AP Chemistry 9.2: Entropy Change

Practice AP Chemistry questions on calculating standard entropy change for a process from the absolute or standard molar entropies of its species.

Exam points

Calculate ΔS°rxn from tabulated molar entropies using ΣS°products − ΣS°reactants and stoichiometric coefficients.
Use ΔS = q_rev/T to determine entropy change from vaporization heat and temperature in J mol−1 K−1.

[Maximum number: 2]

Methanol vapor decomposes to form carbon monoxide gas and hydrogen gas at high temperatures in the presence of a platinum catalyst, as represented by the balanced chemical equation given.

The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.

Table for Question 9.2.A—Calculate the standard entropy change for a chemical or physical process based on the absolute entropies… question 1 — AP Chemistry

Use the data in the table to calculate the value of the standard entropy change, $\Delta S^{\circ}$ , in $\mathrm{J} /\left(\mathrm{K} \cdot \operatorname{mol}_{r x n}\right)$ , for the reaction.

(i) For the correct stoichiometry (may be implicit): 1 point

\begin{aligned} & \Delta S^{\circ}{ }_{r x n}=\Sigma \Delta S^{\circ}{ }_{\text {products }}-\Sigma \Delta S^{\circ}{ }_{\text {reactants }} \\ & \Delta S^{\circ}{ }_{r x n}=\left(\Delta S^{\circ}{ }_{\mathrm{CO}(g)}+2\left(\Delta S^{\circ}{ }_{\mathrm{H}_{2}(g)}\right)\right)-\left(\Delta S^{\circ}{ }_{\mathrm{CH}_{3} \mathrm{OH}(g)}\right) \end{aligned}

For the correct calculated value: 1 point

\Delta S^{\circ}{ }_{r x n}=198+2(131)-240 .=220 \cdot \frac{\mathrm{~J}}{\mathrm{~K} \cdot \mathrm{~mol}_{r x n}}

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9.2.A—Calculate the standard entropy change for a chemical or physical process based on the absolute entropies… question 1