One of the most important industrial reactions is the conversion of nitrogen to ammonia. Because the efficiency of this reaction governs the economies of nations, it is necessary for chemical engineers to understand its thermodynamic properties. Calculate the standard reaction entropy of N₂(g) + 3 H₂(g) → 2 NH₃(g) at 25°C.

ANTICIPATE You should expect a decrease in entropy, because there is a net reduction in the amount of gas molecules.

PLAN Use the chemical equation to write an expression for ΔS°, as shown in Eq. 2, and then substitute values from Table 4H.1 or Appendix 2A.

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TABLE 4H.1 Standard Molar Entropies at 25 °C* Sm/(J.K¹. mol-¹) Gases ammonia, NH3 carbon dioxide, CO₂ hydrogen, H₂ nitrogen, N₂ oxygen, O₂ Sm/J.K¹. mol-¹) 192.4 213.7 130.7 191.6 205.1 Liquids benzene, C6H6 ethanol, C₂H5OH water, H₂O 173.3 160.7 69.9 Solids calcium oxide, CaO calcium carbonate, CaCO3¹ diamond, C graphite, C lead, Pb Sm/(J-K-¹. mol-¹) 39.8 92.9 2.4 5.7 64.8