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Step-by-step answers for all textbook questions in this chapter
Quick Summary
Surface chemistry deals with phenomena occurring at interfaces between phases. In TN HSE, this chapter contributes 6–8 marks. Adsorption types, types of colloids, the Tyndall effect, Hardy-Schulze rule, and heterogeneous catalysis are the most frequently tested topics. Understanding the distinction between true solutions, colloids, and suspensions is the foundation for everything else.
1. Adsorption
Adsorption is the accumulation of molecular species (gas, liquid, or dissolved substance) on the surface of a solid or liquid.
Key vocabulary:
- Adsorbent: The surface on which adsorption occurs (e.g., silica gel, charcoal, alumina)
- Adsorbate: The substance that gets adsorbed
- Desorption: Removal of adsorbed substances from the surface
Adsorption vs Absorption:
| Adsorption | Absorption |
|---|---|
| Surface phenomenon | Bulk phenomenon |
| Accumulation on surface | Uniform distribution throughout |
| Examples: charcoal adsorbing gases, silica gel adsorbing moisture | Examples: sponge absorbing water, anhydrous CaCl₂ absorbing water vapour |
The term sorption is used when both occur simultaneously.
2. Types of Adsorption
Physisorption (Physical Adsorption)
Caused by weak van der Waals forces between adsorbent and adsorbate.
- Enthalpy of adsorption is low (20–40 kJ mol⁻¹)
- Reversible — adsorbed molecules easily released by heating or reducing pressure
- Non-specific — all gases adsorbed on all surfaces
- Multilayer adsorption is possible
- Amount of adsorption increases with decreasing temperature (exothermic process — Le Chatelier's principle)
- Easily liquefied gases (high critical temperature) adsorb more: SO₂ > CO₂ > N₂
Chemisorption (Chemical Adsorption)
Caused by chemical bond formation (covalent or ionic) between adsorbent and adsorbate.
- Enthalpy of adsorption is high (40–400 kJ mol⁻¹)
- Irreversible (or only reversible under forcing conditions)
- Highly specific — particular adsorbate-adsorbent pairs
- Monolayer adsorption only
- Increases with temperature up to a point (needs activation energy to form chemical bond)
Comparison Table:
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces | Van der Waals | Chemical bonds |
| Enthalpy | 20–40 kJ mol⁻¹ | 40–400 kJ mol⁻¹ |
| Reversibility | Reversible | Irreversible |
| Specificity | Non-specific | Specific |
| Layers | Multilayer | Monolayer |
| Temperature effect | Decreases with T | Increases then decreases |
3. Freundlich Adsorption Isotherm
At constant temperature, the relationship between extent of adsorption and pressure (for gases) or concentration (for solutions):
where:
- x = mass of adsorbate
- m = mass of adsorbent
- P = equilibrium pressure of gas
- k and n are constants (n > 1 always)
Taking logarithm:
Plot of log(x/m) vs log P gives a straight line with slope = 1/n and intercept = log k.
Limitations: Empirical equation — does not explain why adsorption levels off at high pressure. The Langmuir isotherm (based on monolayer adsorption) provides a more theoretical treatment.
4. Catalysis
A catalyst is a substance that alters the rate of a reaction without being permanently consumed in it.
Types of Catalysis
1. Homogeneous Catalysis Catalyst and reactants are in the same phase.
Example: Lead chamber process for H₂SO₄
2. Heterogeneous Catalysis (Contact Catalysis) Catalyst and reactants are in different phases.
Examples:
- Haber's process: N₂ + 3H₂ → 2NH₃ (Fe catalyst, gas-phase reactants)
- Ostwald's process: oxidation of NH₃ over Pt catalyst
- Hydrogenation of oils using Ni catalyst
3. Auto-catalysis One of the reaction products acts as a catalyst.
Example: KMnO₄ + H₂C₂O₄ (oxalic acid) in acidic medium — the Mn²⁺ produced is the catalyst. Reaction starts slowly and accelerates as product accumulates.
4. Enzyme Catalysis Biological catalysts (proteins) with high specificity and efficiency.
Example: Amylase catalyses hydrolysis of starch; urease hydrolyses urea to NH₃ and CO₂.
Key feature: enzyme-substrate complex forms at active site; "lock and key" model.
Mechanism of Heterogeneous Catalysis
1. Adsorption of reactant molecules on catalyst surface (active sites) 2. Formation of intermediate — surface bonds weaken the bonds in adsorbate 3. Reaction between adsorbed species at the surface 4. Desorption of products from the surface 5. Diffusion of products away from surface
Effect of catalyst on energy profile:
- Lowers activation energy Ea for both forward and reverse reactions equally
- Does NOT change ΔH (enthalpy), ΔG, or equilibrium constant K
- Does NOT change equilibrium position — only speeds up reaching equilibrium
Promoters vs Poisons:
- Promoters (activators): substances that increase catalyst efficiency. Example: Mo in Haber's process (promotes Fe catalyst)
- Catalytic poisons: substances that block active sites and reduce catalytic activity. Example: CO poisons Fe catalyst in Haber's process; Pb in petrol destroys Pt catalyst in catalytic converters.
5. Colloids
Colloidal state: Intermediate state of matter with particle size between 1 nm and 1000 nm (10⁻⁹ m to 10⁻⁶ m).
| System | Particle size | Character |
|---|---|---|
| True solution | < 1 nm (< 10⁻⁹ m) | Transparent, homogeneous, particles not visible |
| Colloid | 1–1000 nm | Translucent/opalescent, shows Tyndall effect |
| Suspension | > 1000 nm | Turbid, particles settle on standing |
Classification of Colloids
By dispersed phase and dispersion medium:
| Dispersion medium | Dispersed phase | Type | Examples |
|---|---|---|---|
| Gas | Liquid | Liquid aerosol | Fog, clouds, mist |
| Gas | Solid | Solid aerosol | Smoke, dust |
| Liquid | Gas | Foam | Soap froth, whipped cream |
| Liquid | Liquid | Emulsion | Milk, mayonnaise |
| Liquid | Solid | Sol | Paint, Au sol, starch sol |
| Solid | Gas | Solid foam | Pumice stone, styrofoam |
| Solid | Liquid | Gel | Butter, cheese, jellies |
| Solid | Solid | Solid sol | Ruby glass, coloured glass |
By interaction with dispersion medium:
- Lyophilic colloids (solvent-loving): Strong affinity between dispersed phase and solvent; stable; reversible. Examples: starch, gelatin, gum arabic in water. These are also called reversible colloids or intrinsic colloids.
- Lyophobic colloids (solvent-hating): Weak affinity; unstable without stabilising charge; require special methods to prepare; irreversible. Examples: gold sol, ferric hydroxide sol, sulphur sol. Also called extrinsic colloids.
6. Preparation of Colloids
Chemical methods:
- Double decomposition: As₂O₃ + 3H₂S → As₂S₃ (yellow sol) + 3H₂O
- Reduction: AuCl₃ + reducing agent → Au sol
- Oxidation: SO₂ + H₂S → S sol
Peptisation: Converting freshly precipitated material to colloid by adding a small amount of electrolyte (peptising agent). The precipitate adsorbs ions from the electrolyte and becomes colloidal.
Example: Addition of dilute HCl to ferric hydroxide precipitate forms Fe(OH)₃ sol.
Bredig's arc method (for metal sols): Electric arc struck between metal electrodes immersed in water.
7. Properties of Colloidal Solutions
Tyndall Effect
When a beam of light passes through a colloid, colloidal particles scatter the light, making the path of light visible (the Tyndall cone). True solutions do not show this effect because solute particles are too small to scatter visible light.
Applications: Visibility of headlight beams in fog; blue colour of sky (scattering by dust and aerosol particles).
Brownian Motion
Colloidal particles exhibit continuous, random, zig-zag motion. Caused by unequal bombardment from different directions by molecules of the dispersion medium. Brownian motion prevents sedimentation of colloidal particles (provides kinetic stability).
Electrophoresis
Movement of charged colloidal particles under the influence of an applied electric field. Particles move toward the oppositely charged electrode. This confirms that colloidal particles carry charge.
Application: Purification of colloids, determination of charge on colloidal particles.
Electro-osmosis
Movement of dispersion medium (not the colloidal particles) in an electric field. Observed when colloidal particles are prevented from moving.
8. Coagulation (Flocculation)
Settling of colloidal particles by neutralising their charge.
Hardy-Schulze Rule
The greater the valency (charge) of the coagulating ion, the greater its coagulating power.
- To coagulate a negatively charged sol (e.g., As₂S₃ sol, gold sol): use cations. Coagulating power: Al³⁺ > Ca²⁺ > Na⁺
- To coagulate a positively charged sol (e.g., Fe(OH)₃ sol): use anions. Coagulating power: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻
Flocculation value: Minimum concentration of electrolyte (in mmol L⁻¹) required to coagulate a colloid. Smaller flocculation value = more effective coagulant.
