CBSE Class 12 Chemistry Surface Chemistry Notes

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Surface chemistry is an important chapter of CBSE Class 12 Chemistry, and this summary notes page on Unit 5 covers adsorption and its types, adsorption isotherms, catalysts and catalysis, zeolites and enzymes, colloids and their preparation, purification and properties, coagulation, emulsions, and the applications of colloids, with the key definitions, tables and equations.

SUMMARY UNIT-5, SURFACE CHEMISTRY

Introduction
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations. In this Unit, you will be studying some important features of surface chemistry such as adsorption, catalysis and colloids including emulsions and gels.

ADSORPTION

Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The substance which accumulates at the surface is termed adsorbate, and the material on the surface of which the adsorption takes place is called adsorbent.

Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Application of Adsorption:
(i) Production of high vacuum.
(ii) Gas masks.
(iii) Control of humidity.
(iv) Removal of colouring matter from solutions.
(v) Heterogeneous catalysis.
(vi) Separation of inert gases.
(vii) In curing diseases.
(viii) Froth floatation process.
(ix) Adsorption indicators.
(x) Chromatographic analysis.

Distinction between Adsorption and Absorption
In adsorption, the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid.

Mechanism of Adsorption:
1. The molecules present at the surface of adsorbent possess unbalanced or residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.
2. During adsorption, ΔH = -ve and ΔS = -ve. For a process to be spontaneous, ΔG (= ΔH − TΔS) must be negative. ΔG can be negative if ΔH > TΔS, which happens at low temperature. As adsorption proceeds, ΔH becomes less and less negative, ultimately ΔH becomes equal to TΔS and ΔG becomes zero. At this state equilibrium is attained.

Types of Adsorption:

Adsorption Isotherms: The graph of the amount of gas adsorbed by the adsorbent vs pressure at constant temperature is termed as adsorption isotherm.

The Freundlich adsorption isotherm is given by the equation:
\( \log \frac{x}{m} = \log k + \frac{1}{n} \log p \)

[Figure: Adsorption isotherm graph with the x-axis labelled P and the y-axis labelled x/m. Three curves are marked 195 K, 244 K and 273 K, with the lowest curve at 273 K. Beside it is the straight line plot of log x/m against log p, with intercept log k, slope = 1/n and the angle θ marked.]

Adsorption Isobar: Graph of the extent of adsorption vs temperature at a given pressure is called adsorption isobar.

Adsorption from Solution Phase
\( \log \frac{x}{m} = \log k + \frac{1}{n} \log C \)

CATALYSIS

Catalysts: Substances which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalysts, and the phenomenon is known as catalysis.

Promoters and Poisons: Promoters are substances that enhance the activity of a catalyst while poisons decrease the activity of a catalyst. For example, in Haber's process for manufacture of ammonia, Mo acts as a promoter for Fe catalyst and CO acts as poison.

Homogeneous Catalysis: When the reactants and the catalyst are in the same phase the process is said to be homogeneous catalysis.
Example: Hydrolysis of sugar is catalysed by H⁺ ions furnished by H₂SO₄.

Heterogeneous Catalysis: The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
Example: Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.

Adsorption Theory of Heterogeneous Catalysis:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst's surface through formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.
(v) Diffusion of reaction products away from the catalyst's surface.

Activity: The ability of the catalyst to enhance the rate of a chemical reaction is called the activity of the catalyst.

Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product.
(i) CO(g) + 3H₂(g) —Ni→ CH₄(g) + H₂O(g)
(ii) CO(g) + 2H₂(g) —Cu/ZnO-Cr₂O₃→ CH₃OH(g)
(iii) CO(g) + H₂(g) —Cu→ HCHO(g)

Shape-selective Catalysis: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Zeolites are good shape-selective catalysts because of their honeycomb-like structures. They are micro porous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al - O - Si framework.

The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well as synthesized for catalytic selectivity. Zeolites are very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerization. An important zeolite catalyst used in the petroleum industry is ZSM-5. It converts alcohols directly into gasoline (petrol) by dehydrating them to give mixture of hydrocarbons.

Enzyme Catalysis: Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are protein molecules. They catalyze metabolic reactions, taking place in cells of plants and animals. The enzymes are, thus, termed as biochemical catalysts and the phenomenon is known as biochemical catalysis.
Example: Inversion of cane sugar: The invertase enzyme converts cane sugar into glucose and fructose.

