Entropy of Reaction Example Problem

This example problem demonstrates how to find the entropy of reaction from standard molar entropy data on the reactants and products.

Problem:

What is the standard molar entropy change of the following reaction?

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

Given:
S°_NH3 = 193 J/K·mol
S°_O2 = 205 J/K·mol
S°_NO = 211 J/K·mol
S°_H2O = 189 J/K·mol

Solution

The change in the standard molar entropy of a reaction can be found by the difference between the sum of the molar entropies of the products and the sum of the molar entropies of the reactants.

ΔS°_reaction = Σn_pS°_products - Σn_rS°_reactants

ΔS°_reaction = (4 S°_NO + 6 S°_H2O) - (4 S°_NH3 + 5 S°_O2)

ΔS°_reaction = (4(211 J/K·K) + 6(189 J/K·mol)) - (4(193 J/K·mol) + 5(205 J/K·mol))

ΔS°_reaction = (844 J/K·K + 1134 J/K·mol) - (772 J/K·mol + 1025 J/K·mol)

ΔS°_reaction = 1978 J/K·mol - 1797 J/K·mol)

ΔS°_reaction = 181 J/K·mol

We can check our work using the techniques introduced in this example problem. The reaction involves all gasses and the number of moles of products is greater than the number of moles of reactants so the expected change in entropy should be positive.

Answer:

The standard molar entropy change of the reaction is 181 J/K·mol.