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Catalysis

The word catalysis was first introduced by Berzelius in 1836 to describe a number of experimental observations. Catalysis is defined as a phenomenon in which the presence of a foreign substance in a reaction can accelerate its rate without being used up in that reaction. These foreign substances are called as a catalyst. The catalyst can change the speed of a reaction without being used up in that reaction.

 

What is Catalysis?

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In a reaction, a catalyst reacts with one or more reactants and forms intermediates, which subsequently gives the final reaction product and regenerates the catalyst. For example, in a chemical reaction, where C represents the catalyst, A and B are reactants, and D is the product of the reaction between A and B.

A + C → AC --------(1)
B + AC
→ ABC ----------(2)
ABC
→ CD ---------(3)
CD
→ C + D ------- (4)

In the given mechanism, the catalyst is consumed in the first step of the reaction but subsequently produced in the last step, hence the overall reaction will be

A + B → D

The catalyst is regenerated in a reaction and often, only a small amount, is sufficient to increase the rate of the reaction. A catalyst provides an alternate path for the reaction that has a lower energy of activation Ea.

Activation Energy

Characteristics of a Catalyst

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  • The catalyst remains unchanged in mass and chemical composition at the end of the reaction.
  • Usually, a small amount of the catalyst is needed for a chemical transformation, i.e., because the catalyst itself is not consumed in a reaction.
  • In heterogeneous catalysis, the rate of the reaction increases with the increasing surface area of the catalyst.
  • In reversible reactions, the catalyst helps to attain equilibrium quickly but does not affect the composition of the reaction mixture in equilibrium. The value of the equilibrium constant remains unchanged due to the presence of the catalyst.
  • A catalyst can accelerate the rate of reaction but it can’t initiate the reaction.
  • All catalysts are specific for the reactant. A certain catalyst can react with a particular reactant only.
  • A catalyst increases the speed of the reaction but it can’t change the nature and type of product.

Type of Catalysis Process

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Catalysis can be divided into two main classes:

Homogenous catalysis


In homogenous catalysis, the catalyst and reactants are in the same phase or physical state. For example, the formation of sulfur trioxide gas in the presence of nitric acid gas.
NO(g)
2SO2(g) + O2(g)→ 2SO3(g)


Here both reactants (SO2 + O2) and the catalyst (NO) are in a gaseous state. Another example of homogenous catalysis is inversion of sugar, in which both the reactants and catalyst are in liquid state.
H+
C12H22O11 + H2O → C6H12O6 + C6H12O6


The hydrolysis of ester is also an example of homogenous catalysis.
H+
CH3COOCH3 +H2O → CH3COOH + CH3OH


In homogenous catalysis, the function of the catalyst is to bring about a reaction between molecules which do not posses enough energy to enter into a chemical combination by providing an alternative path in which lesser energy f or activation is required. For example ; for the given reaction

A + B → AB in the presence of catalyst X,
the reaction mechanism will be;

Reaction at Equilibrium

$AX+b\overset{k_3}{\rightarrow}AB+X$

Energy of activation

Here Ea1 is the activation energy leading to the formation of an intermediate complex. Ea2 is the activation energy for the change of the intermediate complex into products. Ea3 is the activation energy of the un-catalyzed reaction.

Heterogeneous catalysis


The phase or physical state of a catalyst and a reactant is different in heterogeneous catalysis. For example, the decomposition of potassium chlorate in the presence of solid catalyst MnO2

$2KClO_{3{(s)}}\overset{M_{2{(s)}}}{\rightarrow}2KCl_{(s)} + 30_{2{(g)}}$
Other examples of heterogeneous catalysis are
$2H_2O_{2{(l)}}\overset{\frac{Pt}{Au(s)}}{\rightarrow}2H_{2}O+O_{2}$
$SO_{2{g}}+O_{2{g}}\overset{Pt(s)}{\rightarrow}2SO_{3{(g)}}$

Following are the common steps in heterogeneous catalysis:

  1. Adsorption: The Reactant gets adsorbed on the catalyst surface.
  2. Recombination: Reactant are combined with the catalyst to form an intermediate reactant-catalyst complex.
  3. Dissociation: Reactant converts into the product and the product gets dissociated from an intermediate complex.
  4. Diffusion: The product gets diffused from the catalyst surface and moves away from the reaction site.
  5. Desorption: The product gets desorbed from the metal surface and the catalyst surface gets free for other reactants.
  6. The best example of the heterogeneous catalysis is the hydrogenation of ethene on a solid metal surface like nickel which acts as a catalyst and forms ethane.

Enzyme catalysis

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Enzymes are the complex organic compounds which are produced by living plants and animals. They act as a catalyst for biochemical reactions in living bodies. They disperse in water and form a colloidal solution, hence they follow colloidal properties and heterogeneous catalysis.

Compared to inorganic catalysts, enzymes are very specific for reactants, temperature, pH of medium and other reaction conditions. Some common examples of enzymes are,

Characteristics of enzymes
  • Enzymes are nitrogenous compounds, which get consumed during biochemical reactions. They can’t be synthesized in an artificial manner. ·
  • Certain inorganic substances like ammonium salts are required for the production of enzymes and the presence of co enzyme are essential for enzymes.·
  • The enzymes accelerate the biochemical reactions by decreasing the activation energy. They do not alter the nature and amount of product.·
  • A very small amount of catalyst is enough to accelerate the reaction.
  • They are specific in their action and catalyze reactions at lower temperature. At high temperature, they lose their specific nature and get denatured.

Reaction Rate

  • Enzymes are very sensitive to the pH of the reaction medium. For each enzyme, there is an optimum p value at which the rate of the reaction is at its maximum pH value and at which the rate of the reaction is maximum.
  • Enzymes are very specific for reactants. There are some active sites on the enzyme surface, known as the active site, in which only a certain reactant fits and forms a product. It is same as a Lock and Key. There is always a certain key for a lock. This model of enzyme catalysis in also called the Lock and Key model.

