Question

Determine whether the reactions with the following ΔH and ΔS values are spontaneous or non-spontaneous. State whether the reactions are exothermic or endothermic.

(a) ΔH = -110kJ, ΔS = + 40JK^-1 at 400 K

(b) ΔH = + 40kJ, ΔS = -120JK^-1 at 250K

Answer 1

(a) ΔH = −110 kJ, ΔS = +40 JK−1 at 400 K

ΔH = −ve; heat is evolved in an exothermic reaction

In the above case, ΔH < 0 and ΔS > 0. Under these conditions, both ΔH and TΔS terms will be negative, so ΔG will be negative regardless of the temperature.

So, the reaction will be spontaneous and exothermic.

(b) ΔH = +40 kJ, ΔS = − 120 JK−1 at 250 K

ΔH = +ve; heat is absorbed in an endothermic reaction

In the above case, ΔH > 0 and ΔS < 0. Under these conditions, ΔG is always positive and the reaction is always non-spontaneous.

So, the reaction will be non-spontaneous and endothermic.