Chemistry / Unit 11 Chem 2099 worked example, 60 to 90 seconds
Calculating ΔS°rxn from Standard Molar Entropies
The idea
Calculating a reaction's standard entropy change means summing products' standard molar entropies, each multiplied by its stoichiometric coefficient, and subtracting the same sum for reactants, using values read directly from a standard table rather than derived.
In practice
For N2(g) + 3 H2(g) → 2 NH3(g), with S° values 191.6, 130.7, and 192.8 J/(mol·K) respectively, ΔS°rxn = [2(192.8)] − [191.6 + 3(130.7)] = 385.6 − 583.7 = −198.1 J/K, negative as expected from a net loss of gas moles when four moles of gaseous reactant become two moles of gaseous product. Checking the sign against Chem 2086's qualitative prediction — fewer gas moles means fewer accessible microstates — confirms the arithmetic before trusting the final number.
Where people slip
Forgetting to multiply each S° value by its coefficient before subtracting treats a balanced equation as if every species contributed equally, the same slip that already threatens ΔH°rxn calculations.
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