A complete, examination-ready deep-dive into adsorption, catalysis, colloids, emulsions, and their real-world applications.
Adsorption is the accumulation of molecules (adsorbate) on the surface of a solid or liquid (adsorbent) due to surface forces. It is a surface phenomenon — distinct from absorption where the substance permeates the bulk.
Desorption is the reverse process. The equilibrium between the two is called adsorption equilibrium.
Adsorption = surface process only (skin-deep). Absorption = bulk process (soaking through). The umbrella term for both is Sorption.
The surface of a solid has unbalanced/residual forces (valency forces left unsatisfied at the surface). These attract and hold adsorbate molecules. Adsorption is always an exothermic process (ΔH < 0) — the system releases energy as the adsorbate–adsorbent bond forms.
Entropy decreases on adsorption (ΔS < 0). Since ΔG = ΔH − TΔS must be negative for spontaneity: ΔH must be sufficiently negative. As adsorption proceeds, ΔH → 0 and the process stops at equilibrium.
| Factor | Effect on Adsorption |
|---|---|
| Nature of adsorbent | Activated charcoal, silica gel, zeolites have high surface area → more adsorption |
| Nature of adsorbate | Easily liquefiable gases (SO₂ > NH₃ > N₂) adsorb more; high critical temperature → stronger adsorption |
| Surface area | Greater surface area → greater adsorption (activated charcoal = 1000 m²/g) |
| Temperature | Physisorption decreases; chemisorption may first increase then decrease |
| Pressure | Adsorption increases with pressure (until saturation) |
| Activation (of adsorbent) | Removal of adsorbed gases, creating rough surfaces increases adsorption capacity |
A graph of the quantity of adsorbate adsorbed per unit mass of adsorbent (x/m) vs pressure (or concentration) at constant temperature. It tells us how adsorption varies with pressure.
Proposed by H. Freundlich in 1909. It is an empirical relationship between the extent of adsorption and pressure of adsorbate at constant temperature.
| Constant | Symbol | Meaning | Typical Range |
|---|---|---|---|
| KF | Freundlich Capacity Factor | Measure of adsorption capacity. Higher KF → adsorbent has stronger affinity for adsorbate. Found from y-intercept = log KF | Dimensionally dependent on units |
| 1/n | Freundlich Intensity Factor | Measure of the favourability of adsorption. 1/n < 1 → favourable (heterogeneous sites). If 1/n = 1, linear; if >1, unfavourable. | 0 < 1/n < 1 (typically 0.1–0.8) |
Proposed by I. Langmuir (1916) based on a theoretical model. Unlike Freundlich, it has a physical basis. CBSE: Qualitative understanding required
A catalyst is a substance that alters the rate of a chemical reaction without itself being permanently consumed. It provides an alternative reaction pathway of lower activation energy (Ea). Positive catalyst → increases rate; Negative catalyst (inhibitor) → decreases rate.
The catalyst surface is not uniform. Certain spots called active sites have higher energy and are responsible for catalytic activity. These may be corners, edges, cracks, or points of lattice imperfection on the catalyst surface. Poisons block active sites; promoters enhance them.
| Term | Definition | Example |
|---|---|---|
| Promoter | Substance that increases the activity of a catalyst | Mo (molybdenum) added to Fe in Haber process |
| Inhibitor/Poison | Substance that decreases or destroys catalytic activity | CO poisons Fe catalyst in Haber process; As₂O₃ poisons Pt |
| Autocatalysis | Product of the reaction acts as catalyst | Oxidation of oxalic acid by KMnO₄ (Mn²⁺ product catalyses) |
| Enzyme Catalysis | Biological catalysis by protein molecules | Zymase in yeast for fermentation of glucose |
Zeolites are microporous aluminosilicates with honeycomb-like structures. Their uniform pore sizes allow only molecules of a specific size to enter and react — making them shape-selective catalysts. Example: ZSM-5 zeolite converts methanol to gasoline range hydrocarbons (Mobil process).
Based on particle size of the dispersed phase:
| System | Particle Size | Appearance | Filterability | Example |
|---|---|---|---|---|
| True Solution | < 1 nm | Transparent, homogeneous | Passes through all | NaCl in water |
| Colloidal Solution | 1–1000 nm | Translucent, heterogeneous | Passes filter paper; not semipermeable | Milk, blood, smoke |
| Suspension | > 1000 nm | Opaque, settles | Does not pass filter paper | Sand in water |
Colloids are classified based on the nature of the dispersed phase and dispersion medium:
| Dispersed Phase | Dispersion Medium | Name | Example |
|---|---|---|---|
| Solid | Liquid | Sol | Paint, starch solution, gold sol |
| Solid | Gas | Aerosol | Smoke, dust storm |
| Solid | Solid | Solid sol | Gemstones (coloured glass) |
| Liquid | Gas | Aerosol (liquid) | Fog, mist, cloud |
| Liquid | Liquid | Emulsion | Milk, cream, mayonnaise |
| Liquid | Solid | Gel | Cheese, butter, jelly |
| Gas | Liquid | Foam | Whipped cream, soap froth |
| Gas | Solid | Solid foam | Pumice stone, foam rubber |
Small particles aggregate to colloidal size from true solution.
