Gibbs Free Energy Change

\[\Delta G=\Delta H-T\Delta S\]

Variables

dG[ΔG]Gibbs free energy change (J/mol)

dH[ΔH]enthalpy change (J/mol)

Tabsolute temperature (K)

dS[ΔS]entropy change (J·mol⁻¹·K⁻¹)

Description

What is this formula?

The Gibbs free energy equation determines the thermodynamic driving force of a process by combining enthalpy and entropy effects.

It is one of the most important equations in physical chemistry because it predicts whether a process is thermodynamically spontaneous.

When to use it

Use this formula when evaluating chemical reactions, phase changes, electrochemical processes, biochemical reactions, and thermodynamic feasibility.

Example

Given:

ΔH = -50000 J/mol

ΔS = -100 J·mol^-1·K^-1

T = 298 K

Formula:

ΔG = ΔH - TΔS

Substitution:

ΔG = -50000 - (298 × -100)

Result:

ΔG = -20200 J/mol

The process is thermodynamically spontaneous because ΔG is negative.

Applications

Chemical thermodynamics

Reaction feasibility analysis

Electrochemistry

Biochemistry

Materials science

Phase equilibrium

Note

The sign of ΔG determines thermodynamic spontaneity:

ΔG < 0 → spontaneous process

ΔG = 0 → equilibrium

ΔG > 0 → non-spontaneous process

A negative Gibbs free energy indicates that a process is thermodynamically favorable, but it does not provide information about reaction speed. A reaction may be thermodynamically favorable and still proceed very slowly if kinetic barriers are large.