Question

According to the adsorption theory of catalysis, the rate of reaction increases with the use of catalyst because:
[A] The heat liberated during adsorption increases the rate of reaction.
[B] The kinetic energy of the reactants increases with the increase in the rate of reaction.
[C] The activation energy increases with the increases in rate of reaction.
[D] The concentrations of the reactants at the active centres become high due to adsorption and thus the rate of reaction.

Answer 1

Hint: To answer this, follow the adsorption theory of heterogeneous catalysts. According to it, the reactants are adsorbed on the surface of the catalyst thus probability of occurrence of reaction increases hence the rate increases.

Complete step by step answer:
We know that theories of catalysis explain the effect of addition of a catalyst in the reaction mechanism. The adsorption theory tells us about the process by which a particle is adsorbed on the surface of others through weak van der Waals forces.
In the modern theory of adsorption, there are two classes of catalysts that we have to deal with – homogeneous and heterogeneous. However, the theory of catalyst adsorption explains the process of heterogeneous catalysis in five major steps-
-In the first step, the reactants first get in contact with the outer surface of the catalyst. However, some of them enter the surface by crossing the barrier.
-These molecules are then adhered to the available sites.
-When the reactants are bound on the surface, there is a higher probability of occurrence of a reaction by formation of an intermediate.
-The intermediate is then desorbed and is again available for adsorption.
-And lastly, the intermediate disintegrates and forms the final product which diffuses out of the catalyst.
As we can see from the above discussion that the concentration of the reactants is greater at the surface which increases the probability of the reaction to move forward, thus we can say that the concentration of reactant at the active centre becomes high due to adsorption thus increases the rate of reaction.
Therefore, the correct answer is option [D] the concentrations of the reactants at the active centres become high due to adsorption and thus the rate of reaction.
So, the correct answer is “Option D”.

Note: According to the old adsorption theory of catalysts, in the gaseous or liquid state the reactants get adsorbed on a solid catalyst. The rate of the reaction increases depending upon the surface of the catalyst, the availability of the adsorbed substance to react and the probability of reaction. As we know, adsorption is an exothermic phenomenon (when the reactants are attached to the surface it releases some heat energy) thus the energy released during adsorption is utilised for the reaction between the two species thus increasing the rate of reaction.
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