Question

How does enthalpy relate to internal energy?

Answer 1

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.