Calculate the value of S° for the reaction shown:

2 NH3(g) N2(g) + 3 H2(g)
At 25°C the values of entropy in J K−1 mol−1 are ammonia, 192.77; nitrogen, 191.61; and hydrogen, 130.68.

Question

Calculate the value of S° for the reaction shown:

2 NH3(g)  N2(g) + 3 H2(g)
At 25°C the values of entropy in J K−1 mol−1 are ammonia, 192.77; nitrogen, 191.61; and hydrogen, 130.68.

BlackTom AI · Accepted Answer

We begin by identifying the thermodynamic relation for the standard molar entropy change of a reaction: ΔS° = Σνp S°(products) − Σνr S°(reactants), where νp are stoichiometric coefficients of products and νr of reactants.

Option analysis:

Option A: +390.88 J/K
- Compute the reactants: 2 NH3 with S°(NH3) = 192.77 J/K/mol → 2 × 192.77 = 385.54 J/K.
- Compute the products: N2 with S°(N2) = 191.61 J/K/mol and 3 H2 with S°(H2) = 130.68 J/K/mol → 1 × 191.61 + 3 × 130.68 = 191.61 + 392.04 = 583.65 J/K.
- ΔS° = 583.65 − 385.54 = +198.11 J/K, which does not match +390.88 J/K. This option is incorrect.

Option B: +198.11 J/K
- From the same calculations: reactants total = 385.54 J/K, products total = 583.65 J/K, so ΔS° = 583.65 − 385.54 = +198.11 J/K.
- This matches the computed value exactly. This option is consistent with the entropy change for 2 NH3 ⇌ N2 + 3 H2 at 25°C.

Option C: +259.03 J/K
- This would imply a different balance between product and reactant entropies. Since the actual calculation gives +198.11 J/K, this option does not align with the stoichiometric subtraction.

Option D: +393.20 J/K
- Similar to Option A, this would require a different product/reactant entropy balance. Our precise sum yields +198.11 J/K, not +393.20 J/K, so this is incorrect.

Option E: +969.19 J/K
- This value is far from the computed ΔS°, and would suggest a large discrepancy between products and reactants that isn’t supported by the given S° values and stoichiometry. This option is not correct.

CHM-1020-LD01.2025SU Quiz 6 - thermodynamics

Calculate the value of S° for the reaction shown: 2 NH3(g) N2(g) + 3 H2(g) At 25°C the values of entropy in J K−1 mol−1 are ammonia, 192.77; nitrogen, 191.61; and hydrogen, 130.68.

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