Effect of Temperature on Feasibility of a Process:
According to Gibbs Helmholtz equation, ΔG = ΔH – TΔS. The magnitude of ΔH does not change much with the change in temperature. But the factor TΔS changes appreciably with the change in temperature. Thus, the magnitude of ΔG is influenced by the change in temperature. The effect of temperature is different for endothermic and exothermic reactions.
Effect on Endothermic Reactions:
For endothermic reactions, ΔH is always positive, and since energy has been absorbed, ΔS is also positive. Thus ΔH opposes the spontaneity of the reaction while TΔS favours spontaneity. Thus, the process is spontaneous only when TΔS > ΔH.
- At low temperature- At low temperature TΔS has small magnitude and is less than ΔH. Thus ΔG is positive at low temperature and the process is non-spontaneous.
- At high temperature- At high temperature, TΔS has larger magnitude and is greater than ΔH. Thus ΔG is negative at high temperature and the process is sponatneous.
Thus, an endothermic process is non-spontaneous at low temperature and spontaneous at high temperature.
Effect on Exothermic Reactions:
For exothermic reactions, ΔH is always negative, and since energy has been released, ΔS is also negative. Thus ΔH favours the spontaneity of the reaction while TΔS opposes spontaneity. Thus, the process is spontaneous only when ΔH > TΔS.
- At low temperature- At low temperature, TΔS has small magnitude and is less than ΔH. Thus ΔG is negative at low temperature and the process is spontaneous.
- At high temperature- At high temperature, TΔS has larger magnitude and is greater than ΔH. Thus ΔG is positive at high temperature and the process is non-spontaneous.
Thus, an exothermic process is spontaneous at low temperature and non-spontaneous at high temperature.
