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