Acertain reaction is spontaneous at temperatures below 400. k but is not spontaneous at temperatures above 400. k. if δh° for the reaction is -20. kj mol-1 and it is assumed that δh° and δs° do not change appreciably with temperature, then the value of δs° for the reaction is

ΔS = 0.05 kJ /mol or 50 J /mol

Explanation:

As given that the reaction is spontaneous below 400 K and non spontaneous above 400K. Thus the reaction is at equilibrium at 400 K temperature.

At equilibrium the change in free energy ΔG is zero.

The relation between free energy change, enthalpy change, entropy change and temperature is

ΔG = ΔH - TΔS

Where

ΔH = enthalpy change

T = temperature

ΔS = entropy change

at equilibrium (400 K)

ΔG = 0 = (-20) + 400XΔS

ΔS = 0.05 kJ /mol or 50 J /mol