How does enthalpy relate to internal energy?

1 Answer
Nov 3, 2015

H = U + PV

Explanation:

H = U + PV

where H = enthalpy,
U = internal energy,
P = pressure,
and V = volume

Consider a process occurring at constant pressure and where the only work allowed is pressure-volume work (w = -PΔV):

The change on enthalpy is then given by:

DeltaH=DeltaU+Delta(PV)=>DeltaH=DeltaU+PDeltaV

and DeltaU=q_P+w=> DeltaU=q_P-PDeltaV

Replacing DeltaU by its expression in the expression of DeltaH
we get:

DeltaH=q_P at constant pressure.