# Surface Chemistry

> Adsorption, catalysis, colloids, Tyndall effect, Hardy-Schulze rule and emulsions — NCERT Class 12 NEET-UG Chemistry notes.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/surface-chemistry
- Exam / course: NEET-UG · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Surface Chemistry", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/surface-chemistry

## Direct answer

Adsorption differs from absorption the way a sticker differs from a sponge: the molecule sticks at the surface rather than soaking through the bulk, and the difference matters because adsorption powers catalysis — iron in ammonia synthesis, nickel in hydrogenation, V2O5 in the Contact process. Colloids, particles of 1 to 1000 nanometres dispersed in a medium, show the Tyndall effect, Brownian motion and electrophoresis, and they coagulate when electrolytes neutralise their charge, following the Hardy-Schulze rule that higher-charged ions precipitate more powerfully. Soaps clean by forming micelles above the critical micelle concentration.

## What you must remember

- Physisorption: van der Waals forces, 20–40 kJ/mol, multimolecular, reversible, needs no activation energy, decreases with temperature.
- Chemisorption: chemical bonds, 80–240 kJ/mol, unimolecular, often irreversible, first increases with temperature then falls.
- Freundlich isotherm x/m = k·P^(1/n) with n > 1; it fails at high pressure, where x/m becomes pressure-independent.
- Industrial catalysts: finely divided Fe (Haber), V2O5 (Contact), Ni for hydrogenation of vegetable oils; zeolite ZSM-5 converts methanol to gasoline by shape-selective catalysis.
- Colloid types: sol, emulsion, foam, aerosol; lyophilic (solvent-loving, self-stabilising) versus lyophobic (needing stabilisers, easily coagulated).
- Micelles form above the critical micelle concentration and above the Kraft temperature; soaps and detergents are the standard examples.
- Hardy-Schulze rule: for a negative sol, Al^3+ > Ba^2+ > Na^+; for a positive sol, [Fe(CN)6]^4− > SO4^2− > Cl−.
- Flocculation value is the minimum electrolyte concentration needed to coagulate — the smaller the value, the stronger the coagulating power.

## A reasoned comparison: physisorption versus chemisorption

Line the two up across four properties and the logic shows why temperature behaves so differently. Physisorption is like condensation — any gas adsorbs on any clean surface if cooled enough, enthalpy is small (20–40 kJ/mol), layers can pile up multimolecularly, and no bonds are made or broken, so no activation energy is needed and raising temperature simply drives the molecules off the surface. Chemisorption is a chemical reaction with the surface: enthalpy is high (80–240 kJ/mol), only one layer can bond, specificity appears (hydrogen on nickel, oxygen on charcoal), and an activation barrier must be crossed — which is precisely why chemisorption first rises with temperature (more molecules can clear the barrier) and then falls (desorption takes over). Adsorption is always exothermic, so both types weaken at high enough temperature by Le Chatelier's principle. When a question shows an adsorption-versus-temperature curve peaking and then declining, it is chemisorption asking to be recognised.

## Charge-matching and other classic errors

The sign-matching error dominates coagulation questions: a positively charged sol needs the negative ion for coagulation, and students who apply the Al^3+ ordering blindly to a positive sol get it wrong — first establish the sol's charge from its preparation, then match the opposite ion. Second, the Tyndall effect is not just "light passes": colloidal particles scatter because their size matches the wavelength of visible light, which is why true solutions show no beam. Third, the Cottrell precipitator removes smoke at power plants, and alum purifies muddy water — both are coagulation applications, while a river delta forms when electrolyte-rich seawater coagulates river colloidal clay; examiners recycle all three as assertion-reason items. Finally, the Kraft temperature and the critical micelle concentration are two separate thresholds — heat alone does not make micelles without enough soap molecules present.

## Frequently asked questions

### Why is chemisorption more specific than physisorption?

Chemisorption requires actual bond formation between adsorbate and surface, so only certain gas-surface combinations react, whereas van der Waals attraction is universal.

### How does the Freundlich isotherm behave at high pressure?

x/m approaches a saturation limit and becomes independent of pressure, beyond the isotherm's valid range.

### Which ion coagulates a negatively charged sol most effectively?

Al^3+, per the Hardy-Schulze rule — coagulating power rises steeply with the charge magnitude of the oppositely charged ion.

### What is the Kraft temperature?

The temperature above which surfactant micelles form; below it, micelles disintegrate regardless of concentration.

### Name one oil-in-water and one water-in-oil emulsion.

Milk is oil-in-water; butter and cold cream are water-in-oil — the emulsifier used determines which type forms.