How does pressure affect enthalpy?

1 Answer

Enthalpy is the heat content of a system as a function of entropy and pressure.

As the pressure increases (DeltaP > 0), so does enthalpy, and vice versa. Thus, more compressed molecules such as solids have greater intermolecular forces than less compressed molecules such as liquids or gases; their interactions are harder to separate.

DeltaH = DeltaU + Delta(PV)

= q + w + PDeltaV + VDeltaP + DeltaPDeltaV

= TDeltaS - PDeltaV + PDeltaV + VDeltaP + DeltaPDeltaV

= TDeltaS + VDeltaP + DeltaPDeltaV

Enthalpy can still exist even at constant pressure; that describes the enthalpy of vaporization or fusion.

Notice how the equation changes at constant pressure (DeltaP = 0):

DeltaH = TDeltaS + cancel(VDeltaP + DeltaPDeltaV)^(0)

Thus:

(DeltaH_(vap))/(T_(vap)) = q_(rev, P)/T_(vap) = DeltaS_(vap)

(DeltaH_(fus))/(T_(fus)) = q_(rev,P)/T_(fus) = DeltaS_(fus)

where q_P is heat flow at a constant pressure.