Surface Chemistry Definitions

Surface chemistry: It deals with phenomena that occur at the surfaces or interfaces.

Adsorption: It is the process of accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.

Adsorbate: The molecular species or substance, which concentrates or accumulates at the surface.

Adsorbent: The material on the surface of which the adsorption takes place.

Desorption: The process of removing an adsorbed substance from the surface.

Sorption: The term used when both absorption and adsorption occur simultaneously.

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Distinction between Adsorption and Absorption

Adsorption

It is a surface phenomenon, i.e., it occurs only at the surface of the adsorbent.

In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk.

Its rate is high in the beginning and then decreases till equilibrium is attained.

Absorption

It is a bulk phenomenon, i.e., occurs throughout the body of the material.

In this phenomenon, the concentration is same throughout the material.

Its rate remains same throughout the process.

Types of adsorption

Physical adsorption (Physisorption)

The particles are held to the surface by the physical forces like Van der waals forces.

Chemical adsorption (Chemisorption)

The particles are held to the surface by the chemical forces or by chemical bonds

Factors affecting adsorption of gases on solids

Nature of adsorbent: Greater are the strained forces on the surface, more is the ease with which adsorption takes place on the surface. Activated adsorbents have high adsorbing power.

Surface area of adsorbent: Greater the surface area, more is the adsorption.

Nature of gas being adsorbed: Easily liquefiable gases like NH₃, HCl, Cl₂, SO₂,CO₂, etc. (whose critical temperature is high) are adsorbed to greater extent.

Pressure: Adsorption increases with increase in pressure. The effect of pressure is large at low temperature.

Temperature: Since adsorption is an exothermic process so according to Le-Chatelier’s principle adsorption decreases with increase in temperature

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Freundlich adsorption isotherm

The plot of x/m vs pressure at constant temperature is called Freundlich adsorption isotherm

where, m = mass of the adsorbent x = mass of the adsorbate

Catalysts and Catalysis

Substances, which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction.

Promoters

Promoters are substances that enhance the activity of a catalyst

Poisons

Poisons decrease the activity of a catalyst.

Contact process: small amount of As₂O₃ act as poison for Pt

Haber’s process: H₂S gas act as poison for Fe

Homogeneous catalysis

The reactants and the catalyst are in the same phase

Heterogeneous catalysis

The reactants and the catalyst are in different.

Zeolites: Shape-selective Catalysis

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules. e.g., Zeolites (have honey–comb like structures).

Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation.

An important zeolite catalyst used in the petroleum industry is ZSM-5.

It converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

Enzyme Catalysis

They are complex nitrogeneous organic compounds made up of proteins also called biochemical catalysts

The following characteristics are exhibited by enzyme catalysts:

Most highly efficient
Highly specific nature
Highly active under optimum temperature
Highly active under optimum pH
Increasing activity in presence of activators and co-enzymes
Influence of inhibitors and poisons

Colloid

A colloid is a heterogeneous system in which one substance (dispersed phase) is in another substance called dispersion medium.

The range of diameters of colloidal particles is between 1 and 1000.

Associated colloids (Micelles)

The aggregated particles thus formed are called micelles. These are also known as associated colloids.

Micelles may contain as many as 100 molecules or more.

The formation of micelles takes place only above a particular temperature called Kraft temperature (T_k)

Above a particular concentration called critical micelle concentration (CMC).

Preparation of Colloids

Chemical methods
Electrical disintegration or Bredig’s Arc method
Peptization

Chemical methods

This can be done by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols.

Electrical disintegration or Bredig’s Arc method

This process involves dispersion as well as condensation.

Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared

Peptization

The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte.

The electrolyte used for this purpose is called peptizing agent.

During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface.

This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid.

Purification of Colloidal Solutions

The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution.

(i) Dialysis

(ii) Electro dialysis

(iii) Ultrafiltration

Dialysis & Electro dialysis

It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane (animal membrane (bladder) or parchment paper or cellophane sheet).

Properties of Colloidal Solutions

Properties of Colloidal Solutions

Colligative properties
Tyndall effect
Colour
Brownian movement
Electrical Properties

Colligative properties

These are the properties which depend only on the number of solute (dispersed phase) particles but not on its nature.

The colligative properties are

1. Relative lowering of vapour pressure

2. Elevation in boiling point

3. Depression in freezing point

4. Osmotic pressure

Tyndall effect

Tyndall effect is observed only when the following two conditions are satisfied

The diameter of the dispersed particles is not much smaller than the wavelength of the light used
The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.
The number of colloidal particles can be counted.
To find average mass of particles

Colour

The colour of colloidal solution depends on the wavelength of light scattered by the dispersed particles. The wavelength of light further depends on the size and nature of the particles.
The colour of colloidal solution also changes with the manner in which the observer receives the light.
For example, a mixture of milk and water appears blue when viewed by the reflected light and red when viewed by the transmitted light.
Finest gold sol is red in colour as the size of particles increases, it appears purple, then blue and finally golden.

Brownian movement

The state of continuous zig-zag motion of particles.
Depends on the size of the particles and viscosity of the solution.
Smaller the size and lesser the viscosity, faster is the motion.
The brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sols.

Electrical Properties

Stability of Colloids and Charge on colloidal particles
Charge on colloidal particles: Due to preferential adsorption of ions from solution
Charge on colloidal particles: Due to preferential adsorption of ions from solution

Zeta potential

This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

Electrophoresis

The movement of colloidal particles under an applied electric potential is called electrophoresis.

Positively charged particles move towards the cathode while negatively charged particles move towards the anode.

Electroosmosis

Movement of particles of the dispersion medium begins to move in an electric field.

Coagulation or Flocculation or Precipitation

The process of settling of colloidal particles is called coagulation or precipitation of the sol

The ion responsible for neutralization of charge on the particles is called the coagulating ion.

A negative ion causes the precipitation of positively charged sol and vice versa

The coagulation of the lyophobic sols can be carried out in the following ways:

(i) By electrophoresis

(ii) By mixing two oppositely charged sols

(iii) By boiling

(iv) By persistent dialysis

(v) By addition of electrolytes

Coagulating value

The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value.

The smaller the quantity needed, the higher will be the coagulating power of an ion.

Protective colloids

A protective colloid is a lyophilic colloid that when present in small quantities keeps lyophobic colloids from precipitating under the coagulating action of electrolytes.

When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes.

Lyophilic colloids used for this purpose are called protective colloids.

Gold Number

The Gold Number is the minimum weight (in milligrams) of a protective colloid required to prevent the coagulation (colour change from Red to Violet) of 10 ml of a standard hydro gold sol when 1 ml of a 10% sodium chloride solution is added to it.

Emulsion

These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid.

Stability of Emulsion

Emulsions of oil in water are unstable and sometimes they separate into two layers on standing.

For stabilisation of an emulsion, a third component called emulsifying agent is usually added.

The emulsifying agent forms an interfacial film between suspended particles and the medium.

The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack

Colloids in our daily life

Blue colour of the sky

Fog, Mist and Rain

Food articles: Milk, Butter, Halwa, Ice Creams, Fruit Juices

Blood: Alum and Ferric Chloride solution used to coagulation of blood

Soils

Applications of colloids

1. Electrical precipitation of smoke

2. Purification of drinking water

3. Midicines:

a) Argyrol is a silver sol used as an eye lotion.

b) Colloidal antimony is used in curing kalaazar.

c) Colloidal gold is used for intramuscular injection

d) Milk of magnesia, an emulsion, is used for stomach disorders.

4. Tanning

5. Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.

6. Rubber industry: Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex.

7. Industrial products: Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions.