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Surface Chemistry

Long Answer Type

221. Explain the term adsorption. How does adsorption differ from absorption? What are the different types of adsorption?

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222. Explain the two terms: adsorption isotherm and adsorption isobar. Describe the Freundlich adsorption isotherm. What is the major point of its application?

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223. What do you mean by colloidal solution and colloidal state? How do collodal solutions differ from true solutions with respect to dispersed particles size, homogenecity, Brownian movement and Tyndall effect.

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224. How are colloids classified on the basis of (a) physical states of components, (b) nature of dispersion medium and (c) association between dispersed phase and dispersion medium.

Since the colloidal solutions consist of dispersed phase and dispersion medium, the components of these phases have certain specific characteristics of their own.

a) CLASSIFICATION BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM

Based on the physical state of dispersed phase and dispersion medium, there are eight types of colloidal solutions. Either these phases may be solid, liquid or gas. Since gases gives homogeneous mixture in all proportions, hence colloidal solution can not be obtained in which dispersed phase and dispersion medium are gases.

S.No.	Dispersed Phase	Dispersion Medium	Name of Colloidal Solution	Examples
1.	Gas	Liquid	Foam	Soap lather, whipped cream, soda water, froth on beer.
2.	Gas	Solid	Solid foam	Cake, Bread, Pumice stone lava.
3.	Liquid	Gas	Aerosol	Mist, fog, Clouds.
4.	Liquid	Liquid	Emulsion	Milk, cream, butter, oil in water
5.	Liquid	Solid	Gel.	Curd, Cheese, Jellies, boot polish.
6.	Solid	Gas	Solid Aerosol	Smoke, dust.
7.	Solid	Liquid	Sols	Paints, gold sol, inks.
8.	Solid	Solid	Solid Sols	Alloys, coloured glass, gem stones.

CLASSIFICATION BASED ON APPEARANCE OF THE COLLOIDS

On this basis, the colloidal solutions can be divided into two types;

(a)Sol When a colloidal solution appears as fluid it is referred to as Sol. The sols are generally named after dispersion medium. For Instance, when the dispersion medium is water, the sol is known as Hydrosol or Aquosol. When the dispersion medium is alcohol and benzene, the sols are called Alcosol and Benzosol respectively.

(b)Gels When a colloidal has solid like appearance it is called Gel. The rigidity of gels varies from substance to substance. Some examples of gels are: Jelly, butter, cheese, curd.

c) CLASSIFICATION BASED ON MOLECULAR SIZE OF THE DISPERSED PHASE

On the basis of molecular size the colloids can be divided into three types:
(a)Multimolecular Colloids

In these colloids, the Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10–7cm. For instance a gold sol consists of particles of various size containing several atoms. Likewise, a sulphur sol contains particles each having eight sulphur atoms (S8). In these colloids, the particles are held by weak vanderwaal’s forces.

(b)Macro Molecular Colloids:

In these colloids, the particles of dispersed phase are sufficiently large in size enough to be of colloidal dimensions. These are called Natural Polymers. Some typical examples are starch, cellulose and proteins. Some artificial or synthetic polymers are polythene, polystyrene, nylon and plastics.

These colloids behave as normal electrolytes at low concentrations but behave as colloids at higher concentration. These associated colloids are also referred asMicelles. Sodium Stearate behaves as electrolyte in dilute solution but as colloid at higher concentration. Some other examples of associated colloids are; Soaps, higher alkyl sulphonates, polythene oxide etc.

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225.

(i) Explain the meaning of the statement. ‘Adsorption is a surface phenomenon.’
(ii) State two features of chemical adsorption which are not found in physical adsorption.
(iii) Explain the terms with suitable examples: (i) Gel, (ii) Aerosol and (iii) Hydrosol.

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