Enzymatic Reaction

  • Enzyme catalysis can be explained by using the steady state theory. In enzyme catalysis, an enzyme provides a catalytic surface to reactant molecules and forms a transition state. Since the enzyme catalysis follows a steady state concept, the production and consumption of the transition state proceed at the same rate and the concentration of transition state stays constant.

Mechanism of Catalysis

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Acid- base catalysis


There are many homogenous catalytic reactions which are catalyzed by acid and bases. Such type of reactions are called as acid-base catalyzed reactions. There are three kinds of acid-base catalysis;
·
Reactions catalyzed by hydrogen ion: For example, inversion of sugar.
·
Reactions catalyzed by hydroxyl ion: For example, dimerization of acetone.
·
Reaction catalyzed by both hydrogen and hydroxyl ion: for example, hydrolysis of ester to form acid and alcohols. The Keto-enol tautomerism (conversion of keto form to enol form) is the best example of acid-base catalysis.

Keto Enol Reaction


Keto Enol Reaction Mechanism


Mechanism of Keto Enol Reaction

Aldol condensation, hydrolysis of p-nitrophenylacetate with imidazole are some other examples of acid base catalysis.

Acid Base Catalysis

Covalent catalysis

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In some catalytic reactions, the covalent bonds are formed between enzymes and subtract. Such type of catalysis is known as covalent catalysis. For example, A-B reactants react with enzyme E-H to form an intermediate E-A, which further reacts with water to form the final product A-OH.

$H_2O+A-B\overset{slow}{\rightarrow}AOH+BH$

A-B + E-H ——> E-A + BH

$E-A+H_2O\underset{Fast}{\rightarrow}A-OH+E-H$

In a covalent catalysis, there are some steps,
  • Attack of the nucleophilic reactant on an enzyme
  • Withdrawal of electrons from the reactant
  • Removal of nucleophilic group and regenerate enzyme
Covalent catalys is generally found in enzymes like chymotrypsin and trypsin, and in this process an acyl-enzyme intermediate is formed. The Covalent catalysis of acetylcholine esterase is as follows

Acetylcholine Esterase

Enzymes That Form Covalent Intermediates
Enzymes
Reacting group
Covalent intermediate
Chymotrypsin Elastase Esterases Subtilisin Thrombin Trypsin Serine Acyl Serine
Glyceraldehyde-3-phosphate dehydrogenase Papain Cystine Acyl Cystine
Alkaline phosphatase Phosphoglucomutase Serine Phosphoserine
Phosphoglycerate mutase Succinyl-CoA synthetase Histidine Phosphohistidine
Aldolase Decarboxylases Pyridoxal phosphate-dependent enzymes Amino Group Schiff Base

Phase Transfer Catalysis

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It is a special form of heterogeneous catalysis. The phase transfer catalysis involves changes in the phase of the reactant during the reaction between the reactant and the catalyst. Phase transfer catalysis is mainly used for reactions between anions and organic substances. Since many anions are soluble in water, but not in organic solvents, and organic reactants are not soluble in water, the phase transfer catalyst acts as a shuttling agent between organic solvent and water.

Phase transfer catalyst bound with the anion carries it to the organic solvent so that it can easily react with the organic reactant to form a product. This mechanism is called the extraction mechanism.

Phase Transfer Catalysis

Oxidation Catalyst

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Oxidation metals are catalytic metals like platinum or palladium because these metals convert exhaust gas pollutants into harmless gases by chemical oxidation. These metals oxidize pollutants like carbon monoxide, hydrocarbons and some organic compounds like aldehyde and ketones.

Carbon Monoxide
CO + $\frac{1}{2}$ O2 → CO2
Hydrocarbons CmHn + (m + $\frac{n}{4}$) O2 → m CO2 + $\frac{n}{2}$ H2O
Aldehydes, Ketones, etc. CmHnO + (m + $\frac{n}{4}$ - 0.5) O2 → m CO2 + $\frac{n}{2}$ H2O
Hydrogen H2 + $\frac{1}{2}$ O2 → H2O

These oxidation catalysts are present in a stainless steel canister which contains a honeycomb structure known as a substrate or catalyst support. This canister contains a large amount of interior surface area which is coated with catalytic metals. This canister is also known as the Catalytic Converter or the OxyCat. It converts Hazardous Air Pollutants up to maximum percentage.

NOx
CO
Non-methane hydrocarbons (NMHC)
Volatile organic compounds (VOC)
Formaldehyde (CH2O)
Hazardous Air Pollutants (HAPs)
Nil
70-99% 40-90% 60-99% 60-99% 60-99%

Catalytic Converter

Catalytic oxidation of ammonia

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The catalytic oxidation of ammonia is the basic reaction of the Oswald process for the formation of nitric acid. The oxidation of ammonia in the presence of platinum forms NO, a brown colored gas and water vapour.

4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (g) (ΔH = −950 kJ/mol)

Catalytic Oxidation of Ammonia


The oxidation of ammonia is carried out in the presence of various catalysts like Pt/CuO/Al2O3 or copper-cerium composite catalyst which is prepared by co-precipitation of copper nitrate and cerium nitrate. The oxidation of ammonia is an exothermic reaction which is initiated by some catalyst. In the Ostwald process a wire mesh consisting of platinum and rhodium is used as a catalyst.

Haber Process

Since the reaction is exothermic in nature, a high temperature is required to initiate the reaction. Hence the percentage yield is very low due to the break down of products.

More topics in Catalysis
Catalyzed Reaction Homogeneous Catalyst
Heterogeneous Catalyst Asymmetric Catalysis
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