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Enthalpy of Reaction
[2ΔHf(Cl (g))] - [1ΔHf(Cl2 (g))]
[2(121.29)] - [1(0)] = 242.58 kJ
242.58 kJ (endothermic)
[2ΔSf(Cl (g))] - [1ΔSf(Cl2 (g))]
[2(165.06)] - [1(222.97)] = 107.15 J/K
107.15 J/K (increase in entropy)
Free Energy of Reaction (at 298.15 K)
From ΔGf° values:
[2ΔGf(Cl (g))] - [1ΔGf(Cl2 (g))]
[2(105.31)] - [1(0)] = 210.62 kJ
210.62 kJ (nonspontaneous)
From ΔG = ΔH - TΔS:
210.63 kJ (nonspontaneous)
Equilibrium Constant, K (at 298.15 K)
This process is not favorable at 25°C.
Atkins, Jones, and Laverman. Chemical Principles 6th ed.; W.H. Freeman and Company: New York, NY, 2013; p 428.