The maximum amount of work that can be obtained from a chemical reaction at constant temperature and pressure, assuming perfect reversibility and no external work, is given by the change in Gibbs free energy G of the reaction. Gibbs free energy is a thermodynamic potential that measures the maximum reversible work that can be done by a system at constant temperature and pressure.The equation for Gibbs free energy change G is:G = H - TSwhere H is the change in enthalpy heat content of the reaction, T is the absolute temperature in Kelvin, and S is the change in entropy disorder of the reaction.A negative G value indicates that the reaction is spontaneous and can perform work, while a positive G value indicates that the reaction is non-spontaneous and requires work to proceed. The maximum amount of work that can be obtained from the reaction is equal to the magnitude of the negative G value.