Coupled Reactions
AP Chemistry· 40 min read
1. Core Definition and Validity Rules for Coupled Reactions★★☆☆☆⏱ 8 min
Coupled Reactions
Two or more sequential reactions that share a common intermediate, where a highly exergonic reaction provides the free energy required to drive an endergonic reaction forward.
Reaction 1 (non-spontaneous): A → B, (\Delta G = +22 , \text{kJ/mol}); Reaction 2 (spontaneous): C → D, (\Delta G = -35 , \text{kJ/mol}), no shared intermediate. Can these two reactions be coupled to produce a spontaneous overall process?
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Step 1: Verify if the reactions share a common intermediate. In this case, no shared intermediate exists.
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Step 2: Even though the sum of (\Delta G) values is -13 kJ/mol, there is no mechanism for free energy transfer between the two unrelated processes.
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Step 3: Final conclusion: The paired process cannot be classified as a valid coupled reaction, and will not proceed spontaneously.
2. Calculating Total Gibbs Free Energy for Coupled Reactions★★★☆☆⏱ 10 min
The non-spontaneous formation of glucose-6-phosphate from glucose has (\Delta G = +13.8 , \text{kJ/mol}). ATP hydrolysis to ADP has (\Delta G = -30.5 , \text{kJ/mol}), and the two reactions share a phosphate intermediate. Calculate the total (\Delta G) for the coupled process.
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Step 1: Confirm the shared phosphate intermediate exists, so coupling is valid.
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Step 2: No stoichiometric scaling is required, so sum the two (\Delta G) values directly.
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Step 4: Final result: (\Delta G_{\text{total}} = -16.7 , \text{kJ/mol}), a negative value confirming the coupled process is spontaneous.
Test your understanding of the summation rule
Reaction X has (\Delta G = +42 , \text{kJ/mol}), Reaction Y has (\Delta G = -55 , \text{kJ/mol}), they share a common intermediate. What is the overall (\Delta G)?
+97 kJ/mol
-13 kJ/mol
-42 kJ/mol
+13 kJ/mol
Reveal answer
-13 kJ/mol —Sum the two values directly: +42 + (-55) = -13 kJ/mol, confirming a spontaneous coupled process.
3. Biological Coupled Reactions: ATP as Universal Energy Currency★★★☆☆⏱ 9 min
4. Industrial Electrochemical Coupled Reactions★★★★☆⏱ 7 min
Reduction of iron(III) oxide to iron metal has (\Delta G = +148 , \text{kJ/mol}) per mole of (\text{Fe}_2\text{O}_3). Combustion of carbon to CO has (\Delta G = -272 , \text{kJ/mol}) per 2 moles of C. The two reactions share a gaseous oxygen intermediate. Calculate the total (\Delta G) for the coupled smelting process.
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Step 1: Confirm shared oxygen intermediate exists, so coupling is valid.
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Step 3: Final result: (\Delta G_{\text{total}} = -124 , \text{kJ/mol}), confirming the smelting process proceeds spontaneously at operating temperatures.
5. Common Pitfalls
Wrong move:
Summing (\Delta G) values for two unrelated reactions with no shared intermediate
Why:
No shared intermediate means there is no mechanism to transfer free energy between the two processes
Correct move:
Always confirm a common intermediate exists before adding (\Delta G) values to claim a process is coupled.
Wrong move:
Using (\Delta G) values measured at different temperatures for summation
Why:
(\Delta G) is temperature dependent, so mismatched conditions produce invalid total values
Correct move:
Only add (\Delta G) values specified for identical pressure, temperature, and concentration conditions.
Wrong move:
Adding standard cell potential (E^\circ) values directly the same way you add (\Delta G)
Why:
(E^\circ) is an intensive property that does not scale with reaction stoichiometry, so it cannot be summed directly
Correct move:
Convert (E^\circ) values to (\Delta G) using (\Delta G = -nFE^\circ) before combining for coupled processes.
Wrong move:
Claiming a reaction with total (\Delta G = 0) is a valid spontaneous coupled process
Why:
Coupled processes require net negative (\Delta G) to proceed at measurable rates
Correct move:
Ensure the sum of (\Delta G) values is strictly negative to confirm the coupled reaction is spontaneous.
Wrong move:
Forgetting to multiply (\Delta G) values by molar coefficients when balancing the overall coupled reaction
Why:
(\Delta G) is an extensive property that scales with reaction stoichiometry
Correct move:
Adjust individual (\Delta G) values by their stoichiometric multipliers before summing the total.
6. Quick Reference Cheatsheet
Rule | Formula / Requirement |
|---|---|
Coupling Validity Check | Must share at least one common reaction intermediate |
Total ΔG Calculation | (\Delta G_{\text{total}} = n_1 \Delta G_1 + n_2 \Delta G_2) |
Spontaneity Condition | (\Delta G_{\text{total}} < 0) |
Biological Standard Condition | (\Delta G^{\circ '}) at pH = 7, 298 K |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 · 1
FRQ on ATP coupled amino acid synthesis
- 2021 · 2
Calculation of total coupled ΔG value
- 2019 · 1
Identifying valid coupled reaction pairs
What's Next
Mastering coupled reactions is a critical bridge between Gibbs free energy concepts and real-world applications you will encounter in the electrochemistry portion of Unit 9, including electrolytic cells that use external electrical energy to drive non-spontaneous redox processes. This topic is also frequently paired with reaction kinetics questions about activation energy in multi-step reaction pathways on AP exam free response sections, so you will see it referenced repeatedly as you review for your test. Make sure you practice identifying shared intermediates across different reaction types to avoid the most common distractors on exam day.