Question
Explain the effect of temperature on feasibility for:
(i) endothermic process
(ii) exothermic process in terms of Gibb’s Helmoltz equation.
Solution
Gibb’s Helmoltz equation is
(i) Endothermic process: ∆H is positive, thus it always opposes the process. Now,
(a) If T∆S is negative and opposes the process, then ∆G will be positive and the process is always non-spontaneous.
(b) When T∆S is positive i.e. favourable, then ∆G may either be positive or negative.
At low temperature:T∆S may have a small value and ∆H may be greater than T∆S. Under these conditions, ∆G(=∆ H + T∆S) may be positive and the reaction may not be spontaneous at low temperature.
Increasing temperature: With the increase in temperature, the magnitude of the favourable factor T∆S increases while ∆H does not change much. Hence at high temperature, the magnitude of T∆S will be quite large and more than ∆H so that ∆G becomes negative. This means that endothermic processes are favoured and are more probable at high temperature.
(ii) Exothermic process: ∆H is always negative and, therefore, it is favourable. Now,
(a) If T∆S is positive i.e. favourable then ∆G have only negative value and the process is spontaneous at all temperatures.
(b) If T∆S is negative i.e. unfavourable, then ∆G can have positive or negative value.
At high temperature: –T∆S will have large magnitude and ∆G will be positive and as such, the process may not be spontaneous.
At low temperature. The value of ∆H may become greater than the small value of T∆S and ∆G becomes negative and the process is spontaneous under these conditions. Hence exothermic processes are favoured and more probable at low temperature.
(i) Endothermic process: ∆H is positive, thus it always opposes the process. Now,
(a) If T∆S is negative and opposes the process, then ∆G will be positive and the process is always non-spontaneous.
(b) When T∆S is positive i.e. favourable, then ∆G may either be positive or negative.
At low temperature:T∆S may have a small value and ∆H may be greater than T∆S. Under these conditions, ∆G(=∆ H + T∆S) may be positive and the reaction may not be spontaneous at low temperature.
Increasing temperature: With the increase in temperature, the magnitude of the favourable factor T∆S increases while ∆H does not change much. Hence at high temperature, the magnitude of T∆S will be quite large and more than ∆H so that ∆G becomes negative. This means that endothermic processes are favoured and are more probable at high temperature.
(ii) Exothermic process: ∆H is always negative and, therefore, it is favourable. Now,
(a) If T∆S is positive i.e. favourable then ∆G have only negative value and the process is spontaneous at all temperatures.
(b) If T∆S is negative i.e. unfavourable, then ∆G can have positive or negative value.
At high temperature: –T∆S will have large magnitude and ∆G will be positive and as such, the process may not be spontaneous.
At low temperature. The value of ∆H may become greater than the small value of T∆S and ∆G becomes negative and the process is spontaneous under these conditions. Hence exothermic processes are favoured and more probable at low temperature.
