Catalyst

Learning Objectives

Discuss types of catalyst Differentiate between homogeneous and heterogeneous

catalyst Describe properties of catalyst Study theories of catalysis

Catalyst A catalyst is a substance which speeds up a

reaction, but is chemically unchanged at the end of the reaction.

When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning.

Catalysis is the change in rate of a chemical reaction

 due to the participation of a catalyst

Some examples

Fe

Reaction Catalyst

Decomposition of hydrogen peroxide

Manganese(IV) oxide, MnO2

Nitration of benzene Concentrated sulphuric acid

Manufacture of ammonia by the Haber Process

Iron

Conversion of SO2 into SO3 during the Contact

Process to make sulphuric acidVanadium(V) oxide, V2O5

Hydrogenation of a C=C double bSSSond

Nickel

Types of catalyst

Catalysts can be divided into two main types – heterogeneous and homogeneousi) Homogeneous catalyst - the catalyst is in the same phase

as the reactants.Example- The reaction between persulphate ions and iodide

ions.

S2O82- + 2I- 2 SO4

2- + I2ii) Heterogeneous catalyst - the catalyst is in a different phase

from the reactants.Example- The hydrogenation of a carbon-carbon double bond.

CH2=CH2 + H2 CH3-CH3

Ni

Fe2+

Properties of catalyst Positive Catalysis The catalyst which increases the rate of a chemical reaction is called

positive catalyst and the phenomenon is known as positive catalysis Examples are

(i) 2KClO3 2KCl + 3O2

(ii) H2O2 H2O + [O] Negative Catalysis The catalyst which decreases the rate of reaction is called negative catalyst and phenomenon is called negative catalysis Examples are

(i) H2O2 H2O + O

(ii) Knocking of petrol by tetraethyl lead

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MnO2

Pt

Acetanilide

Properties of catalyst Promotors Sometimes the activity of a catalyst may be increased by

addition of small amount of a second substance. The second substance, which though itself is not a catalyst, promotes the activity of a catalyst, is called the promoter 

For example, N2 + 3H2 2NH3

[Fe] is the catalyst and presence of Mo or Al2O3 increase the activity

of iron and, therefore, acts as promoter.

Fe ,Mo/ Al2O3

Properties of catalyst Poison Sometimes the rate of a catalysed reaction is reduced by

the presence of a small amount of same substance (may be as impurities in the reactants).

Such a substance, which destroys the activity of a catalyst, is called poison and the process is called catalytic poisoning

For example, in reaction

2H2 + O2 2H2O

Activity of catalyst Pt is poisoned by presence of CO N2 + 3H2 2NH3

Presence of H2S reduces the activity of Fe

Pt ,CO

Fe ,H2S

Theories of catalysisThe two main theories of catalysis are: (i) intermediate

compoundformation theory and (ii) adsorption theory

I) Intermediate Compound Formation Theory According to this theory, the catalyst reacts with one of the

reactants to give an intermediate, which reacts with another reactant to yield products and the catalyst as follows:

A + [ Catalyst] [Intermediate] [Intermediate] + B Product + [Catalyst]Examples:

2 SO2 + O2 2 SO3

Proceeds as: 2NO + O2 2NO2

NO2 + SO2 SO3 + NO

[NO]

II) Adsorption Theory The heterogeneous catalysis e.g. gaseous reaction on a

solid surface, is explained by this theory as follows:

A(g) + B(g) C(g) +D(g) Following four steps are involved in the heterogeneous

catalysis:(i)Diffusion of reactants at the surface of the catalyst.(ii)Adsorption of reactants at the surface.(iii)Reaction of reactants at the surface.(iv)Desorption of products from the surface.

Catalyst