A student tried to determine the standard molar entropy of using the reaction and the…

CHEMISTRY · Single correct · A Level · Chemistry · Thermodynamics · Thermodynamics

Problem

A student tried to determine the standard molar entropy of

\text{NO}_2(g)

using the reaction

2\,\text{NO}(g)+\tfrac{1}{2}\,\text{O}_2(g)\rightarrow \text{NO}_2(g)

and the data

S^{\circ}(\text{NO})=210.8\,\text{J K}^{-1}\,\text{mol}^{-1}

S^{\circ}(\text{O}_2)=205.0\,\text{J K}^{-1}\,\text{mol}^{-1}

, and

\Delta S^{\circ}_{\text{rxn}}=-146.5\,\text{J K}^{-1}

. The student obtained

S^{\circ}(\text{NO}_2)=169.3\,\text{J K}^{-1}\,\text{mol}^{-1}

, far below the accepted value of 240.1

\text{J K}^{-1}\,\text{mol}^{-1}

. Which of the following is the most likely cause of the student’s large error? (a) The student mistakenly used enthalpy values instead of entropy values in the calculation. (b) The student divided the resulting value by 2, thinking the answer had to be averaged over the two reactant molecules of NO. (c) The student failed to multiply the entropy of NO by its stoichiometric coefficient of 2. (d) The student treated the coefficient of

\text{O}_2

as 1.0 rather than 0.5 when summing the reactant entropies.

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