Large bulk particles are broken down to colloidal size.
A semipermeable membrane (animal bladder, cellophane) is used. Small ions and molecules pass through the membrane into the surrounding solvent; colloidal particles do not. Useful for removing electrolyte impurities from colloidal sols.
Dialysis is slow. To speed it up, an electric field is applied across the semipermeable membrane. Ions migrate much faster under the electric potential, making purification more efficient.
Uses ultrafilters — special filter papers impregnated with collodion (nitrocellulose) solution, with pores smaller than colloidal particles. Colloid is retained; solvent and electrolyte pass through. Pressure or suction applied to accelerate the process.
A high-speed centrifuge is used to sediment colloidal particles while the dispersion medium is removed by decantation. Useful for determining molecular weights of polymers and proteins.
| Method | Principle | What is Removed |
|---|---|---|
| Dialysis | Diffusion through semipermeable membrane | Ions, small molecules |
| Electrodialysis | Dialysis + electric field (faster) | Ionic impurities (faster) |
| Ultrafiltration | Filtration through ultra-fine pores | Solvent and electrolytes |
| Ultra-centrifugation | High-speed centrifugal separation | Colloidal particles from solvent |
A protective colloid (usually lyophilic) coats lyophobic particles, preventing coagulation. The lyophilic colloid wraps around the lyophobic particles, imparting hydrophilicity and steric stability.
Gold Number: Proposed by Zsigmondy. The minimum mass (in mg) of a protective colloid needed to prevent coagulation of 10 mL of a standard gold sol when 1 mL of 10% NaCl is added. Lower gold number = more protective power.
| Protective Colloid | Gold Number |
|---|---|
| Gelatin | 0.005–0.01 (most protective) |
| Albumin | 0.1–0.2 |
| Starch | 25 |
| Gum arabic | 0.1–0.15 |
Emulsifiers are surface-active agents (surfactants) that stabilize colloids and emulsions by adsorbing at the interface, reducing interfacial tension. Examples: soaps, detergents, proteins, lecithin. They prevent phase separation by creating a protective film around droplets.
When a beam of light passes through a colloidal solution, the path of light becomes visible due to scattering of light by colloidal particles. This is called the Tyndall effect, discovered by John Tyndall (1869).
Why: Colloidal particles (1–1000 nm) are comparable in size to the wavelength of visible light, causing significant scattering. True solutions do not show this effect (particles too small).
The continuous, random, zigzag motion of colloidal particles in a dispersion medium, observed by Robert Brown (1827) using a microscope. Caused by unequal bombardment of colloidal particles by the solvent molecules. Increases with decreasing particle size and decreasing viscosity. It prevents sedimentation.
Under an applied electric field, charged colloidal particles migrate towards the electrode of opposite charge. This phenomenon is called electrophoresis.
When the colloidal particles are prevented from moving (e.g., packed in a tube) and an electric field is applied, the dispersion medium moves instead — this is electroosmosis. It is the opposite of electrophoresis.
The process of aggregation of colloidal particles to form larger masses that settle down (precipitate) is called coagulation or flocculation. Colloidal particles carry the same charge; mutual repulsion prevents settling. When this charge is neutralised, particles come together and coagulate.
The minimum concentration of electrolyte (in millimoles per litre) required to cause coagulation of a colloidal sol in 2 hours is called the coagulation value or flocculation value. Lower coagulation value = more effective coagulating agent.
The stability of a lyophobic colloid is governed by the balance between:
Each colloidal particle is surrounded by two layers: the Stern layer (compact layer of counter-ions) and the diffuse double layer. The potential at the shear plane is the zeta potential (ζ). Higher ζ-potential → more stable sol.
When electrolyte is added, the double layer is compressed (increased ionic strength screens the charge), reducing the energy barrier to aggregation → coagulation occurs.
An emulsion is a colloidal system in which both dispersed phase and dispersion medium are liquids, and they are immiscible with each other. At least one liquid must be water.
The process of making an emulsion from two immiscible liquids is called emulsification. It requires:
Emulsifiers are surface-active substances (surfactants) with a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. They adsorb at the oil-water interface, reducing interfacial tension and forming a protective film around droplets that prevents coalescence.
| Emulsifier | Type of Emulsion Formed | Examples |
|---|---|---|
| Sodium/potassium soaps (alkali metal soaps) | O/W (oil in water) | Sodium stearate, potassium oleate |
| Calcium/magnesium soaps (heavy metal soaps) | W/O (water in oil) | Calcium palmitate, magnesium stearate |
| Proteins (casein, albumin) | O/W | Milk emulsion stabilized by casein |
| Lecithin (phospholipid) | O/W | Mayonnaise stabilized by egg-yolk lecithin |
| Gum arabic | O/W | Food and pharmaceutical emulsions |
| Cetyl alcohol, beeswax | W/O | Cold cream, ointments |
| Test | O/W Emulsion | W/O Emulsion |
|---|---|---|
| Dilution test | Miscible with water | Miscible with oil |
| Conductivity test | Good conductor | Poor conductor |
| Dye test (water-soluble dye) | Dye diffuses uniformly (water is continuous) | Dye forms globules |
| Filter paper test | Leaves wet patch (water spreads) | Leaves oil patch |
Breaking of an emulsion is called demulsification. Methods include heating, centrifugation, addition of electrolyte, freezing, or adding a demulsifying agent. Used in crude oil processing to separate water from petroleum.
