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Coupled Reactions

An unfavorable reaction can be driven by a favorable one when they are coupled through a shared intermediate. The combined free energy is the sum of the steps' ΔG values, so the overall process is favored when the favorable step is negative enough to outweigh the unfavorable one.

UNIT 9 TOPIC 9.7 • COUPLED REACTIONS COUPLED REACTIONS Coupling a favorable reaction to an unfavorable one through a common intermediate makes the overall process spontaneous. STEP 1 · FAVORABLE phosphoryl transfer to A A + ATP → A–P + ADP ΔG₁ < 0 ↓ releases free energy SHARED INTERMEDIATE A–P high-energy intermediate STEP 2 · UNFAVORABLE A–P drives formation of AB A–P + B → AB + Pᵢ ΔG₂ > 0 ↑ requires free energy OVERALL (NET) COUPLED REACTION A + B + ATP → AB + ADP + Pᵢ ΔGₙₑₜ = ΔG₁ + ΔG₂ < 0 → spontaneous ✓ OTHER ENERGY SOURCES ⚡ Electrical — active ion transport ☀ Light — photosynthesis also drive ΔG > 0 processes. A ATP A–P B AB + + ADP + Pᵢ shared intermediate CED ANCHOR Coupling links an unfavorable process to a favorable one through a common intermediate, so the summed ΔGₙₑₜ = ΔG₁ + ΔG₂ < 0 — the overall process is spontaneous. Enthalpy/entropy sum to drive the whole path. AP Chemistry · Unit 9 · Applications of Thermodynamics
Coupling lets a favorable reaction drive an unfavorable one — but only through a shared intermediate. The combined free energy is the sum of the steps' ΔG values, and the driving step must be favorable enough to make the overall ΔG negative.
Reaction Coupler · Open the sandbox →

The trap misunderstands coupling — assuming any nearby favorable reaction can drive an unfavorable one without a shared intermediate, expecting the target's own ΔG to change, or picking a driver that is not favorable enough. Coupling needs a shared species and a large enough combined ΔG.

The work

3 ways in · any order
Lesson
Coupled Reactions ›

A favorable reaction drives an unfavorable one only through a shared intermediate, with ΔG values summing. The lesson works the coupling and the sum, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 9.7 misconception: reaction coupling misunderstood — the shared intermediate, the summed ΔG, and the magnitude needed.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first