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Enthalpy of Reaction
[1ΔHf(PCl3 (g)) + 1ΔHf(Cl2 (g))] - [1ΔHf(PCl5 (g))]
[1(-287.02) + 1(0)] - [1(-374.9)] = 87.88 kJ
87.88 kJ (endothermic)
[1ΔSf(PCl3 (g)) + 1ΔSf(Cl2 (g))] - [1ΔSf(PCl5 (g))]
[1(311.67) + 1(222.97)] - [1(364.6)] = 170.04 J/K
170.04 J/K (increase in entropy)
Free Energy of Reaction (at 298.15 K)
From ΔGf° values:
[1ΔGf(PCl3 (g)) + 1ΔGf(Cl2 (g))] - [1ΔGf(PCl5 (g))]
[1(-267.78) + 1(0)] - [1(-305)] = 37.22 kJ
37.22 kJ (nonspontaneous)
From ΔG = ΔH - TΔS:
37.18 kJ (nonspontaneous)
Equilibrium Constant, K (at 298.15 K)
This process is not favorable at 25°C.
Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; p 181.