Surface Chemistry

Surface Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Surface chemistry studies processes that occur at the boundary between two phases. The two master terms to lock in are adsorption (accumulation of a species at the surface of a solid or liquid) and absorption (uniform penetration into the bulk); their sum is called sorption. The Freundlich isotherm x/m = k·P^(1/n) and the Langmuir isotherm x/m = a·P/(1+b·P) are the two equations that appear most often in numericals. Adsorption is exothermic (ΔH < 0), so an increase in temperature decreases physical adsorption but may increase chemical adsorption past the activation barrier. Colloids are identified by the Tyndall effect (scattering of light by particles 1–1000 nm) and stabilised by Brownian motion plus like-charged surfaces. For coagulation, remember the Hardy–Schulze rule: the coagulating ion must carry the opposite charge, and its coagulation power ∝ valency.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Physisorption vs Chemisorption

Property	Physisorption	Chemisorption
Enthalpy (ΔH)	20–40 kJ mol⁻¹ (van der Waals)	40–400 kJ mol⁻¹ (chemical bond)
Layers	Multi-layer	Mono-layer
Specificity	Non-specific	Highly specific
Reversibility	Reversible	Often irreversible
Effect of T	Decreases with rising T	May rise, then fall (activated)
Activation energy	Nil	Appreciable Eₐ

Adsorption Isotherms

Freundlich (empirical): x/m = k·P^(1/n), where x is mass adsorbed, m is mass of adsorbent, P is equilibrium pressure, k and n are constants (1/n lies between 0 and 1). Taking logs gives a straight line: log(x/m) = log k + (1/n) log P, the form used in JEE plots. It fails at high P (predicts continuous rise).

Langmuir (theoretical): x/m = a·P/(1+b·P), derived from the postulates: (i) fixed number of identical sites, (ii) mono-layer coverage, (iii) no lateral interaction, (iv) dynamic equilibrium between adsorption and desorption. Its linear form P/(x/m) = 1/(a·b) + P/a lets you read off a (saturation uptake) and b (adsorption constant) from slope/intercept.

Classification of Colloids

• Lyophilic (solvent-loving, e.g. gum, starch, proteins): thermodynamically stable, reversible, viscous; coagulated only by adding a non-solvent + electrolyte.
• Lyophobic (solvent-hating, e.g. metals, metal sulphides): need a stabiliser, irreversible, easily coagulated.
• Associated colloids: electrolytes (e.g. sodium stearate) that behave as normal solutes below the Critical Micelle Concentration (CMC) and form micelles above it. CMC ≈ 10⁻³–10⁻² mol L⁻¹ for soaps.

Coagulation and the Hardy–Schulze Rule

The coagulation value is the minimum millimolar concentration of an electrolyte needed to coagulate a sol in 2 hours. Coagulation power ∝ 1/coagulation value ∝ valency of the effective ion. For a negative sol (As₂S₃), the order is Al³⁺ > Mg²⁺ > Na⁺; for a positive sol (Fe(OH)₃), it is PO₄³⁻ > SO₄²⁻ > Cl⁻.

Catalysis

A catalyst lowers the activation energy of the rate-determining step without altering ΔG. In heterogeneous catalysis (e.g. Haber process on Fe, H₂ + vegetable oil on Ni) the five-step mechanism is: diffusion → adsorption → surface reaction → desorption → diffusion away. A promoter enhances activity (Mo in Fe for Haber) whereas a poison (H₂S, CO) blocks active sites. Enzymes are biological catalysts whose activity peaks at an optimum pH and temperature.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Sign Convention and Spontaneity

Adsorption decreases surface free energy and frees one translational degree of the adsorbed molecule, so ΔS < 0 and ΔH < 0. Substituting in ΔG = ΔH − TΔS, the negative ΔH dominates only at low T, hence physisorption is favoured at low temperature and high pressure. The Traube rule states that for homologous fatty acids on water, k in the Freundlich equation multiplies by roughly a factor of three for each –CH₂– group added, because the longer chain reduces surface tension more effectively.

Edge Cases on the Langmuir Isotherm

At very low P, x/m ≈ a·P, so the plot is linear. At very high P, x/m → a/b = x_m, the saturation monolayer capacity. The constant b = k_ads/k_des carries units of pressure⁻¹, so it must be reported with its reference temperature. BET theory (Brunauer–Emmett–Teller) extends Langmuir to multilayer adsorption and is the workhorse for measuring the specific surface area of catalysts using N₂ adsorption at 77 K; the BET plot of P/[V(P₀ − P)] versus P/P₀ gives a straight line in the range 0.05 < P/P₀ < 0.35.

Emulsions, Gels and Foams

Emulsions are liquid-in-liquid colloids classified O/W (milk, vanishing cream) or W/O (butter, cold cream), stabilised by an emulsifier (e.g. soap, casein). Bancroft’s rule states that the phase in which the emulsifier is more soluble becomes the dispersion medium. Gels are semi-solid lyophilic colloids (jellies, silica gel); loss of water on standing is called syneresis, and reversible sol–gel conversion on shaking is thixotropy. Foams are gas-in-liquid/solid colloids stabilised by surfactants and proteins (e.g. beaten egg white, froth flotation).

Practice Prompts

1. Numerical: 2.0 g of charcoal adsorbs 20 mg of acetic acid at 1 bar and 20 mg at 4 bar. Calculate the Freundlich constants k and 1/n. (Hint: solve two simultaneous equations from the linear log–log plot.)
2. Reasoning: An As₂S₃ sol is negatively charged. Rank the coagulating power of NaCl, BaCl₂ and AlCl₃ on an equimolar basis and justify using the Hardy–Schulze rule. Then state what happens when light is shone through the sol and explain the observation.

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