Answer the question on the basis of the passage given below:

Adsorption is a surface phenomenon and it differs from absorption which occurs throughout the body of the substance which absorbs. In physisorption, the attractive forces are mainly van der Waal forces while in chemisorption actual bonding occurs between the particles of adsorbent and adsorbed more easily on the surface of a solid as compared to the gases which are liquified with difficulty. Adsorption increases with increase in pressure and decreases as the temperature is increased.

According to adsorption theory of catalysis, the speed of the reaction increases because

Concentration of the reactant molecules at active centres of the catalyst becomes high due to adsorption.

The correct answer is option 1. Concentration of the reactant molecules at active centres of the catalyst becomes high due to adsorption.

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 1. Concentrations of the reactants at the active centres become high due to adsorption and thus the rate of reaction.