Chemistry

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Emulsions

Emulsions are colloidal solution in which a liquid is dispersed in an another liquid. Generally there are two types of emulsions.

1. Oil in water (O/W)
2. Water in oil (W/O)

Example:

• Milk is example of the oil in water type emulsion.
• Stif greases are emulsion of water in oil i.e. water dispersed in lubricating oil.
• The process of preparation of emulsion by the dispersal of one liquid in another liquid is called Emulsification.
• A colloid mill can be used as a homogeniser to mix the two liquid. To have a stable emulsion a small amount of emulsifier or emulsification agent is added.

Several Types of Emulsifiers are known.

1. Most of the lyophillic colloids also act as emulsifiers. Example: glue, gelatine.
2. Long chain compounds with polar groups like soap and sulphonic acids.
3. Insoluble powders like clay and lamp black also act as emulsifiers.

Identification of Types of Emulsion

The two types of emulsions can be identified by the following tests.

(i) Dye Test

A small amount of dye soluble in oil is added to the emulsion. The emulsion is shaken well. The aqueous emulsion will not take the colour whereas oily emulsion will take up the colour of the dye.

(ii) Viscosity Test

Viscosity of the emulsion is determined by experiments. Oily emulsions will have higher value than aqueous emulsion.

(iii) Conductivity Test

Conductivity of aqueous emulsions are always higher than oily emulsions.

(iv) Spreading Test

Oily emulsions spread readily than aqueous emulsion when spread on an oily surface.

Deemulsification:

Emulsion can be separated into two separate layers. The process is called Deemulsification.

Various Deemulsification Techniques are Given Below

1. Distilling of one component
2. Adding an electrolyte to destroy the charge
3. Destroying the emulsifir using chemical methods
4. Using solvent extraction to remove one component
5. By freezing one of the components
6. By applying centrifugal force
7. Adding dehydrating agents for water in oil (W/O) type
8. Using ultrasonic waves.
9. Heating at high pressures.

Inversion of Phase:

The change of W/O emulsion into O/W emulsion is called inversion of phases.

For example:

An oil in water emulsion containing potassium soap as emulsifying agent can be converted into water in oil emulsion by adding CaCl₂ or AlCl₃. The mechanism of inversion is in the recent developments of research.

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Colloid, Dispersion Phase and Dispersion Medium

Origin of study of colloid starts with Thomas Graham who observed diffusion of that a solution of sugar, urea or sodium chloride through a membrane but not glue, gelatine or gum. He called the former substances as crystalloids and the latter as colloids (In Greek, kola as gum, eidos-like).

Later it was realised that any substance can be converted into a colloid by reducing its particle size to 1-200nm.

Hence, colloid is a homogeneous mixture of two substances in which one substance (smaller proportion) is dispersed in another substance (large proportion).

In a colloid, the substance present in larger amount is called dispersing medium and the substance present in less amount is called dispersed phase.

Classifications of Colloidal Solution

Probably the most important colloidal systems have dispersed phase as solid and the dispersion medium as liquid. If the dispersion medium considered is water, then the colloids are referred as hydrosols or aquasols.

If the dispersion medium is an alcohol, the colloid is termed as alcosol, and if benzene is the dispersion medium, it is called as benzosol.

One more type of classification is based on the forces acting between the dispersal phase and dispersion medium.

In lyophillic colloids definite attractive force or affinity exists between dispersion medium and dispersed phase. Examples: sols of protein and starch. They are more stable and will not get precipitated easily. They can be brought back to colloidal solution even after the precipitation by addition of the dispersion medium.

In a lyophobic colloids, no attractive force exists between the dispersed phase and dispersion medium. They are less stable and precipitated readily, but can not be produced again by just adding the dispersion medium. They themselves undergo coagulation after a span of characteristic life time.

They are called irreversible sols
Examples: sols of gold, silver, platinum and copper.

The following table lists the types of colloids based on the physical states of dispersed phase and dispersion medium.

Classification of Colloids Based on the Physical State of Dispersed Phase and Dispersion Medium.

Preparation of Colloids

Many lyophillic substances are made in their colloidal form by warming with water. Rubber forms colloidal solution with benzene. Soap spontaneously forms a colloidal solution by just mixing with water.

In general, colloidal are prepared by the following methods.

