Gibbs free energy Gibbs free energy change The increase or decrease of the Gibbs free energy of a system. During a reaction, this is equal to the change in enthalpy of the system minus the change in the product of the temperature times the entropy of the system: ΔG = ΔH - T ΔS. A negative value indicates that the reaction will be favoured and will release energy. The magnitude of the value indicates how far the reaction is from equilibrium, where there will be no free energy change. The standard unit of measure is kJ/mol. Symbol: ΔG.