CBSE Class 12 Chemistry Notes: Surface Chemistry – Catalyst

Catalyst Catalysis

Catalyst:- Catalyst is a substance which can change (enhance) the speed of a chemical reaction without itself undergoing any change in its mass and chemical composition. And the phenomena is called Catalysis.

For Example:-

Positive Catalysis:- if a catalyst increases (accelerates) the speed of reaction is a called a positive catalyst and phenomena is called positive Catalysis.
Ex. Zymase, invertase.
Negative Catalysis:- if catalyst decreases the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalysis.
Ex. Decomposition of H₂O₂ in presence of Glycerol or acetanilide.

Poisons:- Cerain substances like arsenic Co, if present lowers the activity of the catalysts. These substance are to be called as catalytic poisons.

Catalytic promoter:- A substance if present along with catalyst enhances the activity of the catalyst, it is called as a Catalytic promoter.
Ex. Molebdenum

Types of catalysis:-

Homogenous Catalysis:-if the catalyst is present in same phase as the reactants, it is called homogenous catalyst and the type of catalysis is called Homogenous Catalysis.

Example:- oxidation of sulphar dioxide to trioxide in presence of nitric oxide.

Heterogeneous Catalysis:-if the catalyst is present in a different phase than that of reactants, it is called a Heterogeneous catalyst and this type of catalysis is known as Heterogeneous Catalysis.

Ex. Manufacture of ammonia from N₂ and H₂ by Haber’s process using iron as catalyst

(ii) Manfacture of sulphuric acid by oxidation of SO₂ to SO₃ using pltinised asbestos or V₂O₅ as catalyst

Theory of Heterogeneous Catalysis:-

(Modern adsorption Theory):- According to this theory, there are free valencies, on the surface of solid catalysts (generally metals) and mechanism involves these steps:-

Diffusion of reactant molecules towards the surface of the catalyst.
Adsorption of the reactant molecules on the surface of the catalyst by forming loose bonds with catalyst due to free valancies.
Occurrence of chemical reaction between reactants and the catalyst forming an in termediate.
Desorption of the product molecule from the surface due to lack os its affinity for the catalyst surface.
Diffusion of product molecules away from the surface of catalyst.

Some important Features of

Solid Catalysts (or heterogeneous Catalysis):-

Activity:- By activity of the catalyst we mean its capacity to increase the speed of reactions.
Ex.
the activity of enzyme depends upon the extent of chemisorption.
Selectivity:– By selectivity of a catalyst we mean its ability to direct the reactin to form particular products excluding other.
Ex. Co and H₂ forms different products in presence of different catalysts.

Zeolites as shape selective Catalysts:- zeolites are aluminosilicates with the general formula M_x/n [(AlO₂)_x(SiO₂)_y]. ZH₂O when n is charge on metal cation Mn⁺. Which is usually Na⁺, K⁺, Ca⁺. they are microporous three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. Giving Al-O-Si frame work.

Zeolites are being very widely used as catalyst in petrochemical industries for craking of hydrocarbons and isomerization an important zeolite catalyst used in petroleum industry is ZSM – 5^•••. It converts alchol directly into gasoline.