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For which of the following processes is the change in entropy negative?

1. Bromine (liquid) -> Bromine (gas)
2. C(s) + H2O(g) -> CO(g) + H2(g)
3. N2(g, 10 atm) -> N2(g, 1 atm)
4. Fe (1 mol, 400 K) -> Fe (1 mol, 300 K)

Correct answer: Fe (1 mol, 400 K) -> Fe (1 mol, 300 K)

Solution

A) Bromine liquid to gas: vaporization increases entropy (Delta S > 0). B) C(s) + H2O(g) -> CO(g) + H2(g): moles of gas increase from 1 to 2, entropy increases. C) N2 at 10 atm to 1 atm: gas expands (lower pressure = greater volume), entropy increases. D) Cooling Fe from 400 K to 300 K at constant composition: Delta S = n*Cp*ln(T2/T1) = 1*Cp*ln(300/400) < 0. Entropy decreases with decreasing temperature.

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