Surface Chemistry (Class 12): Adsorption, Colloids and Catalysis

Adsorption, and how it differs from absorption

Adsorption is accumulation of a substance at a surface; absorption is uniform distribution throughout a bulk. Where both occur together the term is sorption. Being asked to distinguish these with an example is one of the most reliable one-mark questions in the chapter.

The substance that accumulates is the adsorbate; the surface is the adsorbent. Adsorption is always exothermic, and the reason is worth understanding rather than memorising: molecules at a surface lose freedom of movement, so entropy falls, and for the process to be spontaneous ΔH must be negative and large enough to outweigh the TΔS term.

Physisorption and chemisorption

The Freundlich isotherm

Plotting log(x/m) against log p gives a straight line of slope 1/n and intercept log k. The relationship is empirical and fails at high pressure, where the surface saturates and the equation continues to predict indefinite increase — a limitation that is regularly asked about.

Colloids

A colloid has dispersed particles roughly between 1 and 1000 nm — larger than in a true solution, smaller than in a suspension. That size range is what produces every characteristic property in the chapter.

Classification by affinity for the medium

Properties that follow from particle size

• Tyndall effect — scattering of light by particles comparable to its wavelength, making the beam path visible. A true solution does not show it, which makes it the standard distinguishing test.
• Brownian motion — random particle movement from uneven molecular bombardment, which also opposes settling and helps stability.
• Electrophoresis — movement of charged colloidal particles in an electric field, showing the particles carry charge.

Coagulation and the Hardy–Schulze rule

The coagulating power of an ion increases sharply with its charge, and it is the ion of charge opposite to the colloidal particle that matters. So for a negatively charged sol, a trivalent cation is far more effective than a divalent one, which in turn beats a monovalent one. Questions almost always require both parts — the charge dependence and the opposite-sign requirement.

Emulsions

An emulsion is a colloid of one liquid in another, of two types: oil dispersed in water, or water dispersed in oil. Emulsifying agents stabilise them by sitting at the interface.

Catalysis

A catalyst increases rate by providing an alternative pathway of lower activation energy. It is not consumed, and — a point examiners test — it does not shift the position of equilibrium. It speeds forward and reverse reactions equally, so equilibrium is reached sooner but at the same composition.

Heterogeneous catalysis is explained by adsorption theory: reactants adsorb onto the surface, which weakens their bonds and holds them in favourable orientation; they react; the products then desorb, freeing the site. Activity therefore depends on surface area, which is why catalysts are used finely divided or on a support.

Terms worth distinguishing

• Promoter — increases catalyst activity though not itself a catalyst.
• Poison — reduces or destroys activity, typically by binding irreversibly to active sites.
• Shape-selective catalysis — the pore structure admits only molecules of suitable size and shape, as in zeolites.

Frequently asked questions

Why is adsorption always exothermic?

Because it reduces the freedom of the adsorbed molecules, so ΔS is negative. For ΔG to be negative, ΔH must be negative and sufficiently large.

How can a colloid be told apart from a true solution?

By the Tyndall effect. A colloid scatters a light beam so its path becomes visible; a true solution does not, because its particles are too small to scatter effectively.

Does a catalyst change the yield of a reaction?

Not at equilibrium. It changes how quickly equilibrium is reached, not where it lies. In practice a catalyst may improve yield by favouring one pathway over a competing one, but that is selectivity rather than a shift in equilibrium.

Why are finely divided catalysts more effective?

Because catalysis happens at the surface, and dividing a solid finely increases surface area per unit mass enormously, providing many more active sites.

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