AP Chemistry Unit 9.2: Entropy Change

Practice AP Chemistry Unit 9.2 questions on calculating standard entropy change from absolute or standard molar entropies and stoichiometric coefficients.

Covered syllabus points

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

[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 Absolute Entropy and Entropy Change 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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Covered syllabus points

9.2 Absolute Entropy and Entropy Change question 1