PChem312 f22 w12

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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.
Screen Shot 2020-11-13 at 7.55.16 AM.png
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;
Screen Shot 2020-11-13 at 7.58.25 AM.png
At constant V;
Screen Shot 2020-11-13 at 8.05.55 AM.png
where,
Screen Shot 2020-11-13 at 8.06.56 AM.png
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.)