AP Chemistry Flashcards: Coupled Reactions
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
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What are coupled reactions?
Coupled reactions involve pairing a thermodynamically unfavorable reaction with a favorable one that share a common intermediate, allowing the unfavorable process to proceed.
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What are coupled reactions?
Coupled reactions involve pairing a thermodynamically unfavorable reaction with a favorable one that share a common intermediate, allowing the unfavorable process to proceed.
What is a common example of a 'favorable reaction' used for coupling in biological systems?
The hydrolysis of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) is a common and highly favorable reaction used to drive many cellular processes.
What defines a thermodynamically unfavorable process?
A thermodynamically unfavorable process is one that will not occur spontaneously because it has a positive change in Gibbs free energy (ΔG° > 0).
Explain the general principle of using energy to drive unfavorable reactions.
Energy from an external source or a coupled favorable reaction can be harnessed to overcome the positive ΔG° of an unfavorable process, making it possible for the desired product to be formed.
If Reaction 1 has ΔG° = +15 kJ/mol and Reaction 2 has ΔG° = -40 kJ/mol, can they be coupled to make Reaction 1 occur?
Yes, because the overall summed reaction would have a ΔG° of -25 kJ/mol, making the coupled process thermodynamically favorable.
Provide two examples of external energy sources driving unfavorable processes.
Examples include using electrical energy to perform electrolysis or using light energy to power photosynthesis.
What role does a 'common intermediate' play in coupled reactions?
A common intermediate is a substance shared by both the favorable and unfavorable reactions, physically linking them so that energy can be transferred.
How does the hydrolysis of ATP to ADP function in coupled reactions?
The conversion of ATP to ADP is a highly favorable reaction that is coupled to many unfavorable biological processes, providing the energy needed to drive them forward.
What is the overall thermodynamic requirement for a successful coupled reaction?
For a coupled reaction to proceed, the overall, summed reaction must have a negative Gibbs free energy (ΔG° < 0), making the entire process thermodynamically favorable.
Besides coupling, how else can a thermodynamically unfavorable process be driven?
A thermodynamically unfavorable process can be driven by an external source of energy, such as electrical energy or light.