Reactions are spontaneous when delta G is below zero, depending on both enthalpy and entropy terms.
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The change in Gibbs energy is given by delta G equals delta H minus T times delta S, and the sign of this quantity at a given temperature determines spontaneity. An exothermic reaction with a positive entropy change has a negative delta H and a positive entropy term, so delta G is always negative and the reaction is spontaneous at all temperatures. An endothermic reaction with a negative entropy change has a positive delta H and a negative entropy term, so delta G is always positive and the reaction is never spontaneous. When the two terms have the same sign, the outcome depends on temperature and switches at the point where delta G equals zero. This framework explains many reactions that proceed even though they absorb heat.
Key ideas: Use the four sign cases: negative delta H with positive delta S is always spontaneous, positive delta H with negative delta S never, and the two mixed cases depend on temperature. The crossover temperature equals delta H divided by delta S.
Common mistakes and connections: Remember to use kelvin and consistent units. A common error is to say that endothermic reactions are never spontaneous. The framework shows why many dissolution and decomposition reactions proceed with absorption of heat, driven by entropy.
Negative delta H with positive delta S gives negative delta G at all temperatures: always spontaneous. Adjust the sliders to explore other sign combinations. The interactive lesson shows this as a live animation or chart.
Ammonium chloride dissolves spontaneously in water even though the process is endothermic, because the large increase in entropy makes delta G negative.
Reactions are spontaneous when delta G is below zero, depending on both enthalpy and entropy terms.
Gibbs Energy and Spontaneity is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Entropy Changes in Reactions, Entropy of Phase Change, Gibbs Free Energy.
The next topics in Thermodynamics & Energetics are Temperature Dependence of Spontaneity and Standard Free Energy of Formation.
Yes. The interactive lesson includes a self-check question on Gibbs Energy and Spontaneity, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Gibbs Energy and Spontaneity is topic 38 of 91 in Thermodynamics & Energetics.