in the concentration of a species, is applied to a system at equilibrium, what is the fundamental goal of the resulting shift in the reaction?

A) To change the value of the equilibrium constant, K.

B) To bring the reaction quotient, Q, back into equality with K.

C) To stop the reverse reaction from occurring.

D) To increase the temperature of the system.

Correct Answer: B

The content states that a disturbance causes Q to differ from K. In response, 'The system responds by bringing Q back into agreement with K, establishing a new equilibrium state.' Therefore, the goal of the shift is to make Q equal to K again.

Consider a system at equilibrium. If the concentration of a reactant is suddenly increased, what is the immediate effect on the reaction quotient (Q) relative to the equilibrium constant (K)?

A) Q becomes greater than K.

B) Q becomes less than K.

C) Q remains equal to K, but the reaction shifts.

D) K changes to a new, lower value.

Correct Answer: B

The reaction quotient, Q, is a ratio of products to reactants. Increasing the concentration of a reactant increases the denominator of this ratio, causing the value of Q to decrease. This makes Q momentarily less than K, taking the system out of equilibrium.

A reversible reaction is determined to be at equilibrium. Which statement correctly describes the relationship between its reaction quotient (Q) and its equilibrium constant (K)?

A) Q > K

B) Q < K

C) Q = K

D) Q = 1/K

Correct Answer: C

The definition of a system at equilibrium is the state where the reaction quotient (Q) is equal to the equilibrium constant (K). The provided content explains that when Q differs from K, the system is not at equilibrium and will shift until they are equal.

A system is at equilibrium. A product is removed from the system. Which of the following correctly describes the immediate change and the system's subsequent response?

A) Q becomes greater than K; the system shifts left to re-establish equilibrium.

B) Q becomes less than K; the system shifts right to re-establish equilibrium.

C) K decreases to match Q; the system shifts left to re-establish equilibrium.

D) Q remains equal to K; no shift occurs.

Correct Answer: B

Removing a product decreases the numerator of the Q expression, causing Q to become less than K. According to the content, when Q < K, the system will proceed in the forward direction (shift right) to produce more products, thereby increasing Q until it equals K again at a new equilibrium.

After a system at equilibrium is disturbed by a change in concentration, it shifts and establishes a new equilibrium state. In this new state, how do the values of Q and K compare?

A) Q is now permanently greater than K.

B) Q is now permanently less than K.

C) Q and K are once again equal.

D) K has changed to a new value, but Q has not.

Correct Answer: C

The provided text states that after a disturbance, 'species concentrations redistribute to bring Q and K back into equality.' The entire purpose of the shift is to re-establish the condition of equilibrium, which is defined by Q = K.

The principle that a system at equilibrium will respond to a disturbance by re-establishing a new equilibrium state is based on the tendency of the system to:

A) adjust concentrations so that the reaction quotient (Q) once again equals the equilibrium constant (K).

B) change the value of its equilibrium constant (K) to match the new reaction quotient (Q).

C) shift exclusively in the forward direction regardless of the disturbance.

D) stop all molecular motion until the disturbance is removed.

Correct Answer: A

The core concept described is that a disturbance causes Q to differ from K, and 'The system responds by bringing Q back into agreement with K, establishing a new equilibrium state.' This adjustment of concentrations to make Q equal to K is the essence of the principle.