1. Dispersion Methods:
In this method larger particles are broken to colloidal dimension.

2. Condensation Methods:
In this method, smaller atom or molecules are converted into larger colloidal sized particles.

1. Dispersion Methods

(i) Mechanical Dispersion:

Using a colloid mill, the solid is ground to colloidal dimension. The colloid mill consists of two metal plates rotating in opposite direction at very high speed of nearly 7000 revolution/minute.

The colloidal particles of required colloidal size is obtained by adjusting the distance between two plates. By this method, colloidal solutions of ink and graphite are prepared.

(ii) Electro Dispersion:

A brown colloidal solution of platinum was first prepared by George Bredig in 1898. An electrical arc is struck between electrodes dispersed in water surrounded by ice. When a current of 1 amp/100 V is passed an arc produced forms vapours of metal which immediately condense to form colloidal solution.

By this method colloidal solution of many metals like copper, silver, gold, platinum, etc. can be prepared Alkali hydroxide is added as an stabilising agent for the colloidal solution.

Svedberg modified this method for the preparation of non aqueous inflammable liquids like pentane, ether and benzene, etc using high frequency alternating current which prevents the decomposition of liquid.

(iii) Ultrasonic Dispersion

Sound waves of frequency more than 20kHz (audible limit) could cause transformation of coarse suspension to colloidal dimensions.

Claus obtained mercury sol by subjecting mercury to sufficiently high frequency ultrasonic vibrations.

The ultrasonic vibrations produced by generator spread the oil and transfer the vibration to the vessel with mercury in water.

(iv) Peptisation:

By addition of suitable electrolytes, precipitated particles can be brought into colloidal state. The process is termed as peptisation and the electrolyte added is called peptising or dispersing agent

2. Condensation Methods:

When the substance for colloidal particle is present as small sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods. Here care should be taken to produce the particle with colloidal size otherwise precipitation will occur. Various chemical methods for the formation of colloidal particles.

(i) Oxidation

Sols of some non metals are prepared by this method.

(a) When hydroiodic acid is treated with iodic acid, I₂ sol is obtained.
HIO₃ + 5HI → 3H₂O + I₂ (Sol)

(b) When O₂ is passed through H₂Se, a sol of selenium is obtained.
H₂Se + O₂ → 2H₂O + Se (sol)

(ii) Reduction

Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For example: Gold sol is prepared by reduction of auric chloride using formaldehyde.

2 AuCl₃ + 3HCHO + 3H₂O → Au(sol) + 6HCl + 3HCOOH

(iii) Hydrolysis

Sols of hydroxides of metals like chromium and aluminium can be produced by this method.

For Example,
FeCl₃+3H₂O → Fe(OH)₃+3HCl

(iv) Double Decomposition

For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
As₂O₃ +3H₂S → As₂S₃ + 3H₂O

(v) Decomposition

When few drops of an acid is added to a dilute solution of sodium thiosulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth within the size of colloidal dimensions.

3. By Exchange of Solvent:

Colloidal solution of few substances like phosphorous or sulphur is obtained by preparing the solutions in alcohol and pouring them into water. As they are insoluble in water, they form colloidal solution.

P in alcohol + water → Psol.

Purification of Colloids

The colloidal solutions due to their different methods of preparation may contain impurities. If they are not removed, they may destablise and precipitate the colloidal solution. This is called coagulation. Hence the impurities mainly electrolytes should be removed to increase the stabilisation of colloid. Purification of colloidal solution can be done by the following methods.

1. Dialysis
2. Electrodialysis
3. Ultrafilteration.

1. Dialysis

In 1861, T. Graham separated the electrolyte from a colloid using a semipermeable membrane (dialyser). In this method, the colloidal solution is taken in a bag made up of semipermeable membrane. It is suspended in a trough of flowing water, the electrolytes diffuse out of the membrane and they are carried away by water.

2. Electrodialysis

The presence of electric field increases the speed of removal of electrolytes from colloidal solution. The colloidal solution containing an electrolyte as impurity is placed between two dialysing membranes enclosed into two compartments filled with water.

When current is passed, the impurities pass into water compartment and get removed periodically. This process is faster than dialysis, as the rate of diffusion of electrolytes is increased by the application of electricity.

3. Ultrafiltration

The pores of ordinary filter papers permit the passage of colloidal solutions. In ultra filtrations, the membranes are made by using collodion cellophane or visiking. When a colloidal solution is filtered using such a filter, colloidal particles are separated on the filter and the impurities are removed as washings.

