Question

Find out whether the following reaction is spontaneous or not at 127 degrees centigrade? N2(g) + 3H2(g) \(\rightleftharpoons\) 2NH3(g); ΔH = 92.22 kJ/mol and ΔS = -198.75 J/K-mol.

(a) it is spontaneous

(b) it is not spontaneous

(c) it may be spontaneous

(d) cannot predict

This question was posed to me by my school teacher while I was bunking the class.

The doubt is from Gibbs Energy Change and Equilibrium in section Thermodynamics of Chemistry – Class 11

Answer 1

Right answer is (b) it is not spontaneous

Explanation: We have ΔG = ΔH – TΔS; by substituting ΔH = 92.22 kJ/mol and ΔS = -198.75 J/K-mol, we get ΔG = 92.22 kJ/mol – 400k(-198.75 J/K-mol) = 92.22 kJ/mol + 79.5 kJ/mole = 171.72 KJ/mol. As Gibbs free energy is positive the reaction is non spontaneous.