Gibbs free energy tells you whether a process wants to happen on its own under constant temperature and pressure. A negative change in Gibbs free energy means the process is thermodynamically spontaneous in the direction written.

Quick Scoop

  • ΔG\Delta GΔG is the change in Gibbs free energy for a process.
  • ΔG<0\Delta G<0ΔG<0: process is spontaneous in the forward direction.
  • ΔG>0\Delta G>0ΔG>0: process is non‑spontaneous in the forward direction (but spontaneous in the reverse).
  • ΔG=0\Delta G=0ΔG=0: system is at equilibrium ; no net change over time.

The Core Relationship

Gibbs free energy change is given by
ΔG=ΔH−TΔS\Delta G=\Delta H-T\Delta SΔG=ΔH−TΔS
where ΔH\Delta HΔH is enthalpy change, TTT is absolute temperature, and ΔS\Delta SΔS is entropy change.

  • If ΔH\Delta HΔH is very negative (releases heat) and ΔS\Delta SΔS is positive (disorder increases), ΔG\Delta GΔG is strongly negative and the reaction is spontaneous at all temperatures.
  • If both ΔH\Delta HΔH and ΔS\Delta SΔS are positive, spontaneity depends on temperature: higher TTT can make TΔST\Delta STΔS large enough that ΔG\Delta GΔG turns negative.
  • If ΔH>0\Delta H>0ΔH>0 and ΔS<0\Delta S<0ΔS<0, ΔG\Delta GΔG is positive at all temperatures, so the forward process is never spontaneous.

An everyday example is ice melting: at low temperature, freezing (liquid → solid) is spontaneous, but above 0 °C, melting (solid → liquid) becomes spontaneous because the entropy term wins out and makes ΔG\Delta GΔG negative.

Spontaneous vs Fast

  • Spontaneous means “thermodynamically favorable,” not “instant.” A reaction can have ΔG<0\Delta G<0ΔG<0 but still be very slow if it has a large activation energy.
  • Catalysts do not change ΔG\Delta GΔG; they only change how quickly equilibrium is reached.

Direction and Equilibrium

  • If ΔG<0\Delta G<0ΔG<0, the forward reaction is spontaneous and the system tends to form more products until it gets closer to equilibrium.
  • If ΔG>0\Delta G>0ΔG>0, the reverse reaction is spontaneous, so the system tends to form more reactants.
  • At equilibrium, ΔG=0\Delta G=0ΔG=0, and the forward and reverse reactions occur at the same rate with no net change in composition.

In short: Gibbs free energy is the thermodynamic “scoreboard” for spontaneity—negative ΔG\Delta GΔG means the process can happen on its own, positive ΔG\Delta GΔG means it cannot without external help.

Information gathered from public forums or data available on the internet and portrayed here.