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. Entropy is calculated as a change in the level of entropy before and after a chemical reaction. Essentially, it reflects whether the amount of disorder or randomness in the system increased or decreased as a consequence of the reaction.

Standard Molar Entropy Change Problem

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Calculate the Change in Entropy From Heat of Reaction

By Anne Marie Helmenstine, Ph.D.

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

(Note, in this type of problem you'll either be given the molar entropy values of the reactants and products or you'll need to look them up in a table.)
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.