Gibbs Free Energy and Chemical Spontaneity
Gibbs free energy (G) is the thermodynamic potential that predicts whether a chemical reaction will occur spontaneously at constant temperature and pressure. A negative ΔG means the reaction is spontaneous (releases usable energy). A positive ΔG means the reaction is non-spontaneous and requires energy input. ΔG depends on both enthalpy (heat) and entropy (disorder).
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Formula
Gibbs Free Energy Calculator
Calculate ΔG, ΔH, or ΔS using ΔG = ΔH − TΔS.
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Worked Example
Given:
Related Calculators
FAQs
What does spontaneous mean in chemistry?
A spontaneous reaction proceeds without continuous external energy input under given conditions. Spontaneous does not mean fast — diamond converting to graphite is thermodynamically spontaneous but occurs over geological timescales. Kinetic barriers (activation energy) can prevent fast reactions even when ΔG is negative.
How do enthalpy and entropy affect spontaneity?
Reactions tend toward lower energy (negative ΔH, exothermic) and higher disorder (positive ΔS). When both favour spontaneity (ΔH < 0, ΔS > 0), the reaction is always spontaneous. When both oppose it (ΔH > 0, ΔS < 0), it is never spontaneous. When they conflict, temperature determines the outcome.
What happens at the temperature where ΔG = 0?
When ΔG = 0, the system is at equilibrium — forward and reverse reactions proceed at equal rates. Setting ΔG = 0 in the equation: T = ΔH/ΔS gives the temperature at which a reaction transitions between spontaneous and non-spontaneous. This is used to calculate melting and boiling points.