Coupled Reactions
Add a connected coupled pathway or identify the external energy source that drives an unfavorable process.
AP Chemistry Topic 9.7 · CED printed p. 173 / PDF p. 180. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Thermochemical equation addition
- Free-energy signs
- Energy conservation
An unfavorable chemical step can occur if energy arrives through an appropriate physical connection. An electrical source can force an electrolytic cell to carry out a redox reaction in its unfavorable direction or charge a battery. Light can support energy storage in photosynthesis. The step does not suddenly become free: the external source supplies energy, and the complete process includes that source. Thermodynamic conclusions need the full system and the actual direction being driven.
Chemical coupling links an unfavorable target step to a sufficiently favorable step through a shared intermediate and mechanism. Add the balanced step equations, cancel intermediates, and add their ΔG° values. If the sum is negative, the overall standard-state process is favored. ATP-to-ADP chemistry is a familiar biological source of favorable steps, but “ATP is present” alone is not a coupling explanation; a pathway must link its conversion to the desired transformation. Two reactions in separate, disconnected flasks do not drive one another merely because the sum of their tabulated ΔG° values is negative. Ask what intermediate or energy-transfer mechanism actually joins them.
Let's work through it
- A target transformation X + I → Y has ΔG° = +25 kJ mol−1. A linked step Z → I has ΔG° = −40 kJ mol−1.
- Add the steps: X + I + Z → Y + I. Cancel I on both sides to get X + Z → Y.
- ΔG°overall = +25 + (−40) = −15 kJ mol−1. The linked pathway can form Y through a favored overall process.
- Explain the physical requirement: I is produced and used in the same coupled pathway. An unconnected negative-energy reaction elsewhere is not evidence that X became Y.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
What must cancel in a simple coupled-step equation?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
What must cancel in a simple coupled-step equation?
Worked answer and reason
A shared intermediate produced in one step and consumed in another.
It connects the steps without remaining in the overall reaction.
Retrieval 2
How can electrolysis drive an unfavored reaction?
Worked answer and reason
An applied electrical source supplies energy through the cell circuit.
The complete process includes the external source, not just the chemical step.
Catch the tempting mistake
- An unfavorable step cannot ever occur.
- It can be driven by a physically coupled favorable step or external energy input.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 9 problem 9 · ap application · symbolic
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For the teacher
- Require both a balanced net equation and a named physical or chemical connection; reject energy arithmetic without a coupling mechanism.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 9.7.A
- Explain the relationship between external sources of energy or coupled reactions and their ability to drive thermodynamically unfavorable processes.
- 9.7.A.1
- An external source of energy can be used to make a thermodynamically unfavorable process occur. Examples include: i. Electrical energy to drive an electrolytic cell or charge a battery. ii. Light to drive the overall conversion of carbon dioxide to glucose in photosynthesis.
- 9.7.A.2
- A desired product can be formed by coupling a thermodynamically unfavorable reaction that produces that product to a favorable reaction (e.g., the conversion of ATP to ADP in biological systems). In the coupled system, the individual reactions share one or more common intermediates. The sum of the individual reactions produces an overall reaction that achieves the desired outcome and has ΔG° < 0.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §16.4, Free Energy — Enthalpy–entropy competition and ΔG°/equilibrium connection.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Gibbs free energy
Gibbs free energy combines enthalpy and entropy contributions to a system at specified temperature and pressure. For a reaction at constant temperature and pressure, a negative actual reaction free-energy change favors its forward direction, while standard and actual values must be distinguished. For example, a favorable reaction can still be slow if its pathway has a large activation barrier.
Read more: Gibbs Free Energy and Thermodynamic Favorability →
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Electrochemical cell
An electrochemical cell separates oxidation and reduction into electrode processes. A galvanic cell delivers electrical energy from a favored reaction, while an electrolytic cell uses supplied electrical energy to drive an unfavored direction. For example, a zinc–copper galvanic model transfers electrons through the external wire while ions carry charge through the solutions.
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Redox reaction
A redox reaction combines oxidation of one species with reduction of another. Electron loss and gain must balance in the net equation along with atoms and total charge. For example, zinc reducing copper ions involves zinc oxidation and copper-ion reduction.
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Reaction intermediate
A reaction intermediate is produced in one mechanism step and consumed in another. It cancels from the overall equation and differs from a catalyst, which is present before it is consumed and then regenerated. For example, a short-lived species formed in step one and used in step two can be an intermediate.
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Standard state
A standard state is a defined reference for a substance's thermodynamic properties. It specifies activities or reference forms and does not by itself fix the temperature at 25 °C. For example, gases commonly use a one-bar reference pressure while the table states temperature separately.
Read more: Gibbs Free Energy and Thermodynamic Favorability →