JEE Chemistry Surface Chemistry Notes Set 03

Adsorption

Adsorption is the accumulation of a substance (adsorbate) on the surface of a solid or liquid (adsorbent); absorption is uniform penetration into the bulk. Adsorption is exothermic (\( \Delta H<0 \)) with \( \Delta S<0 \), so it is favoured by low temperature. Sorption means adsorption and absorption together. Examples: charcoal adsorbing gases, silica gel removing moisture, animal charcoal decolourising sugar solution.
Physical adsorption (physisorption): weak van der Waals forces, low enthalpy (20 to 40 kJ/mol), reversible, not specific, multilayer, decreases with rise in temperature, increases with pressure and with the ease of liquefaction of the gas. Chemical adsorption (chemisorption): chemical bonds, high enthalpy (40 to 400 kJ/mol), irreversible, specific, unimolecular layer, needs activation energy so it may first rise with temperature.

Adsorption Isotherms

Freundlich isotherm (constant temperature): \( \dfrac{x}{m} = kP^{1/n} \), or \( \log\dfrac{x}{m} = \log k + \dfrac{1}{n}\log P \). A plot of \( \log(x/m) \) against \( \log P \) is a straight line of slope \( 1/n \) and intercept \( \log k \). At low pressure \( x/m \propto P \); at high pressure it becomes independent of pressure (saturation of the surface).
Applications: gas masks (charcoal), removal of colour, heterogeneous catalysis, chromatography, softening of water by ion-exchange resins, froth flotation, dyeing.

Catalysis

A catalyst changes the rate of a reaction without being consumed. It lowers the activation energy, does not alter \( \Delta H \), \( \Delta G \) or K, affects both directions equally, and cannot start a reaction that is not spontaneous. Promoters increase the activity (Mo in Haber's process); poisons reduce it (As2O3 on Pt, CO on Fe).
Homogeneous catalysis: same phase (NO in the lead chamber process, H\(^+\) in ester hydrolysis). Heterogeneous catalysis: different phase (Fe in Haber's process, V\(_2\)O\(_5\) in the contact process, Pt in the Ostwald process, Ni in hydrogenation). Autocatalysis: a product speeds up the reaction (Mn\(^{2+}\) in permanganate and oxalic acid).
Heterogeneous catalysis follows the adsorption theory: reactants are adsorbed at active centres, react, and products desorb. Shape-selective catalysis uses zeolites (ZSM-5 converts alcohols to gasoline). Enzymes are protein catalysts: highly specific, effective at body temperature and near pH 7 (zymase: glucose to ethanol; invertase: sucrose to glucose and fructose; maltase: maltose to glucose; urease: urea to ammonia and CO\(_2\)).

Colloids

Colloidal particles have size 1 to 1000 nm. Dispersed phase and dispersion medium give types: sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid, e.g. milk), foam (gas in liquid), aerosol (solid or liquid in gas, e.g. smoke, fog).
Lyophilic (hydrophilic) sols are solvent-loving, stable, reversible and have higher viscosity (starch, gelatin, gum). Lyophobic sols are solvent-hating, need stabilisers, are irreversible and are coagulated easily by electrolytes (metal sols, As2S3, Fe(OH)3).
Properties: Tyndall effect (scattering of light), Brownian motion (keeps the sol stable), charge on particles (electrophoresis, electro-osmosis). Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge of opposite sign (for a negative sol, Al\(^{3+}\) > Ba\(^{2+}\) > Na\(^+\)). Protective colloids: gold number is the milligrams of protective colloid that just prevent the coagulation of 10 mL of gold sol by 1 mL of 10% NaCl; the smaller the gold number, the greater the protective power.
Preparation: dispersion (Bredig's arc, peptisation) and condensation (reduction, oxidation, double decomposition, hydrolysis). Purification: dialysis, ultrafiltration, electrodialysis.

Emulsions and Surfactants

Oil in water (milk, cream) and water in oil (butter, cold cream) emulsions need an emulsifier such as soap or gelatin. Soaps are sodium or potassium salts of long-chain fatty acids (RCOONa); synthetic detergents have the sulphonate or sulphate head group (for example sodium alkylbenzenesulphonate). Cleansing action: hydrocarbon tails dissolve in grease while ionic heads stay in water, forming micelles.

Worked Examples

Example 1. In a Freundlich plot, \( \log(x/m) \) against \( \log P \) has slope 0.5 and intercept 0.3. Write the isotherm.
Answer:
\( \log\dfrac{x}{m} = \log k + \dfrac{1}{n}\log P \), so \( 1/n = 0.5 \) and \( \log k = 0.3 \) (k = 2).
\( \dfrac{x}{m} = 2P^{1/2} \).

Example 2. A protective colloid (0.05 g) just prevents coagulation of 10 mL of gold sol on adding 1 mL of 10% NaCl. Find its gold number.
Answer:
Gold number = milligrams of colloid per 10 mL sol = 0.05 g = 50 mg.