For oxidation of iron, 4Fe(s) + 3O2(g) → 2Fe2O3 (s) entropy change is – 549.4 JK-1 mol-1 at 298 K. Inspite .of negative entropy change of this reaction, why is the reaction spontaneous? (ΔrH° = -1648 × 103 J mol-1)
The spontaneity of a reaction is determined from ΔStotal (which is = Δsys + Δsutu) Now ΔSsurr = – ΔrH0 T at constant pressure = −1648×103298 K J mol-1 = 5530 JK-1 mol-1 Thus total entropy change for the reaction is Δr Stotal = 5530 + (- 549.4) = 4980.6 JK-1 mol-1. Since the total entropy change is positive, therefore, the above reaction is spontaneous.
