Question 1
Multiple choiceMedium1 markFor the reaction shown, the standard molar entropies are S°(H₂O₂(l)) = 109.6, S°(H₂O(l)) = 69.9 and S°(O₂(g)) = 205.0 J K⁻¹ mol⁻¹. Which option gives ΔᵣS° per mole of reaction as written and the resulting change in the system’s entropy?
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Answer: C. For a reaction, ΔᵣS° = ΣνS°(products) − ΣνS°(reactants). Using the coefficients read from the figure: [2(69.9) + 205.0] − [2(109.6)] = +125.6 J K⁻¹ mol⁻¹. The positive value means the system’s entropy increases.
A: reverses products minus reactants, giving −125.6 J K⁻¹ mol⁻¹ and a decrease.
B: omits the coefficient 2 for water, giving +55.7 J K⁻¹ mol⁻¹.
D: omits the coefficient 2 for hydrogen peroxide, giving +235.2 J K⁻¹ mol⁻¹.
Remember: multiply each standard molar entropy by its stoichiometric coefficient before summing.