This process is quickened by application of pressure. The separation of sol particles from electrolyte by filteration through an ultrafilter is called ultrafiltration. Collodion is 4% solution of nitrocellulose in a mixture of alcohol and water.

Properties of Colloids

1. Colour

The colour of a sol is not always the same as the colour of the substance in the bulk. For example bluish tinge is given by diluted milk in reflected light and reddish tinge in transmitted light.

Colour of the sol, generally depends on the following factors.

• Method of preparation
• Wavelength of source of light.
• Size and shape of colloidal particle
• Whether the observer views the reflected light or transmitted light.

2. Size

The size of colloidal particles ranges from 1nm (10^-9m) to 1000 nm (10^-6m) diameter.

3. Colloidal Solutions are Heterogeneous in Nature Having two Distinct Phases

Though experiments like dialysis, ultrafiltration and ultracentrifuging clearly show the heterogeneous nature in the recent times colloidal solution are considered as border line cases.

4. Filtrability

As the size of pores in ordinary filter paper are large the colloidal particles easily pass through the ordinary filter papers.

5. Non-Setting Nature

Colloidal solutions are quite stable i.e. they are not affcted by gravity.

6. Concentration and Density

When the colloidal solution is dilute, it is stable. When the volume of medium is decreased coagulation occurs. Generally, density of sol decreases with decrease in the concentration.

7. Diffusability

Unlike true solution, colloids diffuse less readily through membranes.

8. Colligative Properties

The colloidal solutions show colligative properties i.e. elevation of boiling point, depression in freezing point and osmotic pressure. Measurements of osmotic pressure is used to find molecular weight of colloidal particle.

9. Shape of Colloidal Particles

It is very interesting to know the various shapes of colloidal particles. Here are some examples

10. Optical Property

Colloids have optical property. When a homogeneous solution is seen in the direction of light, it appears clear but it appears dark, in a perpendicular direction.

But when light passes through colloidal solution, it is scattered in all directions. This effect was first observed by Faraday, but investigations are made by Tyndall in detail, hence called as Tyndall effect.

The colloidal particles absorb a portion of light and the remaining portion is scattered from the surface of the colloid. Hence the path of light is made clear.

11. Kinetic Property

Robert Brown observed that when the pollen grains suspended in water were viewed through ultra microscope, they showed a random, zigzag ceaseless motion.

This is called Brownian movement of colloidal particles.

This can be explained as follows

The colloidal sol particles are continuously bombarded with the molecules of the dispersion medium and hence they follow a zigzag, random, continuous movement.

Brownian Movement Enables Us,

I. To calculate Avogadro number.

II. To confirm kinetic theory which considers the ceaseless rapid movement of molecules that increases with increase in temperature.

III. To understand the stability of colloids:

As the particles in continuous rapid movement they do not come close and hence not get condensed. That is Brownian movement does not allow the particles to be acted on by force of gravity.

12. Electrical Property

(i) Helmholtz Double Layer

The surface of colloidal particle adsorbs one type of ion due to preferential adsorption. This layer attracts the oppositely charged ions in the medium and hence at the boundary separating the two electrical double layers are setup. This is called as Helmholtz electrical double layer.

As the particles nearby are having similar charges, they cannot come close and condense. Hence this helps to explain the stability of a colloid.

(ii) Electrophoresis:

When electric potential is applied across two platinum electrodes dipped in a hydrophilic sol, the dispersed particles move toward one or other electrode. This migration of sol particles under the influence of electric field is called electrophoresis or cataphoresis.

If the sol particles migrate to the cathode, then they posses positive (+) charges, and if the sol particles migrate to the anode then they have negative charges(-). This from the direction of migration of sol particles we can determine the charge of the sol particles. Hence electrophoresis is used for detection of presence of charges on the sol particles.

Few Examples of Charges of Sols Detected by Electrophoresis are Given Below:

(iii) Electro Osmosis

A sol is electrically neutral. Hence the medium carries an equal but opposite charge to that of dispersed particles. When sol particles are prevented from moving, under the influence of electric field the medium moves in a direction opposite to that of the sol particles. This movement of dispersion medium under the influence of electric potential is called electro osmosis.

