Enthalpy, H, is defined by the relationship H=U+pV. The enthalpy change, ΔH, for finite changes at constant pressure is given by the expression ΔH = ΔU+pΔV, so making the enthalpy change for a process equals to the heat exchange in a system at constant pressure. For a chemical system which releases or absorbs a gas at constant pressure, the enthalpy change is related to the internal energy change by ΔH=ΔU+Δn.RT, where Δn is the molar change in gaseous component some other process occurring.

The enthalpy, H, is defined by the expression; H=U+pV, Hence for a finite change at constant pressure:

ΔH=ΔU+ pexΔV