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.