Coupled Reactions

Learning Objective 9.7.A Explain the relationship between external sources of energy or coupled reactions and their ability to drive thermodynamically unfavorable processes.

Quick Notes

Thermodynamically unfavorable reactions (+ΔG°) don’t occur spontaneously.
They can be driven (made to happen) by:
- Using external energy sources (e.g., electricity, light).
- Coupling them with a thermodynamically favorable reaction (ΔG° < 0).
The overall process must have -ΔG° to be thermodynamically favorable.

Full Notes

Not all important chemical reactions are thermodynamically favorable and many have a positive standard Gibbs free energy change (ΔG° > 0). Even when reactions are not favorable on their own, they can often still occur if external energy is supplied or if they are coupled with another, highly favorable process.

Driving Unfavorable Reactions with External Energy

One way to make a non-spontaneous process occur is to supply energy from outside the system. This added energy helps "push" the reaction forward and overcome the positive standard Gibbs free energy change.

Electrolysis
Electrical energy can be used to force redox reactions that wouldn't otherwise occur.

Example: Electrolysis of sodium chloride solution:

AP Chemistry diagram of electrolysis of NaCl(aq), showing oxidation at the anode (2Cl− → Cl₂ + 2e−) and reduction at the cathode (2H⁺ + 2e− → H₂).

2Cl⁻(aq) + 2H⁺(aq) → H₂(g) + Cl₂(g) ΔG° > 0
This reaction doesn't happen spontaneously — it requires electricity to overcome the attraction between ions and water molecules and to oxidize the chloride ions and reduce the hydrogen ions in the solution.

Photosynthesis
This essential biological process also has an overall positive standard Gibbs Free Energy (+ΔG°). Plants absorb sunlight and use that energy to convert carbon dioxide and water into glucose and oxygen.

AP Chemistry diagram of photosynthesis: CO₂ and H₂O converted into glucose and O₂ using sunlight energy.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ ΔG° > 0
Light energy, captured by chlorophyll, provides the energy input needed to drive the process forward.

Coupling with Thermodynamically Favorable Reactions

Another strategy is to pair an unfavorable reaction with one that is highly favorable. The reactions must share a common intermediate, and the overall combination results in a net favorable process (-ΔG°).

This is a common strategy in biological systems, where reactions that would not proceed on their own are powered by coupling them to ATP hydrolysis – a highly exergonic reaction:

AP Chemistry diagram of ATP hydrolysis: ATP → ADP + Pi, releasing energy (ΔG° ≈ −30.5 kJ/mol).

ATP → ADP + Pi ΔG° ≈ −30.5 kJ/mol

This reaction can be coupled to an energy-requiring step, like the phosphorylation of glucose:

Unfavorable: Glucose + Pi → Glucose-6-phosphate (+ΔG°)

AP Chemistry diagram of unfavorable phosphorylation: glucose + phosphate → glucose-6-phosphate.

Favorable: ATP → ADP + Pi (−ΔG°)

Overall (coupled): Glucose + ATP → Glucose-6-phosphate + ADP (−ΔG°)

AP Chemistry diagram showing coupled reaction: ATP hydrolysis drives phosphorylation of glucose to glucose-6-phosphate.

The favorable breakdown of ATP "pays for" the energy cost of the unfavorable reaction, allowing both to proceed.

Key Principle: Overall Gibbs Free Energy Change

The total Gibbs free energy change for a multi-step or coupled process is simply the sum of the ΔG° values of the individual steps:

ΔG°_total = ΔG°₁ + ΔG°₂ + …

If this total is negative, the overall process is thermodynamically favorable and can occur spontaneously – even if some individual steps are not.

Summary

Reactions with ΔG° > 0 do not proceed on their own, but they can still happen:
- If external energy is supplied (like electricity or sunlight), or
- If coupled with a more favorable process that offsets the energy cost.
This is how cells build complex molecules, how batteries recharge, and how plants power life through photosynthesis.
Understanding how energy flows and is managed in these systems is central to both chemistry and biology — and underscores the importance of Gibbs free energy in determining what’s possible.