Where \ Delta H ^ { 0 } _ { T } is the standard enthalpy of reaction at temperature T and pressure p _ { 0 } = 1 \ mathrm { bar }, for all fuels \ Delta G ^ { 0 } _ { T } so the exergy efficiency must always be greater than the energy efficiency.
12.
However, under conditions of constant pressure, as in reactions in vessels open to the atmosphere, the measured heat change is not always equal to the internal energy change, because pressure-volume work also releases or absorbs energy . ( The heat change at constant pressure is called the enthalpy change; in this case the enthalpy of reaction, if initial and final temperatures are equal ).
