AP Chemistry Practice Quiz: Coupled Reactions
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
All Questions (9)
A) To increase the rate of a thermodynamically favorable reaction.
B) To drive a thermodynamically unfavorable process.
C) To change the equilibrium constant of a reaction.
D) To decrease the activation energy of any chemical reaction.
Correct Answer: B
The text explicitly states that an external source of energy or a coupled reaction can be used to 'make a thermodynamically unfavorable process occur' or 'drive thermodynamically unfavorable processes.'
A) The conversion of ATP to ADP.
B) A reaction with a shared common intermediate.
C) The use of light for photosynthesis.
D) A summed reaction with a negative ΔG°.
Correct Answer: C
The text provides two specific examples of external energy sources: 'using electrical energy for electrolysis or light for photosynthesis.' The conversion of ATP to ADP is an example of a favorable reaction used in coupling, not an external energy source.
A) The overall ΔH° must be negative.
B) The overall ΔS° must be positive.
C) The overall ΔG° must be less than zero.
D) The overall ΔG° must be greater than zero.
Correct Answer: C
The provided content states that for a coupled reaction to form the desired product, 'the overall, summed reaction has ΔG° < 0.' This indicates that the overall process must be thermodynamically favorable (spontaneous).
A) +45 kJ/mol, non-spontaneous
B) +15 kJ/mol, non-spontaneous
C) -15 kJ/mol, spontaneous
D) -45 kJ/mol, spontaneous
Correct Answer: C
The overall ΔG° is the sum of the Gibbs free energy changes for the individual reactions. Overall ΔG° = (+15 kJ/mol) + (-30 kJ/mol) = -15 kJ/mol. Since the overall ΔG° is less than zero, the coupled reaction is spontaneous.
A) It acts as a catalyst to speed up both reactions.
B) It is the final desired product of the overall reaction.
C) It provides an external source of energy like light or electricity.
D) It links the unfavorable reaction to the favorable one, allowing for energy transfer.
Correct Answer: D
The text states that in a coupled reaction, 'The reactions share a common intermediate.' This intermediate is the chemical species that physically links the two reactions, allowing the energy released from the favorable reaction (e.g., ATP hydrolysis) to drive the unfavorable one.
A) Using sunlight to split water into hydrogen and oxygen.
B) Using electricity to plate a metal spoon with silver.
C) Using the energy from ATP hydrolysis to synthesize a protein.
D) Burning natural gas to heat a home.
Correct Answer: C
The text distinguishes between using an external energy source (like sunlight in A or electricity in B) and coupling reactions. The use of ATP hydrolysis (a favorable reaction) to power another process (protein synthesis, an unfavorable reaction) is the specific example of a coupled reaction provided.
A) The First Law of Thermodynamics (Conservation of Energy).
B) The Second Law of Thermodynamics, which dictates the direction of spontaneous processes.
C) Hess's Law, which concerns the enthalpy of reactions.
D) The Law of Mass Action, which relates to chemical equilibrium.
Correct Answer: B
A negative ΔG° signifies a spontaneous (thermodynamically favorable) process. The Second Law of Thermodynamics governs spontaneity, stating that the total entropy of an isolated system can only increase over time. ΔG is a measure of this for a system at constant temperature and pressure, and a process can only occur spontaneously if the overall ΔG is negative.
A) The hydrolysis of ATP to ADP is an example of a favorable reaction used in coupling.
B) For a coupled reaction to be successful, the ΔG° of the favorable reaction must be more negative than the ΔG° of the unfavorable reaction is positive.
C) External energy sources and coupled reactions are two distinct methods to achieve the same goal.
D) Any favorable reaction can be coupled with any unfavorable reaction to make it occur.
Correct Answer: D
The text specifies a key requirement for coupled reactions: 'The reactions share a common intermediate.' This implies that not just any two reactions can be coupled; they must be chemically linked via a shared species for the energy transfer to occur. Therefore, the statement that 'any' favorable reaction can be coupled with 'any' unfavorable one is incorrect.
A) ΔG° < 0
B) ΔG° > 0
C) ΔG° = 0
D) ΔG° is unrelated to thermodynamic favorability.
Correct Answer: B
A thermodynamically unfavorable process is one that is non-spontaneous under standard conditions. The sign of Gibbs free energy (ΔG°) indicates spontaneity: a negative value means spontaneous (favorable), a positive value means non-spontaneous (unfavorable), and zero means the system is at equilibrium.