13. Coagulation or Precipitation

The flocculation and settling down of the sol particles is called coagulation.
Various method of coagulation are given below:

• Addition of electrolytes
• Electrophoresis
• Mixing oppositively charged sols.
• Boiling

Addition of Electrolytes

A negative ion causes the precipitation of positively charged sol and vice versa. When the valency of ion is high, the precipitation power is increased. For example, the precipitation power of some cations and anions varies in the following order

Al³⁺ > Ba²⁺ > Na⁺, Similarly [Fe(CN)₆]^3- > SO₄^2- > Cl^–

The precipitation power of electrolyte is determined by finding the minimum concentration (millimoles/lit) required to cause precipitation of a sol in 2 hours. This value is called flocculation value. The smaller the flocculation value greater will be precipitation.

Electrophoresis

In the electrophoresis, charged particles migrate to the electrode of opposite sign. It is due to neutralization of the charge of the colloids. The particles are discharged and so they get precipitated.

By Mixing two Oppositively Charged Sols

When colloidal sols with opposite charges are mixed mutual coagulation takes place. It is due to migration of ions from the surface of the particles.

By Boiling

When boiled due to increased collisions, the sol particles combine and settle down.

14. Protective Action

Generally, lyophobic sols are precipitated readily even with small amount of electrolytes. But they are stabilised by addition of a small amount of lyophillic colloid.

A small amount of gelatine sol is added to gold sol to protect the gold sol.

Zsigmondy introduced the term ‘gold number’ as a measure of protecting power of a colloid. Gold number is defined as the number of milligrams of hydrophilic colloid that will just prevent the precipitation of 10ml of gold sol on the addition of 1ml of 10% NaCl solution. Smaller the gold number greater the protective power.

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Zeolite Catalysis

The details of heterogeneous catalysis will be in complete, if zeolites are not discussed. Zeolites are microporous, crystalline, hydrated, alumino silicates, made of silicon and aluminium tetrahedron. There are about 50 natural zeolites and 150 synthetic zeolites. As silicon is tetravalent and aluminium is trivalent, the zeolite matrix carries extra negative charge.

To balance the negative charge, there are extra framework cations for example, H⁺ or Na⁺ ions. Zeolites carrying protons are used as solid acid catalysts and they are extensively used in the petrochemical industry for cracking heavy hydrocarbon fractions into gasoline, diesel, etc., Zeolites carrying Na⁺ ions are used as basic catalysts.

One of the most important applications of zeolites is their shape selectivity. In zeolites, the active sites namely protons are lying inside their pores. So, reactions occur only inside the pores of zeolites.

Reactant Selectivity:

When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal, this selectivity is called reactant shape selectivity.

Transition State Selectivity:

If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed.

Product Selectivity:

It is encountered when certain product molecules are too big to diffuse out of the zeolite pores.

Phase Transfer Catalysis:

Suppose the reactant of a reaction is present in one solvent and the other reactant is present in an another solvent. The reaction between them is very slow, if the solvents are immiscible. As the solvents form separate phases, the reactants have to migrate across the boundary to react. But migration of reactants across the boundary is not easy.

For such situations a third solvent is added which is miscible with both. So, the phase boundary is eliminated, reactants freely mix and react fast. But for large scale production of any product, use of a third solvent is not convenient as it may be expensive.

For such problems phase transfer catalysis provides a simple solution, which avoids the use of solvents. It directs the use a phase transfer catalyst (a phase transfer reagent) to facilitate transport of a reactant in one solvent to the other solvent where the second reactant is present. As the reactants are now brought together, they rapidly react and form the product.

Example:

Substitution of Cl^– and CN^– in the following reaction.
R-Cl + NaCN → R-CN + NaCl

organic phase aqueous phase organic phase aqueous phase

R – C l = 1 – chlorooctane
R – C N = 1 – cyanooctane

By direct heating of two phase mixture of organic 1-chlorooctane with aqueous sodium cyanide for several days, 1-cyanooctane is not obtained. However, if a small amount of quaternary ammonium salt like tetraalkylammoniumchloride is added, a rapid transition of 1-cyanooctane occurs in about 100% yield after 1 or 2 hours.

In this reaction, the tetraalkylammonium cation, which has hydrophobic and hydrophilic ends, transports CN^– from the aqueous phase to the organic phase using its hydrophilic end and facilitates the reaction with 1-chloroocatne as shown below:

So phase transfer catalyst, speeds up the reaction by transporting one reactant from one phase to another.

