A Catalyst is defined as a substance which alters the rate of a chemical reaction, but remains chemically unchanged at the end of the reaction. A catalyst is often present in a very small proportion.
For example; the reaction between H2 and
O2 to form water is very slow at ordinary temperature, but
proceed more rapidly in the presence of platinum. Platinum acts as a catalyst.
Similarly, KClO3 decomposes
much more rapidly in the presence of a small amount of MnO2. HCl is
oxidized to Cl2 in the presence of CuCl2.
4HCl + O2 → 2H2 O + 2Cl2
The process, which takes place in the presence of a catalyst, is called catalysis.
A catalyst provides a new reaction path with a low activation energy barrier, as shown in the above figure. A greater number of molecules are now able to get over the new energy barrier and reaction rate increases.
Types of Catalysis:
There are two types of
catalysis:
- Homogeneous Catalysis
- Heterogeneous Catalysis
a) Homogeneous Catalysis:
In this process, the catalyst and the reactants are in the same phase and the reacting system is homogeneous throughout. The catalyst is disturbed uniformly throughout the system. For example:
The formation of SO3 (g) and O2 (g) in the lead chamber process for the manufacture of Sulphuric acid, needs NO (g) as a catalyst. Both the reactants and the catalyst are gases.
2SO2 (g) +
O2 (g) ↔ 2SO3 (g)
Esters are hydrolyzed in
the presence of H2 SO4. Both the reactants and
the catalyst are in the solution state.
CH3COOC2H5
(aq) + H2O (l) ↔ CH3COOH (aq) +
C2H5OH (aq)
b) Heterogeneous Catalysis:
In such systems, the
catalyst and the reactants are in different phases. Mostly, the catalyst is in
the solid phase, while the reactants are in the gaseous or liquid phase. For
example:
Oxidation of Ammonia to
NO in the presence of platinum gauze helps us to manufacture HNO3.
4NH3 (g) +
5O2 (g) ↔ 4NO (g) + 6H2O (g)
Hydrogenation of
unsaturated organic compounds is catalyzed by finely divided Ni, Pd or Pt.
CH2 =
CH2 (g) + H2 (g) ↔ CH3 – CH3 (g)
Characteristics of a Catalyst:
There are many types of catalyst with varying chemical compositions, but the followings features are common to most of them.
- A catalyst remains unchanged in mass and chemical composition at the end of reaction. It may not remain in the same physical state. MnO2 is added as a catalyst for the decomposition of KClO3 in the form of granules. It is converted to fine powder at the end of reaction. It has been found in many cases that the shining surfaces of the solid catalyst become dull.
- Sometimes, we need a trace of a metal catalyst to affect very large amount of reactants. For example, 1 mg of fine platinum powder can convert 2.5 dm3 of H2 and 1.25 dm3 of O2 to water. Dry HCl and NH3 don’t combine, but in the presence of trace of moisture, they give dense white fumes of NH4Cl. Thousands of dm3 of H2O2, can be decomposed in the presence of 1 g of colloidal platinum.
- A catalyst is more effective, when it is present in a finely divided form. For example, a lump of platinum will have much less catalytic activity than colloidal platinum. In the hydrogenation of vegetable oils finely divided nickel is used.
- A catalyst cannot start a reaction, which is not thermodynamically feasible. It is now considered that a catalyst can initiate a reaction. The mechanism of a catalyzed reaction is different from that of an uncatalyzed reaction.
- A catalyst is specific in its action. When a particular catalyst works for one reaction, it may not necessarily work for any other reaction. If different catalysts are used for the same reactant, then the products may change. For example; formic acid is decomposed by Al2O3 to H2O and CO while Cu causes its decomposition to H2 and CO2.
HCOOH → H2 + CO2
- Temperature affects the role of catalyst. Some catalysts are physically altered by a change in temperature and hence their catalytic power will be decreased. For example, colloidal catalysts like platinum may be coagulated will the rise in temperature.
- Catalytic poisoning happens due to presence of trace amounts of foreign substances which render them ineffective. Such substances are called poisons. The poisoning, the poison reacts chemically with the catalysts. For example;
- The presence of CO as an impurity with hydrogen decreases the catalytic activity of catalyst in the Haber’s process for the manufacture of NH3.
- The manufacture of H2SO4 in the contact process needs platinum as a catalyst. The traces of arsenic present as impurities in the reacting gases makes platinum ineffective. That’s why arsenic purifier is employed in the contact process.
Activation of Catalyst:
Such a substance which
promotes the activity of a catalyst is called a promotor or activator. It is
also called as “catalyst for a catalyst.” For example,
Hydrogenation of
vegetable oils is accelerated by nickel. The catalytic activity of nickel can
be increased by using copper and tellurium.
In Haber’s process for
the manufacture of ammonia, iron is used as a catalyst. If small amounts of
some high melting oxides like aluminum oxide, chromium oxide or rare earth
oxides are added, they increase the efficiency of iron.
Negative Catalysis:
When the rate of
reaction is retarded by adding a substance, then it is said to be a negative
catalyst or inhibitor. For example; tetraethyl lead is added to petrol, because
it saves the petrol from pre – ignition.
Auto catalyst:
In some of the reactions, a product formed acts as a catalyst.
This phenomenon is called auto catalyst. For example;
When copper is allowed to react with nitric acid, the reaction is
slow in the beginning. It gains the speed gradually and finally becomes very
fast. This is due to the formation of nitrous acid during the reaction, which
accelerates the process.
The reaction of oxalic acid with acidified KMnO4 is slow at the beginning, but after sometimes, MnSO4 produced in the reaction makes it faster.
2KMnO4 + 3H2SO4 + 5(COOH)2 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O
Enzyme Catalysis:
Enzyme are the complex protein molecules and catalyze the organic
reactions in the living cells. Many enzymes have been identified and obtained
in the pure crystalline state. However, the first enzyme was prepared in the
laboratory in 1969. For example;
Urea undergoes hydrolysis into NH3 and
CO2 in the presence of enzyme urease present in soya bean.
(NH2)2 CO + H2O → 2NH3 +
CO2
Concentrated sugar solution undergoes hydrolysis into glucose and
fructose by an enzyme called invertase, present in the yeast.
C12H22O11 +
H2O → C6H12O6 + C6H12O6
Glucose is converted into ethanol by the enzyme zymase in the
yeast.
C6H12O6 → 2C2H5OH
+ 2CO2
Enzymes have active centers on their surfaces. The molecules of a
substrate fit into their cavities just as a key fits into a lock as shown in
the figure. The substrate molecules enter the cavities, form the complex,
reactants and the products get out of the cavity immediately. Michaulis and Menter (1913) proposed the following
mechanism for enzyme catalysis.
E + S ↔ ES → P + E
Where E = enzyme, S = substrate (reactant), ES = activated complex, P = product.
Characteristics of Enzyme Catalysis:
The role of enzyme as catalysts is like inorganic heterogeneous catalysts. They are unique in their efficiency and have a high degree of specificity. For example;
- Enzyme are the most efficient catalysts known and they lower the energy of activation of a reaction.
- Enzyme catalysis is highly specific, for example, urease catalysis the hydrolysis of urea only and it cannot hydrolyze any other amide even methyl urea.
- Enzyme catalytic reactions have the maximum rates at an optimum temperature.
- The Ph of the system also controls the rates of the enzyme catalyzed reaction and the rate passes through a maximum at a particular Ph, known as optimum Ph. The activity of enzyme catalyst is inhibited by a poison.
- The catalytic activity of enzymes is greatly enhanced by the presence of a co-enzyme or activator.
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