Question

How does pressure affect enthalpy?

Answer 1

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.