The enthalpy (H) is defined as H = E + PV. Where E is the internal energy, P is the pressure, and V is the volume.

At constant pressure, H = E + PV. The same result can be obtained from the first law of thermodynamics:

E = q-w, so

q = E + w, but w = PV when V is changed against a constant P, so

q = E + PV

Thus, when the heat of a reaction is measured at constant pressure, it is really H that is measured. Furthermore, most biological processes occur at constant pressure, so H gives a more accurate measure of the energy available from a biological process than E does. Finally, because E and PV are functions of state (not path), H is also a function of state. Thus, H depends only on the initial and final states of the process for which it is calculated.