N2(g) + 3H2(g) = 2NH3(g) A: 4 gas molecules go to 2 gas molecules and fewer gas molecules means fewer possible configurations which means less disorder Therefore: our prediction is Entropy ( S) < 0 Standard entropy values are listed in appendix 4 Notice that indeed entropy increases for a substance in going from a solid to a liquid to a gas. Ex. S can also be expressed as Enthalpy H for a process occurring at constant pressure Therefore S = _ H T 16.4 Free Energy 16.5 Entropy Changes in Chemical Reactions Does Entropy increase or decease in the following?

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