**Classify the following phase changes by the signs of the system’s ΔH and ΔS.**

**Answer:**

If dH = positive; then dH = Hfinal - Hinitial > 0, i.e. Hfinal > Hinitial, so there must be a change of phase which requires energy; examples: solid to liquid, liquid to gas, solid to gas

If dH negative, then dH = Hfinal - Hinitial < 0, i.e. Hfinal < Hinitial, so there must be a change of phase which releases energy; examples: liquid to solid , gas to liquid, gas to solid

For entropy, recall entropy is related to chaos, so the more excited the state, the more enrtopy

solid < liquid < gas, therefore

**Case 1)**

**positive S + positive H**

solid to liquid

solid to gas

liquid to gas

**Case 2)**

**negative S + negative S**

gas to liquid

liquid to solid

gas to solid

positive S + negative H

**Case 3)**

**positive S + negative H**

Impossible, since you can’t have chaos creation without positive H

**Case 4)**

**negative S + positive H**

Impossible, since enthalpy addition will provide chaos