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Problem 7

(a) What is the entropy of a perfect crystal at 0 $\mathrm{K}$ ? (b) Does entropy increase or decrease as the temperature rises? (c) Why is $\Delta H_{\mathrm{f}}^{\circ}=0$ but $S^{\circ}>0$ for an element? (d) Why does Appendix $\mathrm{B}$ list $\Delta H_{\mathrm{f}}^{\circ}$ values but not $\Delta S_{\mathrm{f}}^{\circ}$ values?

Answer

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Chapter 20

Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

## Discussion

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## Recommended Questions

A reaction has $\Delta H_{\mathrm{rxn}}^{\circ}=-112 \mathrm{kJ}$ and $\Delta S_{\mathrm{rxn}}^{\circ}=354 \mathrm{J} / \mathrm{K}$ . At what temperature is the change in entropy for the reaction equal to the change in entropy for the surroundings?

Given the values of $\Delta H_{\mathrm{rxn}}, \Delta S_{\mathrm{rxn}}$ and $T,$ determine $\Delta S_{\text { univ }}$ and predict whether or not each reaction is spontaneous. (Assume that all reactants and products are in their standard states.)

\begin{equation}\begin{array}{l}{\text { a. } \Delta H_{\mathrm{rxn}}^{\circ}=-95 \mathrm{kJ} ; \Delta S_{\mathrm{rxn}}^{\circ}=-157 \mathrm{J} / \mathrm{K} ; T=298 \mathrm{K}} \\ {\text { b. } \Delta H_{\mathrm{rxn}}^{\circ}=-95 \mathrm{kJ} ; \Delta S_{\mathrm{rxn}}^{\circ}=-157 \mathrm{J} / \mathrm{K} ; T=855 \mathrm{K}} \\ {\text { c. } \Delta H_{\mathrm{rxn}}^{\circ}=+95 \mathrm{kJ} ; \Delta S_{\mathrm{rxn}}^{\circ}=-157 \mathrm{J} / \mathrm{K} ; T=298 \mathrm{K}} \\ {\text { d. } \Delta H_{\mathrm{rxn}}^{\circ}=-95 \mathrm{kJ} ; \Delta S_{\mathrm{rxn}}^{\circ}=+157 \mathrm{J} / \mathrm{K} ; T=398 \mathrm{K}}\end{array}\end{equation}