Revision: Class 12 >> Surface Chemistry NEET (UG) Surface Chemistry

The substance adsorbed on the surface of another substance is called as adsorbate or absorbed phase.

A substance which adsorbs another substance on its surface is called as adsorbent.

Adsorption is the phenomenon of accumulation of a higher concentration of one substance on the surface of another (in bulk) due to unbalanced/unsatisfied attractive forces on the surface.

The phenomenon of accumulation of substance at the surface of another molecular species rather than in its bulk is called adsorption. e.g. Adsorption of gases like nitrogen, carbon dioxide by activated charcoal.

When both adsorption and absorption occur simultaneously, it is known as sorption.

The process of removal of an adsorbed substance from the surface of adsorbent is known as desorption.

The relationship between the extent of adsorption (x/m) and the pressure of gas at constant temperature is called an adsorption isotherm.

The phenomenon in which the rate of a chemical reaction is reduced by an inhibitor is called inhibition.

A catalyst is a substance that alters the rate of a chemical reaction without being consumed. The phenomenon is called catalysis.

The phenomenon of increasing the rate of a chemical reaction with the help of a catalyst is known as catalysis.

The movement of colloidal particles under an applied electric potential is called electrophoresis.

A substance is said to be in colloidal state when one substance of heterogeneous system is dispersed (dispersed phase) as very fine particles in another substance (dispersion medium).

Multimolecular colloids are species having size in the colloidal range (diameter < 1 nm) that are formed by the aggregation of large number of atoms or small molecules of substances when dissolved in the dispersion medium.

The process used for reducing the amount of impurities to a requisite minimum is called as purification of colloidal solution.

Dialysis process is used for removing a dissolved substance from a colloidal solution by diffusion through a suitable membrane.

Multimolecular colloids are species having size in the colloidal range (diameter < 1 nm) that are formed by the aggregation of large number of atoms or small molecules of substances when dissolved in the dispersion medium.

The process used for reducing the amount of impurities to a requisite minimum is called as purification of colloidal solution.

Dialysis process is used for removing a dissolved substance from a colloidal solution by diffusion through a suitable membrane.

The potential difference between a colloidal particle's fixed layer (primary layer) and diffused layer (secondary layer) is known as the zeta potential. It's also referred to as electrokinetic potential.

It is a process of aggregating together the colloidal particles so as to change them into large-sized particles, which ultimately settle as a precipitate.

Coagulation is the process by which colloidal particles aggregate and settle as a precipitate. It is brought about by the addition of electrolyte.

Emulsions are colloidal solutions of two immiscible liquids in which one liquid acts as the dispersed phase and the other as the dispersion medium.

Freundlich Adsorption Isotherm:

Empirical equation: \[\frac{x}{m}=kP^{1/n}\quad(n>1)\]

where:

• x = mass of the gas adsorbed
• m = mass of the adsorbent
• `x/m` = mass of gas adsorbed per unit mass of adsorbent
• P = equilibrium pressure
• k and n = constants depending on the nature of the adsorbate, adsorbent, and temperature

For solutions: \[\frac{x}{m}=kC^{1/n}\] where C = concentration of the solution

Logarithmic (linear) form: \[\log\frac{x}{m}=\log k+\frac{1}{n}\log P\]

In the log x/m vs log P graph:

• Slope = `1/n` (value between 0 and 1)
• Y-intercept = log k
• When `1/n` = 0: adsorption is independent of pressure
• When `1/n` = 1: adsorption is directly proportional to pressure

1. Nature of adsorbent — Substances with large surface area per unit mass (porous solids like silica gel and charcoal) are more effective adsorbents
2. Surface area — Greater the surface area of the adsorbent, greater is the extent of adsorption
3. Temperature — Adsorption is exothermic, so it decreases with rise in temperature (and vice versa)
4. Pressure of the gas — At constant temperature, extent of gas adsorption increases with increase in pressure
5. Nature of adsorbate (gas) — Gases that are more easily liquefiable (e.g., SO₂, Cl₂, NH₃) get adsorbed to a larger extent compared to gases like N₂, O₂, H₂

The reaction occurs on the surface of the solid catalyst. Steps involved:

1. Diffusion — Reactant molecules diffuse towards the surface of the catalyst
2. Adsorption — Reactant molecules adsorb onto the surface of the catalyst
3. Intermediate formation — Chemical reaction on the catalyst surface proceeds via intermediate formation
4. Desorption — Products are desorbed (released) from the catalyst surface
5. Product formation — Products diffuse away from the surface

Heterogeneous catalytic reactions are also called surface reactions. Heterogeneous catalysts are used in automobile catalytic converters.

(i) Based on Physical State of Dispersed Phase and Dispersion Medium

(ii) Based on Interaction / Affinity of Phases

(iii) Based on Molecular Size

1. Chemical Methods

• Oxidation: \[\mathrm{SO}_2+2\mathrm{H}_2\mathrm{S}\xrightarrow{\mathrm{Oxidation}}3\mathrm{S}\downarrow+2\mathrm{H}_2\mathrm{O}\] (sulphur sol)

• Reduction: \[2\mathrm{AuCl}_{3}+3\mathrm{HCHO}+3\mathrm{H}_{2}\mathrm{O}\xrightarrow{\mathrm{Reduction}}2\mathrm{Au}\downarrow+3\mathrm{HCOOH}+6\mathrm{HCl}\] (gold sol)

• Hydrolysis: \[\mathrm{FeCl}_3+3\mathrm{H}_2\mathrm{O}\xrightarrow{\text{Hydrolysis}}\mathrm{Fe(OH)}_3\downarrow+3\mathrm{HCl}\] (iron hydroxide sol)

2. Electrical Disintegration — Bredig's Arc Method

• An electric arc is struck between electrodes of the metal immersed in the dispersion medium
• The intense heat produces metal vapour that condenses to form colloidal particles

3. Peptization

• A precipitate is converted into colloidal sol by shaking it with the dispersion medium in the presence of a small amount of electrolyte (the peptizing agent)

Emulsions show all properties of sols. Their particle size is 1000 Å to 10,000 Å; they scatter light (Tyndall effect).

Types of Emulsions: