CATALYSI

S

By :- Gaurav Dave

CATALYST AND CATALYSIS

Catalyst is a substance that increases the rate of the reaction at

which a chemical system approaches equilibrium , without being

substantially consumed in the process.

And the process is known as Catalysis.

Catalyst affects only the rate of the reaction ,i.e.Kinetics.

It changes neither the thermodynamics of the reaction nor the

equilibrium composition.

CATALYSIS…

Catalysts generally react with one or more reactants to form

intermediates that subsequently give the final reaction product, in

the process regenerating the catalyst. The following is a typical

reaction scheme, where C represents the catalyst, X and Y are

reactants, and Z is the product of the reaction of X and Y:

X + C → XC (1)

Y + XC → XYC (2)

XYC → CZ (3)

CZ → C + Z (4)

TYPES OF CATALYSIS

Catalysts can be divided into two main types

1. Homogeneous

2. Heterogeneous

3. Enzyme catalysis

HOMOGENEOUS CATALYSIS

This has the catalyst in the same phase as the reactants.

Typically everything will be present as a gas or contained in a

single liquid phase.

Examples:

1. influence of H+ on the esterification of carboxylic acids, such as

the formation of methyl acetate from acetic acid and methanol

2. The reaction between persulphate ions and iodide ions

3. oxidation of sulphur dioxide to sulphur trioxide in presence of

nitric oxide as catalyst...

2SO2(g) + O2(g) + NO(g) 2SO3(g) ...Here NO is a homogeneous

catalyst as it is present in gaseous phase.

HETEROGENEOUS CATALYSIS

Heterogeneous catalysts act in a different phase than the reactants.

Normally, the catalyst is a solid and reactants are fluids (liquids or gases). It is characterized by the presence of “active sites” on the catalyst surface

The reaction in which the reactants are in gaseous phase while the catalyst is a solid is called CONTACT catalysis since the reaction occurs by contact of reactants with the catalyst surface.

In contact process the catalyst is usually a finely divided metal or a gauze.

Examples:

1. The hydrogenation of a carbon-carbon double bond in the reaction between ethene and hydrogen in the presence of a nickel catalyst.

2. in the oxidation of sulfur dioxide on vanadium(V) oxide for the production of sulfuric acid.

SO2(g) + ½O2(g) + V2O5(s) SO3(g)

CHERECTARISTICS OF CATALYTIC

REACTIONS

1. A catalyst remains unchanged in mass and chemical composition at the end of the reaction.

2. A small quantity of catalyst is generally needed to produce almost unlimited reaction.

3. A catalyst is more effective when finely divided.

4. A catalyst is specific in its actio.

5. A catalyst cannot, in general, initiate a reaction.

6. A catalyst does not effect the final position of equilibrium, although it shortens the time required to establish the equilibrium.

7. Change of temperature alters the rate of a catalytic reactionas it would do for the samereaction without a catalyst.

PROMOTERS

A substance which, though itself not a catalyst, promotes the

activity of a catalyst is called a promoter.

Example:

Molybdenum (Mo) or aluminium oxide (Al2O3) promotes the activity

of iron catalyst in the Haber’s process for manufacture of amonia.

N2 + 2H2 + Fe + Mo 2NH3

(catalyst) (promoter)

CATALYTIC POISONING

A substance which destroys the activity of the catalyst to

accelerate a reaction, is called a poison and the process is called

catalytic poisoning.

Examples:

1. The platinum catalyst used in the oxidation of Sulphur dioxide

(contact process), is poisoned by arsenic oxide (As2O3)

SO2 + O2 + Pt + As2O3 2SO3

2. The iron catalyst used in Haber’s process of ammonia is

poisoned by H2S.

N2 + 2H2 + Fe + H2S 2NH3

(catalyst) (poison)

ACTIVATION ENERGY

AND CATALYSIS

The minimum amount of energy

required to cause a chemical reaction

is known as activation energy.

The Activation Energy determines

how fast a reaction occurs, the higher

Activation barrier, the slower the

reaction rate.

The lower the Activation barrier, the

faster the reaction.

Catalyst lowers the activation

energy for both forward and

reverse reactions.

THEORIES OF CATALYSIS

Intermediate compound formation theory The adsorption theory

In homogeneous catalysis,

catalyst forms an intermediate

compound with one of the

reactants.

The highly reactive

intermediate compound then

reacts with the second

reactants to yield the product,

releasing the catalyst.

A + B + C AB (C-catalyst)

1. A + C AC(AC-

intermediate)

2. AC +B AB + C

Here the catalyst functions by adsorption of the reacting molecules on its surface.

The process can be explained in four steps:

1. ADSORPTION OF REACTANT MOLECULES

2. FORMATION OF ACTIVATED COMPLEX

3. DECOMPOSITION OF ACTIVATED COMPLEX

4. DESORPTION OF PRODUCTS

There are two main theories of catalysis: Intermediate compound

formation theory and The adsorption theory

ACID-BASE CATALYSIS

A number of homogenous catalytic reactions are catalyzed by

acids or bases, these are often referred to as Acid-Base catalysts.

Arrhenius pointed out that acid catalysis was brought about by H+

ions supplied by strong acids, while base catalysis was caused by

OH- ions supplied by strong bases.

Example:

1. Inversion of cane sugar.

C12H22O11 + H2 + H+ C6H12O6 + C6H12O6

2. Decomposition of Nitramide.

NH2NO2 +H+ N2O + H2O

1. Not only H+ ions but all Bronsted acids (proton donors) cause

acid catalysis.

Thus the general acid catalysts are H+ , undissociated acids

(CH3COOH), cations of weak bases (NH4+ ) and water.

2. Not only OH- ions but all Bronsted bases(proton accepters) act

as base catalyst.

Thus the general base catalysts are OH- , undissociated bases,

anions of weak acids ( CH3COO- ) and water.

THE CATALYSIS BROUGHT ABOUT BY GENERAL ACIDS AND

BASES IS TERMED GENERAL ACID-BASE CATALYSIS.

GENERAL ACID-BASE CATALYSIS

MECHANISM OF ACID BASE CATALYSIS

In acid catalysis the proton donated Bronsted acid forms an

intermediate complex with the reactant, which then reacts to give

back the proton.

IN base catalysis the Bronsted base acceps a proton from

reactant to form an intermediate complex which then reacts or

decomposes to regenerate the base.

Example:

ENZYME CATALYSIS

Enzyme catalysis is the catalysis of chemical reactions by

specialized proteins known as enzymes.

Enzyme catalysis of chemical reactions occur with high selectivity

and rate in a small part of the enzyme macromolecule known as

the active site.

Each enzyme is produced in a particular living cell to catalyse a

reaction occurring in that cell.

The first enzyme was prepared by synthesis in the laboratory in

1969.

MECHANISM of enzyme

catalysis

The molecules of substrate which

have complementary shape fit into

the cavities just as key fits into a lock

( lock and key theory).

Due to the presence of active groups

the enzyme forms an activated

complex with the substrate which at

once decomposes to yield the

products.

E + S ES P + E

( complex )

Where E=enzyme , S= substrate ,

ES= activated complex , P=

products

CHARACTERISTICS OF ENZYME

CATALYSIS

1. Enzymes are the most efficient catalysts known

2. Enzyme catalysis is marked by absolute specificity.

3. The rate of enzyme catalysed reaction is maximum at the

optimum temperature.

4. The rate of enzyme catalysed reactions is maximum at the

optimum pH.

5. Enzymes are markedly inhabited or poisoned.

6. Catalytic activity of enzymes is greatly enhenced by the

presence of activators or coenzyme.

THANK YOU…