Chapter 19 - Spontaenous Change: Entropy and Gibbs Energy - Example 19-3 - Calculating Entropy Changes from Standard Molar Entropies - Page 831: Practice Example A

$-198.7\,J\,mol^{-1}K^{-1}$

We find: $\Delta S^{\circ}=\Sigma n_{p}S^{\circ}(products)-\Sigma n_{r}S^{\circ}(reactants)$ $=[2S^{\circ}(NH_{3},g)]-[S^{\circ}(N_{2},g)+3S^{\circ}(H_{2},g)]$ $=[2(192.5\,J\,mol^{-1}K^{-1})]-[(191.6\,J\,mol^{-1}K^{-1})+3(130.7\,J\,mol^{-1}K^{-1})]$ $=-198.7\,J\,mol^{-1}K^{-1}$