Coupled Reactions

Coupled Reactions

A pair of reactions where a thermodynamically favorable reaction (negative ΔG°) is used to drive a thermodynamically unfavorable reaction (positive ΔG°).

Thermodynamically Unfavorable Process

A reaction with a positive ΔG° (>0); it will not occur spontaneously without an external energy source or coupling.

External Energy Sources

Methods to force unfavorable reactions to occur, such as using electricity (electrolysis/charging batteries) or light (photosynthesis).

Common Intermediate

A chemical species that is produced in one step of a coupled reaction sequence and consumed in the next step; it links the two reactions.

Condition for a Successful Coupled Reaction

The sum of the ΔG° values for the individual reactions must be negative (ΔG°_{total} < 0).

Calculation of Coupled ΔG°

ΔG°_{overall} = ΔG°_{reaction1} + ΔG°_{reaction2}.

Biological Coupling Example

The conversion of ATP to ADP (highly favorable/exergonic) releases energy that drives unfavorable biological processes (like forming Glucose-6-Phosphate).

Electrolytic Cell

A system that uses an external source of electrical energy to drive a non-spontaneous chemical reaction.