The surface of the catalyst has free valencies which provide sites for chemical forces of attraction.
The reactants in the gaseous state or in solution are adsorbed on the surface of the solid catalyst.
As the reactant molecules are adsorbed, its bonds are weakened and the reaction can proceed quickly because the bonds are more quickly broken.
For example: In the Haber process for the synthesis of ammonia is the adsorption of N2 molecules on iron and the weakening of the strong NΞN triple bond. Since the adsorption is an exothermic process, the heat of adsorption is utilised in weakening the bonds in the reactants and hence enhancing the rate of reaction.
Modern Adsorption Theory
It involves the following steps:
1) Diffusion of the reactants to the surface of the catalyst.
2) Adsorption of the molecules of the reactant at the active sites.
3) Occurrence of the chemical reactions on the surface of the catalyst through the formation of an intermediate.
4) Desorption of product molecules from the surface and thereby making the surface available again for more reactions to occur.
5) Diffusion of products away from the surface of the catalyst.
The adsorption helps the reaction in the following ways:
1) Adsorption increases the concentration of reactants on the surface of the catalyst. Due to increased concentration of the reactants, the reactions proceed rapidly.
2) Adsorbed molecules get dissociated to form active species like free radicals which react faster than molecules.
3) The adsorbed molecules are not free to move about and, therefore, they collide with other molecules on the surface.
4) The heat of adsorption evolved acts as energy of activation for the reaction (chemisorption).
Important Features of Solid Catalysts
The effectiveness of a catalyst depends upon the two important aspects- activity and selectivity.
(i) Activity of a catalyst
The ability of a catalyst to increase the rate of a chemical reaction is called activity.
The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactant must adsorb reasonably strongly for the catalyst be active but must not adsorb so strongly that they become immobilise and the other reactants do not get space on the catalyst surface for adsorption.
A catalyst may accelerate a reaction to as high as 1010 times.
For example : The mixture of H2 and O2 can be stored for any period but in the presence of
platinum, the reaction occurs with explosive violence.
2H2 (g) + O2 (g) —–> 2 H2O (g)
(ii) Selectivity of a catalyst
The ability of the catalyst to direct a reaction to give a particular product is called selectivity. For example: different catalysts give different products for the reaction between CO and H2 as shown below:
(1) CO(g) + H2(g) ——> CH4 (g) + H2O(g) Ni
(2) CO(g) + 2H2(g) ——> CH3OH (g) Cu, ZnO, Cr2O3
(3) CO(g) + H2(g) ——> HCHO (g) Cu
Acetylene on reaction with H2, in the presence of Pt catalyst gives ethane while in the presence of Lindlar’s catalyst (palladium and BaSO, poisoned with quinoline or sulphur) gives ethylene.
H-C≡C-H + H2 ——-> C2H6 Pt catayst
H-C≡C-H + H2 ——-> C2H4 Lindlar’s catalyst
A given catalyst can act as catalyst only in a particular reaction and not in all reactions. Substance which acts as a catalyst in one reaction may fail to catalyse other reaction. Thus, catalyst is highly selective in nature.