JEE Chemistry Surface Chemistry Notes Set 02

Adsorption and Absorption

Adsorption is the build-up of a substance (adsorbate) on the surface of a solid or liquid (adsorbent). Absorption is uniform uptake into the bulk, as water enters a sponge. Adsorption is fast at first and then slows, and it is accompanied by release of heat, so it is exothermic (ΔH negative). The surface is the site, so finely divided solids and porous solids adsorb strongly.

Physisorption and Chemisorption

Factors Affecting Adsorption of Gases

1. A gas that liquefies easily (higher critical temperature) is adsorbed more because its van der Waals forces are stronger.
2. Greater surface area of the solid (powder, porous form) means more adsorption; activating a solid increases its surface.
3. Adsorption falls as temperature rises (exothermic) and rises with pressure.

Adsorption Isotherms

Freundlich isotherm for gases: \[ \dfrac{x}{m} = kp^{1/n} \quad (n > 1) \]
Taking logarithms, \( \log\dfrac{x}{m} = \log k + \dfrac{1}{n}\log p \), a straight line with slope 1/n and intercept log k. At low pressure the amount adsorbed is proportional to p; at high pressure it becomes independent of p.
For adsorption from solution \( \dfrac{x}{m} = kC^{1/n} \).
Langmuir isotherm assumes a monolayer: \( \dfrac{x}{m} = \dfrac{ap}{1+bp} \), giving x/m ∝ p at low pressure and a constant value at high pressure.

Applications of Adsorption

Gas masks (charcoal), removal of colour from sugar solutions, production of high vacuum, froth flotation, heterogeneous catalysis, softening of water by ion exchange resins and zeolites, and chromatography.

Catalysis

A catalyst changes the rate of a reaction without being used up, by providing a path of lower activation energy; it does not change ΔH or the equilibrium composition. Catalysts are usually specific, work in small amounts, and are more effective when finely divided.
1. Homogeneous catalysis: catalyst and reactants in the same phase, for example NO in the lead chamber process.
2. Heterogeneous catalysis: different phases, for example Fe in the Haber process with Mo as a promoter, V₂O₅ in the contact process and Ni in hydrogenation.
A promoter increases the activity of a catalyst and a poison decreases it. Steps of surface catalysis: diffusion of reactants to the surface, adsorption, reaction on the surface, desorption of products and diffusion away. Enzymes are highly specific biological catalysts.

Colloids

A colloid is a heterogeneous system in which particles of size 1 to 1000 nm (the dispersed phase) are spread through a medium. Types by phases: sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid), foam (gas in liquid or solid), aerosol (solid or liquid in gas).
Lyophilic (solvent-loving) sols are strongly solvated, stable and reversible; lyophobic sols are not solvated, need a stabiliser and coagulate irreversibly.
By particle type: multimolecular (aggregates, e.g. gold sol), macromolecular (giant molecules such as starch or proteins) and associated colloids (micelles formed by soaps and detergents above the critical micelle concentration).

Preparation and Purification

Methods: chemical (double decomposition, oxidation, reduction, hydrolysis), Bredig's arc method for metal sols, and peptisation of a fresh precipitate with a little electrolyte. Dialysis removes dissolved ions and molecules through a semi-permeable membrane.

Properties of Colloids

1. Tyndall effect: scattering of light by the particles makes the path of a light beam visible.
2. Brownian motion: random zig-zag motion caused by collisions with the medium keeps the sol from settling.
3. Charge: particles carry a charge and move in an electric field (electrophoresis). Fe(OH)₃ and Al(OH)₃ sols are positive; As₂S₃, gold and clay sols are negative.
4. Coagulation: neutralising the charge precipitates the sol. By the Hardy-Schulze rule the coagulating power of an ion of opposite charge rises with its valence, for example Al³⁺ > Ba²⁺ > Na⁺ for a negative sol and [Fe(CN)₆]³⁻ > SO₄²⁻ > Cl⁻ for a positive sol.
5. Protection: a lyophilic colloid protects a lyophobic sol. The gold number is the minimum mass in mg of protective colloid that stops coagulation of 10 mL of gold sol by 1 mL of 10% NaCl; the smaller the gold number, the greater the protecting power.

Emulsions, Gels and Everyday Examples

Emulsions are of two types: oil in water (milk) and water in oil (butter). An emulsifier such as soap stabilises them. Soaps clean because their hydrocarbon tails dissolve grease inside a micelle while the ionic heads face the water. A gel loses liquid slowly by syneresis. Blood is stopped by ferric chloride (coagulation), deltas form where river water meets sea water, and smoke is removed with an electrostatic precipitator (Cottrell).

Worked Example

Example. A log-log plot of x/m against p has slope 1 and ln k = 0.693. Find x/m at 0.5 atm.
Slope 1 means 1/n = 1, so x/m = kp. Since k = e^0.693 = 2, x/m = 2 × 0.5 = 1.