Nano Catalysis:

Nano materials such a metallic nano particles, metal oxides, etc., are used as catalyst in many chemical transformation, Nanocatalysts carry the advantages of both homogeneous and heterogeneous catalyses.

Like homogeneous catalysts, the nanocatalysts give 100% selective transformations and excellent yield and show extremely high activity. Like the heterogeneous catalysts, nanocatalysts can be recovered and recycled. Nanocatalysts are actually soluble heterogeneous catalysts. An example for nanoparticles catalysed reaction is given below.

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Enzyme Catalysis

Enzymes are complex protein molecules with three dimensional structures. They catalyse the chemical reaction in living organism. They are often present in colloidal state and extremely specific in catalytic action. Each enzyme produced in a particular living cell can catalyse a particular reaction in the cell.

Some common examples for enzyme catalysis

1. The peptide glycyl L-glutamyl L-tyrosin is hydrolysed by an enzyme called pepsin.

2. The enzyme diastase hydrolyses starch into maltose
2(C₆H₁₀O₅)_n + nH₂O → nC₁₂H₂₂O₁₁

3. The yeast contains the enzyme zymase which converts glucose into ethanol.
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

4. The enzyme micoderma aceti oxidises alcohol into acetic acid.
C₂H₅OH + O₂ → CH₃COOH + H₂O

5. The enzyme urease present in soya beans hydrolyses the urea.
NH₂ – CO – NH₂ + H₂O → 2NH₃ + CO₂

Mechanism of Enzyme Catalysed Reaction

The following mechanism is proposed for the enzyme catalysis
E + S ⇄ ES → P + E

Where E is the enzyme, S the substrate (reactant), Es represents activated complex and P the products.

Enzyme Catalysed Reaction show Certain General Special Characteristics.

(i) Effective and efficient conversion is the special characteristic of enzyme catalysed reactions. An enzyme may transform a million molecules of reactant into product in a minute.
For eg. 2H₂O₂ → 2H₂O + O₂

For this reaction, the activation energy is 18k cal/mole without a catalyst. With colloidal platinum as a catalyst the activation energy is 11.7 kcal/mole.

But with the enzyme catalyst the activation energy of this reaction is less than 2kcal/mole.

(ii) Enzyme catalysis is highly specific in nature.

H₂N-CO-NH₂ + H₂O → 2NH₃ + CO₂

The enzyme urease which catalyses the reaction of urea does not catalyse the following reaction of methyl urea

H₂N-CO-NH-CH₃ + H₂O → No reaction

(iii) Enzyme catalysed reaction has maximum rate at optimum temperature. At first rate of reaction increases with the increase of temperature, but above a particular temperature the activity of enzyme is destroyed. The rate may even drop to zero. The temperature at which enzymic activity is high or maximum is called as optimum temperature.

For Example:

• Enzymes involved in human body have an optimum temperature 37°C/98°F
• During high fever, as body temperature rises the enzymatic activity may collapse and lead to danger.

(iv) The rate of enzyme catalysed reactions varies with the pH of the system. The rate is maximum at a pH called optimum pH.

(v) Enzymes can be inhibited i.e. poisoned. Activity of an enzyme is decreased and destroyed by a poison. The physiological action of drugs is related to their inhibiting action.

Example: Sulpha drugs. Penicillin inhibits the action of bacteria and used for curing diseases like pneumonia, dysentery, cholera and other infectious diseases.

(vi) Catalytic activity of enzymes is increased by coenzymes or activators. A small non protein (vitamin) called a coenzyme promotes the catalytic activity of enzyme.

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Catalysis

In 1836 Berzelius identified certain substances loosen the bond in the reacting molecules and increased the rate of the reaction. But he also found these substances didn’t undergo any change chemically. In order to indicate the property, he gave them the name catalyst. (In greek, kata-wholly, lein-to loosen).

Later it was identified that there were many substances which retarded the speed of a reaction.

Hence a catalyst is defined as a substance which alters the rate of chemical reaction without itself undergoing chemical change. The phenomenon which involves the action of a catalyst is called catalysis.

Positive and Negative Catalysis:

In positive catalysis, the rate of a reaction is increased by the presence of catalyst but in negative catalysis, the rate of reaction is decreased by the presence of a catalyst. The two main types of catalysis

1. Homogeneous Catalysis and
2. Heterogeneous Catalysis

Homogeneous Catalysis

In a homogeneous catalysed reaction, the reactants, products and catalyst are present in the same phase.