FeCl₃ coagulates negatively charged blood particles to stop bleeding.
Alum (Al³⁺) coagulates colloidal clay in water; also activated charcoal for decolorisation.
Activated charcoal adsorbs poisonous gases (CO, Cl₂) in charcoal gas masks.
Adsorption of dye molecules on cloth fibres and paper surface.
Differential adsorption separates components in column, TLC, HPLC.
Activated charcoal (in tablets) adsorbs poisons in the stomach.
Silica gel and zeolite adsorbents in desiccants and gas purification.
Silica gel (SiO₂) adsorbs water vapour — used in packaging as desiccant.
| Field | Colloid Involved | Application |
|---|---|---|
| Medicine | Colloidal gold, sulphur, silver (Argyrol) | Drug delivery; antiseptic; colloidal medicines easier absorbed |
| Photography | AgBr in gelatin | Photographic emulsion coated on film/paper |
| Food Industry | Emulsions, gels, foams | Mayonnaise, ice-cream, bread, processed cheese |
| Sewage Treatment | Colloidal sewage particles | Coagulated by electrolytes (lime, alum); Hardy-Schulze rule applied |
| Smoke Precipitation | Smoke aerosol | Cottrell precipitator uses high-voltage electrodes to coagulate smoke particles |
| Rubber Industry | Latex (rubber sol) | Coagulation by acid (acetic acid) to obtain rubber |
| Tanning of Leather | Positively charged hide + tannin (−ve) | Mutual coagulation to harden/tan leather |
| Detergency | Micelles (colloidal aggregates) | Oil in water emulsified by soap/detergent micelles, removing grease |
Fe catalyst + Mo promoter: N₂ + 3H₂ → 2NH₃ (fertilisers, explosives)
V₂O₅ catalyst: 2SO₂ + O₂ → 2SO₃ → H₂SO₄ (industrial acid)
Zeolite catalysts crack heavy petroleum fractions into petrol range hydrocarbons.
Pt/Pd/Rh convert CO, NO, hydrocarbons to CO₂, N₂, H₂O in exhaust.
Ni catalyst: C=C bonds in vegetable oils + H₂ → margarine (solid fat)
Enzymes (biological catalysts): zymase ferments glucose to ethanol + CO₂
Q1. Which type of adsorption involves formation of chemical bonds between adsorbate and adsorbent?
Q2. In the Freundlich adsorption isotherm log(x/m) vs log P plot, the slope gives:
Q3. The Tyndall effect is shown by:
Q4. According to Hardy-Schulze rule, which coagulating agent is most effective for a negatively charged sol?
Q5. Milk is an example of which type of emulsion?
Q6. The catalyst used in the Haber's process for manufacture of ammonia is:
Q7. The process of converting a freshly prepared precipitate into a colloidal sol by addition of an electrolyte is called:
Q8. Gold number is a measure of:
| Topic | Key Point | Example / Value |
|---|---|---|
| Physisorption | van der Waals, multilayer, reversible, ~20–40 kJ/mol | N₂ on charcoal |
| Chemisorption | Chemical bond, monolayer, specific, ~80–240 kJ/mol | H₂ on Fe catalyst |
| Freundlich: x/m = K·P^(1/n) | Log plot: slope = 1/n, intercept = log K | 0 < 1/n < 1 |
| Langmuir | Monolayer, homogeneous, active sites, saturation | θ → 1 at high P |
| Homogeneous catalysis | Same phase, e.g., NO catalyst in SO₂ oxidation | H⁺ in esterification |
| Heterogeneous catalysis | Different phase, surface reaction, active sites | Fe in Haber, V₂O₅ in Contact |
| Colloidal size | 1–1000 nm | Milk, blood, paint |
| Tyndall effect | Light scattering by colloidal particles | Sunbeam in dusty room |
| Brownian motion | Zigzag, prevents sedimentation | Pollen in water |
| Electrophoresis | Charged particles migrate in electric field | Fe(OH)₃⁺ → cathode |
| Hardy-Schulze Rule | Coagulating power ∝ valency | Al³⁺ > Mg²⁺ > Na⁺ |
| Gold number | Lower = more protective | Gelatin: 0.005–0.01 |
| O/W emulsion | Oil drops in water; dilutable with water | Milk, vanishing cream |
| W/O emulsion | Water drops in oil; dilutable with oil | Butter, cold cream |
| Peptization | Precipitate → colloid by electrolyte | Fe(OH)₃ + FeCl₃ |
| Dialysis | Remove ions using semipermeable membrane | Kidney dialysis machine |