COLLOIDS

Colloids: A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. The range of diameters of particles of dispersed phase is 1 to 1000 nm.

Colloids are classified on the basis of the following criteria:
(i) Physical state of dispersed phase and dispersion medium: Solid sol, Sol, Aerosol, Gel, Emulsion, Foam.
(ii) Nature of interaction between dispersed phase and dispersion medium: (i) Lyophilic colloids, (ii) Lyophobic colloids.
(iii) Type of particles of the dispersed phase: (i) Multimolecular colloids, (ii) Macromolecular colloids, (iii) Associated colloids (Micelles).

Mechanism of Micelle Formation:

[Figure: Structure of a soap or detergent molecule with a hydrophilic head (labelled Hydrophilic head) and a hydrophobic tail (labelled Hydrophobic tail). Below it is a circular diagram of an ionic micelle with the ionic groups arranged around the outside.]

The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions.

PREPARATION OF COLLOIDS

(a) Chemical Method: Colloidal sols are prepared by the following chemical reactions:
As₂O₃ + 3H₂S → As₂S₃(sol) + 3H₂O (Double decomposition)
SO₂ + 2H₂S → 3S(sol) + 2H₂O (Oxidation)
2AuCl₃ + 3HCHO + 3H₂O → 2Au(sol) + 3HCOOH + 6HCl (Reduction)
FeCl₃ + 3H₂O → Fe(OH)₃(sol) + 3HCl (Hydrolysis)

(b) Electrical Disintegration or Bredig's Arc Method

(c) Peptization: Peptization may be defined as 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.

PURIFICATION OF COLLOIDAL SOLUTIONS

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: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. Since particles (ions or smaller molecules) in a true solution can pass through animal membrane or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis. The apparatus used for this purpose is called dialyser.

(ii) Electro-dialysis: The dialysis in the presence of electrical field is called electrodialysis.

(iii) Ultra Filtration: Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles. Colloidal particles can pass through ordinary filter paper because the pores are too large. However, the pores of filter paper can be reduced in size by impregnating with collodion solution to stop the flow of colloidal particles. The usual collodion is a 4% solution of nitrocellulose in a mixture of alcohol and ether. An ultra-filter paper may be prepared by soaking the filter paper in a collodion solution, hardening by formaldehyde and then finally drying it.

PROPERTIES OF COLLOIDAL SOLUTIONS

Properties of Colloidal Solutions:
(i) Colligative properties.
(ii) Tyndall effect.
(iii) Colour.
(iv) Brownian movement.
(v) Charge on colloidal particles.
(vi) Electrophoresis: The movement of colloidal particles under an applied electric potential is called electrophoresis.
(vii) Coagulation or precipitation.

Charge on Colloidal Particles: The potential difference between the fixed layer and the diffused layer of opposite charges is called the electro kinetic potential or zeta potential.

Coagulation or Precipitation: The process of settling of colloidal particles is called coagulation or precipitation of the sol. This 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.

Hardy-Schulze Rule: 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. Greater the valence of the flocculating ion added, the greater is its power to cause precipitation. In the coagulation of a positive sol, the flocculating power is in the order:
[Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻

Coagulating Power of an Ion: 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.

Protection of Colloids: Lyophilic sols are more stable than lyophobic sols. This is due to the fact that lyophilic colloidal particles are covered by a sheath of the liquid in which they are dispersed. Lyophilic colloids are capable of protecting lyophobic colloids. 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.

EMULSIONS

Emulsions: These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid.
Types of Emulsions: (i) Oil dispersed in water (O/W type) and (ii) Water dispersed in oil (W/O type).

EXAMPLES AND APPLICATIONS OF COLLOIDS

Examples of Colloids:
(i) Blue colour of the sky.
(ii) Fog, mist and rain.
(iii) Food articles.
(iv) Blood.
(v) Soils.
(vi) Formation of delta.

Applications of Colloids:
(i) Electrical precipitation of smoke: Cottrell precipitator.
(ii) Purification of drinking water.
(iii) Medicines.
(iv) Tanning.
(v) Cleansing action of soaps and detergents.
(vi) Photographic plates and films.
(vii) Rubber industry.
(viii) Industrial products.