Illustration (1):

In this reaction the catalyst NO, reactants, SO₂ and O₂, and product, SO₃ are present in the gaseous form.

Illustration (2):

In the decomposition of acetaldehyde by I₂ catalyst, the reactants and products are all present in the vapour phase.

CH₃CHO_(g) + [I₂]_(g) → CH_4(g) + CO_(g) + [I₂]_(g)

Let us consider some examples in which the reactants, products and catalyst are present in aqueous solution.

(1) Hydrolysis of cane sugar with a mineral acid as catalyst

(2) Ester hydrolysis with acid or alkali as catalyst

Heterogeneous Catalysis

In a reaction, the catalyst is present in a different phase i.e. it is not present in the same phase as that of reactants or products. This is generally referred as contact catalysis and the catalyst present is in the form of finely divided metal or as gauze.

Illustration

(i) In the manufacture of sulphuric acid by contact process SO₃ is prepared by the action of SO₂ and O₂
in the presence of Pt or V₂O₅ as a catalyst.

(ii) In the Haber’s process for the manufacture of ammonia, iron is used as a catalyst for the reaction between Hydrogen and Nitrogen.

(iii) Oxidation of ammonia is carried out in presence of platinum gauze

(iv) The hydrogenation of unsaturated organic compounds is carried out using finely divided nickel as a catalyst.

(v) Decomposition of H₂O₂ occurs in the presence of the Pt catalyst

(vi) In the presence of anhydrous AlCl₃, benzene reacts with ethanoyl chloride to produce acetophenone

Characteristics of Catalysts

1. For a chemical reaction, catalyst is needed in very small quantity. Generally, a pinch of catalyst is enough for a reaction in bulk.
2. There may be some physical changes, but the catalyst remains unchanged in mass and chemical composition in a chemical reaction.
3. A catalyst itself cannot initiate a reaction. It means it can not start a reaction which is not taking place. But, if the reaction is taking place in a slow rate it can increase its rate.
4. A solid catalyst will be more effective if it is taken in a finely divided form.
5. A catalyst can catalyse a particular type of reaction, hence they are said to be specific in nature.
6. In an equilibrium reaction, presence of catalyst reduces the time for attainment of equilibrium and hence it does not affect the position of equilibrium and the value of equilibrium constant.
7. A catalyst is highly effective at a particular temperature called as optimum temperature.
8. Presence of a catalyst generally does not change the nature of products.

For example: 2SO₂ + O₂ → SO₃
This reaction is slow in the absence of a catalyst, but fast in the presence of Pt catalyst

Promoters and Catalyst Poison

1. In a catalysed reaction the presence of a certain substance increases the activity of a catalyst. Such a substance is called a promoter.
2. For example in the Haber’s process of manufacture of ammonia, the activity of the iron catalyst is increased by the presence of molybdenum.
3. Hence molybdenum is called a promoter. In the same way Al₂O₃ can also be used as a promoter to increase the activity of the iron catalyst.

On the other hand, certain substances when added to a catalysed reaction decreases or completely destroys the activity of catalyst and they are often known as catalytic poisons.

Few examples,

In the reaction, 2SO₂ + O₂ → 2SO₃ with a Pt catalyst, the poison is As₂O₃
blocks the activity of the catalyst. So, the activity is lost.

In the Haber’s process of the manufacture of ammonia, the Fe catalyst is poisoned by the presence of H₂S.

In the reaction, 2H₂ + O₂ → 2H₂O,
CO acts as a catalytic poison for Pt – catalyst

Auto Catalysis

In certain reactions one of the products formed acts as a catalyst to the reaction. Initially the rate of reaction will be very slow but with the increase in time the rate of reaction increases.

Auto catalysis is observed in the following reactions.

CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH

Acetic acid acts as the autocatalyst

2AsH₃ → 2As + 3H₂

Arsenic acts as an autocatalyst

Negative Catalysis

In certain reactions, presence of certain substances, decreases the rate of the reaction. Ethanol is a negative catalyst for the following reaction.

(i) 4CHCl₃ + 3O₂ → 4COCl₂ + 2H₂O + 2Cl₂

Ethanol decreases the rate of the reaction

(ii) 2H₂O₂ → 2H₂O + O₂

In the decomposition of hydrogen peroxide, dilute acid or glycerol acts as a negative catalyst.

Theories of Catalysis

For a chemical reaction to occur, the reactants are to be activated to form the activated complex. The energy required for the reactants to reach the activated complex is called the activation energy. The activation energy can be decreased by increasing the reaction temperature. In the presence of a catalyst, the reactants are activated at reduced temperatures in otherwords, the activation energy is lowered.

The catalyst adsorbs the reactants activates them by weakening the bonds and allows them to react to form the products. As activation energy is lowered in presence of a catalyst, more molecules take part in the reaction and hence the rate of the reaction increases.

The action of catalysis in chemical reactions is explained mainly by two important theories. They are

• The intermediate compound formation theory
• The adsorption theory.

The Intermediate Compound Formation Theory

A catalyst acts by providing a new path with low energy of activation. In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or decompose to give products and the catalyst is regenerated.

Consider the reactions:

A + B → AB (1)
A + C → AC (intermediate) (2)
C is the catalyst
AC + B → AB + C (3)

Activation energies for the reactions (2) and (3) are lowered compared to that of (1). Hence the formation and decomposition of the intermediate accelerate the rate of the reaction.

Example 1:

The mechanism of Fridel craft reaction is given below

The action of catalyst is explained as follows

CH₃Cl + AlCl₃ → [CH₃]⁺[AlCl₄]^–

It is an intermediate.

C₆H₆ + [CH₃⁺] [AlCl₄]^– → C₆H₅CH₃ + AlCl₃ + HCl

Example 2:

Thermal decomposition of KClO₃ in presence of MnO₂ proceeds as follows.

Steps in the reaction
2KClO₃ → 2KCl + 3O₂ can be given as
It is an intermediate.
6MnO₃ → 6MnO₂ + 3O₂

Example 3:

Formation of water due to the reaction of H₂ and O₂ in the presence of Cu proceeds as follows.
Steps in the reaction H₂ + \(\frac{1}{2}\)O₂ → H₂O can be given as

2Cu + \(\frac{1}{2}\)O₂ → Cu₂O

It is an intermediate.

Cu₂O + H₂ → H₂O + 2Cu

Example 4:

Oxidation of HCl by air in presence of CuCl₂ proceeds as follows. Steps in the reaction

4HCl + O₂ → 2H₂O + 2Cl₂ can be given as

2CuCl₂ → Cl₂ + Cu₂Cl₂
2Cu₂Cl₂ + O₂ → 2Cu₂OCl₂

It is an intermediate.

2Cu₂OCl₂ + 4HCl → 2H₂O + 4CuCl₂

This theory describes

• The specificity of a catalyst and
• The increase in the rate of the reaction with increase in the concentration of a catalyst.

Limitations

• The intermediate compound theory fails to explain the action of catalytic poison and activators (promoters).
• This theory is unable to explain the mechanism of heterogeneous catalysed reactions.

Adsorption Theory

Langmuir explained the action of catalyst in heterogeneous catalysed reactions based on adsorption. The reactant molecules are adsorbed on the catalyst surfaces, so this can also be called as contact catalysis. According to this theory, the reactants are adsorbed on the catalyst surface to form an activated complex which subsequently decomposes and gives the product.

The various steps involved in a heterogeneous catalysed reaction are given as follows:

1. Reactant molecules diffuse from bulk to the catalyst surface.
2. The reactant molecules are adsorbed on the surface of the catalyst.
3. The adsorbed reactant molecules are activated and form activated complex which is decomposed to form the products.
4. The product molecules are desorbed.
5. The product diffuse away from the surface of the catalyst.

Active Centres

The surface of a catalyst is not smooth. It bears steps, cracks and corners. Hence the atoms on such locations of the surface are co-ordinatively unsaturated. So, they have much residual force of attraction. Such sites are called active centres. So, the surface carries high surface free energy.

The presence of such active centres increases the rate of reaction by adsorbing and activating the reactants. The adsorption theory explains the following

1. Increase in the surface area of metals and metal oxides by reducing the particle size increases acting of the catalyst and hence the rate of the reaction.

2. The action of catalytic poison occurs when the poison blocks the active centres of the catalyst.

3. A promoter or activator increases the number of active centres on the surfaces.