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Chemisorption is exothermic process, but still it increases with increase in temperature. Why?
I know that there needs to be a minimum activation energy to be achieved in order to initiate chemisorption. Also physiosorption can change to chemisorption in some cases on merely rising the temperature.
But why does it increase with an increase in temperature? How is this related to Le Chatelier's principle?
To answer that first think about a reaction like combustion reaction. Even though combustion reactions are exothermic you need to provide heat to the system by increasing its temperature. See, every reaction or chemical change happening requires a certain minimum amount energy called the "activation energy" for the reaction to start. This energy is required to break the bonds of the reactants so that they can react to form product. That's why we say that at low temperatures, kinetics controls the reaction rather than thermodynamics. Now, once you are at a temperature which is able to provide the sufficient activation energy to the reactants, the thermodynamics comes into play which essentially says in this case that if a reaction is exothermic, increasing the temperature will shift the reaction in backward direction.
Now coming to chemisorption, in this case too you need to increase the temperature to provide sufficient activation energy so that the reaction can start. So increasing the temperature increases the rate of reaction. After a certain temperature when thermodynamics become more important, the reaction shifts backward.
Hope it helps!
This behavior is expected because, like all chemical reactions, some activation energy is required for chemisorption. At low temperature, x/m is small. As temperature is increased, the molecules of the adsorbate gain energy and become equal to activation energy so that proper bonds are formed with the adsorbent molecules. Therefore, initially, the amount of gas adsorbed increases with rise in temperature. Further increase of temperature will increase the energy of the molecules which have already been adsorbed. This would increase the rate of desorption, thereby decreasing the extent of adsorption
No doubt, chemisorption is (mostly) exothermic. So if we increase the temperature, Le Chatelier tells us that the extent of adsorption decreases, i.e., equilibrium will lie less towards the adsorbed side. But kinetics tells us that the temperature is a "universal catalyst", hence will increase the rate.
So our equilibrium favouring less adsorption will be achieved in less time.
It is the interplay of both the factors that gives the familiar hump-shaped plot of chemisorption with temperature. But I've never come across a graph online which labels the y-axis accurately. It must be the /extent of chemisorption achieved in a "feasible" time scale/, instead of just extent of chemisorption.
We know that in case of chemical adsorption, the forces acting are chemical bonds (ionic or covalent). We also know that temperature is directly proportional to activation energy.
So, when increasing the temperature, the activation energy is increased, so more bonds are formed. Thus, the magnitude of adsorption increases on increasing the temperature.
The increase in temperature is required to provide sufficient activation energy so that the reaction can start . So increasing the temperature increases rate of reaction initially . Then after a certain temperature by Le-Chatelier's principle further increase will shift the equilibrium to the left side and thus desorption will prevail. Moreover with further increase the energy of the molecules of adsorbate and in turn increases desorption.
Chemisorption first increase on increasing in temp. then decrease because initially breaking of old bond takes place the formation of new bond will be there on the surface of adsorbent after that these bond also start at very high temperature.
