Gibbs Free Energy (G) is a single property that combines that entropy calculations for both a system and its surroundings and is measured in Joules; It is incorrect to assume that exothermic reactions are spontaneous.

The Gibbs free energy of a reaction dictates where a reaction is spontaneous (-ΔG) or nonspontaneous (+ΔG). This is due to the components of the free energy equation (ΔG = ΔH -TΔS) that add up to be negative or positive.

While an exothermic reaction (-ΔH) may suggest spontaneous reaction, a high temperature (T) or large entropy (ΔS) may negate the negative enthalpy and result in a positive ΔG value. Likewise, an endothermic reaction may be spontaneous from low temperature and small entropy change.

The designation of ΔG⁰ indicates the thermodynamic favorability of the reaction at equilibrium (with a positive value being unfavorable). This is a property of the reaction.

Practice Questions

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Key Points

• Gibbs Free energy can be calculated from the equation ΔG = ΔH -TΔS and is in Joules.

• Exothermic reactions can be nonspontaneous

• Endothermic reactions can be spontaneous

Key Terms

spontaneous reaction: reaction that occurs without external activity

non-spontaneous reaction: reaction that requires the input of additional action to proceed

Gibbs free energy: the difference between the internal energy of a system and the product of its entropy and absolute temperature

exothermic: net release of heat.

endothermic: net absorbance of heat