Question 8.13: PREDICTING THE SIGNS OF ΔH, ΔS, AND ΔG FOR A REACTION What a...

PREDICTING THE SIGNS OF ΔH, ΔS, AND ΔG FOR A REACTION

What are the signs of ΔH, ΔS, AND ΔG for the following nonspontaneous transformation?

STRATEGY
First, decide what kind of process is represented in the drawing. Then decide whether the process increases or decreases the entropy of the system and whether it is exothermic or endothermic.

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The drawing shows ordered particles in a solid subliming to give a gas. Formation of a gas from a solid increases molecular randomness, so ΔS is positive. Furthermore, because we’re told that the process is nonspontaneous, ΔG is also positive. Because the process is favored by ΔS (positive) yet still nonspontaneous, ΔH must be unfavorable (positive). This makes sense, because conversion of a solid to a liquid or gas requires energy and is always endothermic.

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