When a chemical reaction occurs spontaneously and isothermally the loss of useful work is equal to the decrease in free energy of the system. If the same reaction, supposed for example to be exothermic, occurs adiabatically the temperature rises and the system can be used as a source of heat to operate a heat engine: as heat is withdrawn the temperature of the system falls and eventually returns to its initial value. Part of the free energy of the reaction has been recovered as useful work, part lost in the spontaneous irreversible step. Expressions are derived for the loss of useful work and the entropy production in the adiabatic irreversible step, and it is shown that the work lost is equal to T0× (entropy produced) where T0 is the temperature ofthesurroundings. Some consequences of these ideas in relation to the definition of the efficiency of internal combustion engines are considered briefly.
