Q.(a) Give two differences between macromolecular colloids and associated colloids.

Surface chemistry is the physics and chemistry of what happens at an interface — the thin boundary where two phases meet — and its central fact is that a surface is a place of unsatisfied forces. An atom in the bulk of a solid is pulled equally in every direction by its neighbours; an atom sitting at the surface has neighbours on one side only, so it carries a residual, unbalanced attraction outward. That leftover pull is the engine behind almost everything in this concept: it makes gases stick to solids, it holds colloidal particles together and charged, and it lets a metal speed up a reaction merely by offering its face. Learn to see every phenomenon here as the surface trying to satisfy those dangling forces and the subject stops being a list to memorise.

The first consequence is adsorption — the accumulation of a species at a surface — and it must be sharply distinguished from absorption, its bulk-penetration cousin. In adsorption the concentration of the adsorbate is higher at the surface than in the bulk (water on silica gel); in absorption the substance spreads uniformly through the whole volume (water into anhydrous CaCl₂); when both occur together the word is sorption. The material offering the surface is the adsorbent, the species held is the adsorbate, and the reverse process is desorption. Because adsorption creates order — a freely-moving gas becomes a tethered film — it always lowers entropy (ΔS < 0) and always releases heat (ΔH < 0), so from ΔG = ΔH − TΔS it can only be spontaneous at onset because the negative ΔH outweighs the positive −TΔS term. As the surface fills, ΔH becomes less negative until ΔG reaches zero at equilibrium.

The single most examined distinction in this concept is physisorption versus chemisorption, and every feature follows from the strength of the bond doing the holding. Physisorption uses only weak van der Waals attraction, so its enthalpy of adsorption is small, roughly -20 to -40 kJ/mol; chemisorption forms an actual chemical (often covalent) bond to the surface, so its enthalpy is large, roughly -80 to -240 kJ/mol. That one difference cascades: physisorption is non-specific (any gas sticks to any surface), reversible, builds up in multiple layers, and needs essentially no activation energy; chemisorption is highly specific (a gas bonds only where its chemistry allows), effectively irreversible, is limited to a single monolayer because a chemical bond can form only in direct contact, and needs appreciable activation energy to make that bond. The classic trap statement in a multiple-correct set inverts exactly one of these — "chemisorption is multilayer" or "physisorption requires high activation energy" — so you must check every feature independently rather than eliminate on one.

Temperature separates the two adsorptions even more cleanly, and their opposite curves are a favourite of the paper. Since physisorption is exothermic and needs no activation, raising the temperature simply supplies enough energy to shake the weakly-held molecules loose, so its extent falls monotonically as T rises — heat a physisorbed gas and it desorbs. Chemisorption, by contrast, first rises then falls with temperature: at low T the reaction is starved of the activation energy needed to form the surface bond, so warming initially increases adsorption until an optimum, after which the usual exothermic-desorption effect takes over and the extent drops. Recognising which curve — a steady decline versus a rise-then-fall hump — belongs to which adsorption is a standard single-correct item.

At fixed temperature the extent of adsorption depends on pressure, and the empirical Freundlich isotherm captures that dependence. It states x/m = k·p to the power (1/n), where x is the mass adsorbed on mass m of adsorbent, p is the equilibrium gas pressure, and n > 1 so that the exponent 1/n lies between 0 and 1 (typically 0.1 to 0.5). Taking logarithms linearises it: log(x/m) = log k + (1/n)·log p, so a plot of log(x/m) against log p is a straight line of slope 1/n and intercept log k — which is exactly how a numerical item asks you to extract 1/n from a slope or from two (p, x/m) data pairs. The equation also has two revealing limits: at low pressure 1/n approaches 1 so x/m ∝ p (first order in pressure), while at high pressure 1/n approaches 0 so x/m becomes constant (zero order, the surface saturates). For adsorption from solution the identical law holds with concentration replacing pressure: x/m = k·C to the power (1/n).

The Langmuir isotherm supplies the theoretical monolayer picture that Freundlich lacks. Langmuir treats adsorption as a dynamic equilibrium between molecules landing on and leaving a surface that can hold at most one complete layer, giving x/m = a·p/(1 + b·p): linear in p at low pressure and saturating to a fixed monolayer coverage at high pressure. Where Freundlich is purely empirical and never saturates in its bare power-law form, Langmuir is built on an explicit monolayer assumption with a genuine saturation term — the cleanest way to answer a "which isotherm predicts saturation coverage" comparison. The factors that raise the extent of adsorption then follow naturally: a larger surface area (activated charcoal, finely divided metals) offers more sites; higher pressure drives more gas onto the surface; lower temperature favours the exothermic physisorption. The Advanced hook is the nature-of-the-gas rule — a gas that is more easily liquefied, meaning it has a higher critical temperature, is adsorbed more strongly, so the order of extent runs SO₂ (Tc 630 K) > CH₄ (Tc 190 K) > H₂ (Tc 33 K).

The concept's second block, colloids, occupies the size window between true solutions and suspensions — particles from about 1 to 1000 nm. Below 1 nm you have a true solution; above 1000 nm the particles settle as a suspension; in between they stay dispersed as a colloid, small enough to resist gravity yet large enough to scatter light. Colloids are classified three ways. By the physical states of the dispersed phase and dispersion medium there are eight combinations with distinct names: a solid or liquid dispersed in a gas is an aerosol (smoke, fog), a gas in a liquid is a foam (whipped cream), a liquid in a liquid is an emulsion (milk), a solid in a liquid is a sol (paint, gold sol), a liquid in a solid is a gel (cheese, butter), a gas in a solid is a solid foam (pumice), and a solid in a solid is a solid sol (coloured glass) — with no gas-in-gas colloid because two gases simply mix molecularly.

The other two classifications — by interaction and by particle type — control stability, and stability is what the questions probe. By interaction a sol is either lyophilic (solvent-loving: starch, gum, gelatin) — reversible, self-stabilised by strong solvation, and hard to coagulate — or lyophobic (solvent-hating: metal and metal-sulphide sols) — irreversible, needing an added stabiliser and easily thrown out of solution. By particle type there are multimolecular colloids (aggregates of many small atoms or molecules, e.g. gold or sulphur sol), macromolecular colloids (single genuinely large molecules — starch, cellulose, proteins, polymers), and associated colloids or micelles (electrolytes such as soaps that behave normally when dilute but aggregate above a threshold concentration). Colloids are made either by dispersion — Bredig's arc method (an electric arc struck between metal electrodes under water for Au, Ag, Pt sols) or peptisation (converting a fresh precipitate to a sol with a small amount of electrolyte, e.g. a little FeCl₃ peptising Fe(OH)₃) — or by condensation of smaller particles, via oxidation (2 H₂S + SO₂ → 3 S + 2 H₂O for a sulphur sol), reduction, hydrolysis (FeCl₃ + 3 H₂O → Fe(OH)₃ sol + 3 HCl), double decomposition (As₂O₃ + 3 H₂S → As₂S₃ sol) or exchange of solvent. Once made, the sol is purified of the small ions that would coagulate it by dialysis (diffusion through a parchment or cellophane membrane), electrodialysis (the same, accelerated by an applied potential) or ultrafiltration (a colloid-retaining membrane). …