Gibbs free energy

Science

Definition

Gibbs free energy (G) is a thermodynamic quantity that predicts whether a chemical reaction or process will occur spontaneously at constant temperature and pressure. A negative ΔG means the reaction is spontaneous (favorable), while a positive ΔG means it is non-spontaneous.

Examples

  • The combustion of glucose has a large negative ΔG, making it a spontaneous energy-releasing process
  • Ice melting above 0°C is spontaneous (ΔG < 0) because the entropy increase outweighs the enthalpy input
  • Photosynthesis has a positive ΔG and requires continuous energy input from sunlight to proceed
Key Fact

ΔG = ΔH − TΔS; if ΔG < 0, the reaction is spontaneous; if ΔG = 0, the system is at equilibrium.

Related Terms

Enthalpy Entropy Free energy and equilibrium Equilibrium constant

Study This Concept

Practice Gibbs free energy with free review games in these units:

AP Chemistry Unit 6: Thermodynamics