Question

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

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

(b) ΔH = +50 kJ and ΔS = -130 JK^-1 at 250 K.

Answer 1

(a) Given : 

ΔH = -110 kJ 

ΔS = 40 JK^-1 = 0.04 kJK 

Temperature = T = 400 K 

ΔG = ?

Since ΔH is negative, 

The reaction is exothermic.

ΔG = ΔH – TΔS 

= -110 – 400 × 0.04 

= -110 – 16 

= -126 kJ

Since ΔG is negative, 

The reaction is spontaneous.

(b) Given : 

ΔH = 50 kJ, 

ΔS = -130 JK^-1 = -0.13 kJ K^-1 

Temperature = T = 250 K 

ΔG = ?

Since ΔH is positive, 

The reaction is endothermic. 

ΔG = ΔH – TΔS 

= 50 – 250 × (-0.13) 

= 50 + 32.5 

= 82.5 kJ

Since ΔG > 0, 

The reaction is non-spontaneous.

∴ (a) ΔG = -126 kJ; 

The reaction is exothermic and spontaneous.

(b) ΔG = 82.5 kJ; 

The reaction is endothermic and non-spontaneous.