PChem312 f22 w12

Entropy of Phase Changes...

...continuing numbering from notes...

5) Solid --> Liquid phase change (fusion/melting/freezing)

- under constant pressure (ΔH)

- under constant volume (ΔU)

For processes involving solids and liquids, ΔH ~ ΔU, hence ΔS is the same for both constant P and constant V.

6) Liquid --> gas phase change (vaporization/boiling/condensation)

- under constant pressure (ΔH)

- under constant volume (ΔU)

For processes involving gases, ΔH ≠ ΔU, hence ΔS is not the same for constant P and constant V processes.

At constant P;

At constant V;

where,

and Δn is the change in moles of gas (Products - reactants; ie. increase in the number of moles of gas)

(for a vaporization, Δn will always be 1.)