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 = -110 kJ, ΔS = +40 JK^-1 at 400 K

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

Answer 1

(a) Given : 

ΔH = -110 kJ, 

ΔS = +40 JK^-1 at T = 400K 

ΔG = ΔH – TΔS

= -110 – 16 

= -126 kJ

Since ΔG is negative, 

The reaction is spontaneous.

Since ΔH is negative, 

The reaction is exothermic.

(b) Given : 

ΔH = + 40 kJ, 

ΔS = -120 JK^-1 at T = 250 K 

ΔG = ΔH – TΔS

= 40 + 30 

= 70 kJ

Since ΔG is negative, 

The reaction is spontaneous.

Since ΔH is negative, 

The reaction is exothermic.