9.7 Coupled Reactions

Explain the relationship between external sources of energy or coupled reactions and their ability to drive thermodynamically unfavorable processes.

• 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.
• 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.