Other Coagulation Methods
1. Dialysis — removal of electrolyte ions through semipermeable membrane 2. Electrophoresis — particles migrate to electrode and get neutralised 3. Heating — increases kinetic energy; particles collide and aggregate 4. Mixing oppositely charged colloids — charges cancel, coagulation occurs
9. Emulsions
An emulsion is a colloidal dispersion of one liquid in another immiscible liquid.
- Oil in water (O/W): Oil is dispersed phase, water is medium. Examples: milk, vanishing cream, mayonnaise.
- Water in oil (W/O): Water is dispersed phase, oil is medium. Examples: butter, cold cream, margarine.
Emulsifying agents (emulsifiers): Substances that stabilise emulsions by forming a protective film around droplets. Examples: soap (sodium stearate), proteins, gum arabic, lecithin.
Demulsification: Breaking an emulsion by removing the emulsifier — heating, centrifugation, addition of electrolyte.
10. Worked Examples
Example 1 — Identifying Lyophilic vs Lyophobic Colloids
Question: Classify each as lyophilic or lyophobic: (a) gum arabic in water, (b) gold sol, (c) gelatin in water, (d) sulphur sol.
- (a) Gum arabic in water — lyophilic (gum has strong attraction to water)
- (b) Gold sol — lyophobic (gold has no attraction to water; stabilised only by charge)
- (c) Gelatin in water — lyophilic (gelatin dissolves in water; reversible gel-sol transformation)
- (d) Sulphur sol — lyophobic (sulphur repels water; coagulated easily by electrolytes)
Example 2 — Hardy-Schulze Rule Application
Question: A colloidal dispersion of As₂S₃ carries a negative charge. Which electrolyte will be most effective for its coagulation: NaCl, CaCl₂, or AlCl₃? Justify.
According to the Hardy-Schulze rule, the coagulating power of an electrolyte depends on the valency of the oppositely charged ion. For a negatively charged As₂S₃ sol, we need cations to neutralise the negative charge. The cations here are:
- NaCl → Na⁺ (valency 1)
- CaCl₂ → Ca²⁺ (valency 2)
- AlCl₃ → Al³⁺ (valency 3)
Coagulating power: Al³⁺ > Ca²⁺ > Na⁺
AlCl₃ has the lowest flocculation value (smallest concentration needed), so it is most effective.
Example 3 — Tyndall Effect Explanation
Question: Why does a beam of light become visible when passing through colloidal sulphur solution but not through a true solution of sugar?
This difference in light scattering is used to distinguish colloids from true solutions.
11. Important Board Questions with Model Answers
One-Mark Questions
Q1. Define adsorption. Adsorption is the accumulation of molecular species on the surface of a solid or liquid due to surface forces.
Q2. What is the Tyndall effect? The Tyndall effect is the scattering of light by colloidal particles, making the path of a beam of light visible when it passes through a colloid.
Q3. State the Hardy-Schulze rule. The coagulating power of an electrolyte increases with the valency of the ion carrying the charge opposite to that of the colloidal particles.
Q4. What is a promoter in catalysis? A promoter (activator) is a substance that increases the efficiency and activity of a catalyst without itself being a catalyst. Example: molybdenum (Mo) in Haber's process increases the activity of the iron catalyst.
Q5. Give one example of O/W type emulsion. Milk — oil (fat) droplets dispersed in water.
Short-Answer Questions (3 marks)
Long-Answer Questions (5 marks)
Common Mistakes
- Confusing Tyndall effect with Brownian motion — Tyndall is about light scattering; Brownian is about particle movement
- Stating that Hardy-Schulze rule is about the valency of the salt (it is about the valency of the coagulating ion)
- Confusing O/W and W/O emulsions — remember milk is O/W (oil dispersed in water) and butter is W/O
- Thinking a catalyst shifts equilibrium position (it does not — only speeds reaching equilibrium)
- Saying lyophilic colloids are unstable (they are the stable ones; lyophobic are unstable)
Revision Checklist
- [ ] Adsorption vs absorption distinction with examples
- [ ] Physisorption vs chemisorption comparison table (6 points)
- [ ] Freundlich isotherm equation and log-form
- [ ] Types of catalysis with one industrial example each
- [ ] Mechanism of heterogeneous catalysis (4 steps)
- [ ] Promoters and catalytic poisons with examples
- [ ] Classification of colloids by dispersed phase and medium (8 types)
- [ ] Lyophilic vs lyophobic colloids with examples
- [ ] Tyndall effect, Brownian motion, electrophoresis — mechanism of each
- [ ] Hardy-Schulze rule — coagulating power order for both positive and negative sols
- [ ] O/W vs W/O emulsions with examples
- [ ] At least five 1-mark definitions ready
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