AP® Chemistry · Unit 9 · Topic 9.7
Coupled Reactions
Coupled reactions link an unfavorable process to a favorable one through a shared intermediate, so their free energy changes add together, letting the combined reaction run on its own even though one half could not.
Free energy, delta G, tells you whether a reaction happens on its own. Negative means favorable, positive means you'd have to push it. Coupling solves the problem of a positive delta G reaction by hooking it to a very negative delta G reaction so the sum comes out negative. But this only works if the two reactions share a common intermediate: a substance produced by one reaction and consumed by the other. That shared species links them mechanistically into a single overall process, and because delta G is a state function, the free energies of the two steps simply add.
Take a reaction with delta G = +30 kJ/mol coupled to one with delta G = -50 kJ/mol. Add them: +30 + (-50) = -20 kJ/mol. The overall process is favorable, even though the first reaction alone never would have happened. Smelting iron ore works the same way. Reducing Fe_2O3 to Fe alone is unfavorable. But burning carbon to CO2, or forming CO, is strongly favorable. Written so that CO produced in one step is consumed in the other, the two reactions combine into one overall equation with a negative delta G, and iron forms.
This is the same logic that shows up later as cell potential in electrochemistry: splitting a redox reaction into two half-reactions is really coupling two processes through shared electrons instead of a shared molecule. Seeing coupling here, as free energies adding through a shared intermediate, makes voltage addition in galvanic cells feel like the same idea wearing different units.
Key terms
- Coupled reactions
- Two reactions linked through a shared intermediate so their free energy changes add into one overall process.
- Common intermediate
- A species produced by one reaction and consumed by the other, physically connecting the two steps.
- Delta G (free energy change)
- A measure of whether a reaction is thermodynamically favorable; negative values mean the reaction proceeds on its own.
- State function
- A property, like delta G, whose value depends only on initial and final states, allowing values from separate steps to be added.
Watch it explained
- How it worksHow do two reactions sharing a common intermediate add their free energies together?
- Worked exampleFind whether coupling a plus 30 kJ reaction to a minus 50 kJ reaction gives a favored overall process.
- How it worksWalk me through how smelting iron ore with carbon couples two reactions into one favorable one.
- Common mistakeWhy isn't simply mixing two reactions together enough to couple them?
Check yourself
End of topic.