Worked example

ΔH=-100 kJ, ΔS=-200 J/K, T=298 K → ΔG=-40.4 kJ.

Gibbs Energy

Predict spontaneity and maximum non-PV work.

Equation

$\Delta G = \Delta H - T\Delta S$

Inline: $\Delta G = \Delta H - T\Delta S$$

Variables

Symbol	Quantity	Unit	Notes
$ΔG$	Gibbs change	kJ·mol⁻¹	—
$ΔH$	enthalpy change	kJ·mol⁻¹	—
$ΔS$	entropy change	J·mol⁻¹·K⁻¹	—
$T$	temperature	K	—

Conditions and limitations

Constant T,P; ΔG<0 spontaneous.

Rearrangements

$T= (ΔH-ΔG)/ΔS$

Dimensional check: kJ - K·J/K = kJ ✓ (convert units)

Worked example

ΔH=-100 kJ, ΔS=-200 J/K, T=298 K → ΔG=-40.4 kJ.

Steps: write equation → substitute with units → check dimensional consistency → report with correct significant figures.

Common errors

• Unit mismatch J vs kJ

Related resources

• thermochemistry-thermodynamics topic
• Calculators — linked only when a real calculator implements this formula.
• Constants — sourced constants with version when values are required.

Sources

• NIST Physical Constants 2022 — CODATA.

Reviewed 2026-08-21 by Editorial Team; due 2027-08-21.