⚗️ Calculating Standard Entropy Changes

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Q1. Calculate ΔS°rxn for the balanced chemical equation.

4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)

2. Calculate ΔS°rxn for the balanced chemical equation.

2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g)

Begin by looking up (in Appendix IIB) the standard entropy for each reactant and product. Always note the correct state—(g), (l), (aq), or (s)—for each reactant and product.

Calculate ΔS°rxn by substituting these values into Equation 17.11. Remember to include the stoichiometric coefficients in your calculation.

ΔS°rxn = ΣnpS°(products) − ΣnrS°(reactants)
= [4(S°NO(g)) + 6(S°H2O(g))] − [4(S°NH3(g)) + 5(S°O2(g))]
= [4(210.8 J ∕ K) + 6(188.8 J ∕ K)] − [4(192.8 J ∕ K) + 5(205.2 J ∕ K)]
= 1976.0 J ∕ K − 1797.2 J ∕ K
= 178.8 J ∕ K