Class 11 Chemistry — Chapter 306: Adsorption and Colloids

Key concepts: Adsorption and Colloids (Class 11 Chemistry)

This chapter is about what happens at surfaces and about the state of matter that sits between a true solution and a suspension. It begins by separating adsorption, a surface phenomenon, from absorption, a bulk phenomenon, and distinguishes weak physisorption held by van der Waals forces from strong, specific chemisorption held by chemical bonds, summarising the pressure dependence in the Freundlich adsorption isotherm x/m = k p^(1/n). Surface chemistry then explains catalysis, both homogeneous and heterogeneous, through the adsorption theory of active centres. The second half studies colloids: how they are classified as lyophilic or lyophobic and as multimolecular, macromolecular or associated, how micelles form above the critical micelle concentration, how sols are prepared and purified by dialysis, electrodialysis and ultrafiltration, and how their characteristic behaviour — the Tyndall effect, Brownian motion, electrophoresis and coagulation governed by the Hardy-Schulze rule — arises from particle size and surface charge.

Adsorption

The accumulation of a substance at the surface of a solid or liquid, so that its concentration at the surface is higher than in the interior. It occurs because atoms at a surface have unbalanced attractive forces, and it is always exothermic since it lowers the surface energy of the adsorbent.

Absorption and sorption

Absorption is a bulk phenomenon in which a substance is taken up uniformly throughout another, as water is by a sponge or anhydrous CaCl₂. When adsorption and absorption happen together and cannot easily be told apart, as with a dye on cotton, the combined process is called sorption.

Adsorbate and adsorbent

The adsorbate is the substance that gets held on the surface, and the adsorbent is the solid that provides that surface. Good adsorbents such as activated charcoal, silica gel and alumina all have very large surface areas per gram, which is why finely divided or porous forms adsorb best.

Physisorption

Physical adsorption in which the adsorbate is held by weak van der Waals forces. Its enthalpy is only about 20 to 40 kJ mol⁻¹, it needs no activation energy, it is not specific to any particular gas, it can form multiple layers, and it is easily reversed by heating or by reducing the pressure.

Chemisorption

Chemical adsorption in which real bonds form between adsorbate and adsorbent, releasing roughly 80 to 240 kJ mol⁻¹. It is highly specific, needs activation energy so it increases with temperature up to a point, forms only a unimolecular layer, and is essentially irreversible.

Factors affecting adsorption

The amount adsorbed rises with the surface area of the adsorbent and with the pressure of the gas, and falls with temperature for physisorption. It also rises with the ease of liquefaction of the gas, so a gas of high critical temperature such as SO₂ is adsorbed far more than H₂ or N₂.

Freundlich adsorption isotherm

An empirical relation x/m = k p^(1/n) linking the mass adsorbed per gram of adsorbent to the equilibrium pressure at constant temperature, with 1/n between 0 and 1. At low pressure x/m is nearly proportional to p, and at high pressure the surface saturates and x/m becomes independent of p.

Applications of adsorption

Adsorption underlies gas masks and activated-charcoal filters, the decolourising of raw sugar syrup with animal charcoal, silica gel as a drying agent, chromatographic separations, froth flotation of sulphide ores, ion exchange for softening water, and the action of most solid catalysts.

Catalysis

A catalyst alters the rate of a reaction by offering an alternative path of lower activation energy and is recovered chemically unchanged. It cannot shift the position of equilibrium or change ΔH; it only lets the reaction reach the same equilibrium sooner, speeding both directions equally.

Homogeneous and heterogeneous catalysis

In homogeneous catalysis the catalyst is in the same phase as the reactants, as when dilute H₂SO₄ hydrolyses an ester in solution or NO catalyses the oxidation of SO₂ in the lead chamber process. In heterogeneous catalysis the phases differ, as with iron in the Haber process or V₂O₅ in the contact process.

Adsorption theory of catalysis

A solid catalyst works because reactant molecules are adsorbed at active centres on its surface. Adsorption concentrates them, holds them in a favourable orientation and weakens their bonds, lowering the activation energy; the product then desorbs, freeing the site for another cycle.

Colloidal state

A colloid is a two-phase system in which particles of the dispersed phase, between 1 nm and 1000 nm across, are spread through a continuous dispersion medium. The particles are too large to pass a semipermeable membrane but too small to be retained by ordinary filter paper or to settle under gravity.

Lyophilic and lyophobic sols

Lyophilic sols such as starch, gelatin or gum in water are solvent-loving, stable and reversible, since simply adding the medium re-forms them. Lyophobic sols such as metal or metal sulphide sols are solvent-hating, unstable, need a stabiliser and are irreversible once coagulated.

Multimolecular, macromolecular and associated colloids

In multimolecular colloids many small molecules or atoms aggregate into a colloidal particle, as in sulphur and gold sols. In macromolecular colloids one giant molecule such as starch or a protein is already of colloidal size. In associated colloids such as soap, molecules cluster into micelles only above a certain concentration.

Micelles and critical micelle concentration

Surfactant molecules have a polar head and a long hydrocarbon tail. Above the critical micelle concentration and above the Kraft temperature they associate with tails pointing inward and heads outward, forming micelles; the oily interior dissolves grease, which is how soaps and detergents clean.

Purification of colloids

Dialysis removes dissolved electrolytes by letting small ions diffuse out through a semipermeable membrane into flowing water. Electrodialysis applies an electric field to drag those ions out far faster, and ultrafiltration uses filter paper impregnated with collodion so that its pores retain the colloidal particles.

Tyndall effect and Brownian motion

Colloidal particles scatter light sideways, making the path of a beam visible as a luminous cone; a true solution does not. Under an ultramicroscope the same particles are seen in continuous zig-zag Brownian motion, caused by unequal bombardment by the molecules of the medium, which also keeps the sol from settling.

Electrophoresis and coagulation

All the particles of a given sol carry the same sign of charge, so under an electric field they migrate together to the oppositely charged electrode, which is electrophoresis. Neutralising that charge with an electrolyte, or mixing two oppositely charged sols, makes the particles aggregate and settle, which is coagulation.

Hardy-Schulze rule

The ion that coagulates a sol is the one carrying charge opposite to that of the sol particles, and its coagulating power increases sharply with the magnitude of that charge. So Al³⁺ > Ba²⁺ > Na⁺ for a negative sol, and PO₄³⁻ > SO₄²⁻ > Cl⁻ for a positive sol.

Emulsions

An emulsion is a colloid of one liquid dispersed in another immiscible liquid. Milk is oil-in-water, with fat droplets held in water by casein, while butter is water-in-oil; an emulsifying agent is needed in both cases, and its removal makes the two liquids separate again.