Question 23.1: Predict the sign of the entropy change, ΔSsys, for the follo......
Predict the sign of the entropy change, ΔS_{sys}, for the following processes:
(a) H_{2}O(g) (1 bar, 25°C) →H_{2}O(g) (0.01 bar, 25°C)
(b) The dissolution of sodium chloride in water
(c) NaCl(aq) (4.8 M, 20°C) →NaCl(aq) (1.0 M, 20°C)
(d) C_6H_6(g) (10 Torr, 25°C) →C_6H_6 (adsorbed on a metal surface at 25°C)
(a) A gas at a lower pressure, and thus with greater volume, is more disordered than a gas at a higher pressure at the same temperature; therefore,
ΔS_{gas} (constant-temperature expansion) > 0
(b) Upon dissolving, the sodium and chloride ions are dispersed throughout the solvent and are much more disordered than in the crystal, so
ΔS_{solid} (dissolution) > 0
(c) Dilution with solvent increases the positional disorder of the solute, so
ΔS_{solute} (dilution) > 0
(d) The adsorption of a gas onto a surface produces a decrease in the entropy of adsorbed molecules because of the decreased freedom of movement (see Figure 23.15), so
ΔS_{ benzene} (adsorption) < 0
