Surface Chemistry studies what happens at the surface where two phases meet, covering adsorption, catalysis, and the colloidal state. You learn how substances pile up on a surface (physisorption vs chemisorption and the Freundlich isotherm), how catalysts speed up reactions by lowering activation energy, and how colloids behave (Tyndall effect, Brownian motion, coagulation). It is a high-scoring, fact-heavy NEET chapter where most questions are direct concept recall rather than long numericals.
Class 12 Chemistry · Chapter 18 – swipe through all 10 cards to understand the whole chapter.
Adsorption: a surface game
Adsorption is the piling-up of a substance on the surface of a solid, not throughout its bulk.
Absorption = uniform into the bulk; sorption = both together.
- Surface atoms have unbalanced residual forces that grab passing particles
- Adsorbate = caught substance; adsorbent = the surface (charcoal, silica gel)
- More surface area (porous/powdered) → more adsorption
Why adsorption is spontaneous
Adsorption is always exothermic and lowers randomness, yet still happens.
At equilibrium ΔG = 0; no further net adsorption.
- ΔH < 0 (heat released as surface energy falls) — never call it endothermic
- ΔS < 0 because a free gas loses freedom on the surface
- Negative ΔH must outweigh the positive −TΔS term for ΔG < 0
Physisorption vs Chemisorption
Adsorption splits by the forces holding the adsorbate — weak van der Waals or true chemical bonds.
With T: physisorption falls; chemisorption rises (needs Eₐ) then falls.
- Physisorption: van der Waals, reversible, non-specific, multilayer, low T
- Chemisorption: chemical bonds, irreversible, highly specific, monolayer
- High critical-temp gases (CO2) physisorb more; can convert to chemisorption on heating (H2/Ni)
Freundlich Adsorption Isotherm
At constant temperature, adsorption per gram varies with pressure as a fractional power.
Log-log plot: slope = 1/n, intercept = log k; 0 < 1/n < 1.
- Low P: x/m ∝ P1 ; high P: x/m ∝ P0 (surface saturates)
- x = mass of gas adsorbed, m = mass of adsorbent
- Empirical only — fails at very high pressure; for solutions use C in place of P
Catalysis
A catalyst gives an alternative path with lower activation energy and is regenerated unchanged.
Heterogeneous: diffusion → adsorption → reaction → desorption.
- Homogeneous = same phase (NO in lead-chamber); heterogeneous = different phase
- Memorise: Haber → Fe(+Mo) ; Contact → V2O5 ; hydrogenation → Ni
- Activity = how much it accelerates; selectivity = steers to one product
Enzymes & shape-selective catalysis
Biological and pore-based catalysts achieve extreme specificity.
ZSM-5 zeolite converts alcohols to gasoline.
- Enzymes are protein catalysts — highly specific (maltase, urease, zymase)
- Zeolites admit only molecules of a particular size/shape
- Both are exam favourites for specificity questions
Colloids & their classification
A colloid is a heterogeneous system with particle size between a true solution and a suspension.
Below 1 nm = true solution; above 1000 nm = suspension.
- By state: sol, aerosol, foam, gel, emulsion (read as phase-in-medium)
- Lyophilic = stable + reversible (starch, gelatin); lyophobic = unstable + needs stabiliser (metal sols, As2S3)
- By particle: multimolecular (S, Au sol), macromolecular (proteins), associated colloids (micelles)
Associated colloids & cleansing
Soaps act as normal electrolytes at low concentration but cluster into micelles above a threshold.
Tail traps grease inward, head faces water outward — micelle washes away.
- CMC = critical micelle concentration; below it, soap behaves as an electrolyte
- Micelle = water-hating tails inward, water-loving heads outward
- This associated-colloid behaviour is how soap cleans grease
Properties of colloids
Colloidal particles scatter light, jitter, and carry charge — telling them apart from true solutions.
Tyndall effect distinguishes a colloid from a true solution.
- Tyndall = light scattering makes the beam visible
- Brownian motion (uneven bombardment) gives stability, prevents settling
- Electrophoresis = charged particles drift to an electrode
Coagulation & Hardy–Schulze rule
Neutralising the colloid’s charge with an electrolyte makes it clump and settle.
coagulating power ∝ 1 / coagulating value (smallest value = strongest).
- Negative sol (As2S3): Al3⁺ > Ba2⁺ > Na⁺ (compare cations)
- Positive sol (Fe(OH)3): PO43⁻ > SO42⁻ > Cl⁻ (compare anions)
- Protective colloids (lyophilic) shield sols; smaller gold number = better protection
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Related Chapters in Class 12 Chemistry
- Chemical Kinetics Class 12 Notes
- Electrochemistry Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
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Frequently Asked Questions
Adsorption is the accumulation of a substance only at the surface of a solid or liquid, while absorption is the uniform spreading of a substance throughout the bulk. In adsorption the caught substance is the adsorbate and the surface is the adsorbent, for example a gas concentrating on charcoal; when both happen together it is called sorption.
The Freundlich isotherm describes how the amount adsorbed per gram of adsorbent varies with pressure at constant temperature, given by x by m equals k times P raised to 1 by n, where n is greater than 1. Taking log gives a straight line log(x by m) equals log k plus (1 by n) log P, so a log-log plot has slope 1 by n and intercept log k.
Physisorption is held by weak van der Waals forces, is reversible, non-specific, forms multilayers, has low enthalpy of about 20 to 40 kJ per mol, and is favoured at low temperature. Chemisorption involves actual chemical bonds, is irreversible, highly specific, forms only a monolayer, and has much higher enthalpy of about 80 to 240 kJ per mol.
A catalyst provides an alternative reaction path with lower activation energy and is regenerated unchanged at the end. It only makes the reaction faster and does not change the enthalpy, Gibbs energy, or the equilibrium constant of the reaction.
Yes, Surface Chemistry is part of the NEET syllabus and usually contributes around one or two direct questions. The questions are mostly concept and fact based, such as comparing physisorption and chemisorption, the Hardy-Schulze rule, the Tyndall effect, and catalyst examples like iron in the Haber process and vanadium pentoxide in the Contact process.