Chapter 10 — Surface Chemistry Exercise Solutions
This chapter is mainly theory-based. Definitions, comparisons, and examples carry the most marks.
Key Concepts Quick Recap
| Term | Definition |
|---|
| Adsorption | Accumulation of molecules on the surface of an adsorbent |
| Absorption | Uniform distribution of molecules throughout the bulk |
| Adsorbate | Substance that gets adsorbed |
| Adsorbent | Surface on which adsorption occurs |
| Physisorption | Weak van der Waals forces; reversible; multilayer |
| Chemisorption | Strong chemical bonds; irreversible; monolayer |
| Catalyst | Substance that changes the rate of a reaction without being consumed |
| Colloid | Heterogeneous mixture where dispersed phase has particles 1–1000 nm |
| Sol | Solid dispersed in liquid |
| Emulsion | Liquid dispersed in liquid |
| Foam | Gas dispersed in liquid |
| Lyophilic colloid | Solvent-attracting; stable; can be reformed after precipitation |
| Lyophobic colloid | Solvent-repelling; unstable; cannot be reformed |
| Tyndall effect | Scattering of light by colloidal particles |
| Brownian motion | Erratic, random motion of colloidal particles due to collision with solvent molecules |
| Coagulation | Aggregation of colloidal particles and their settling |
| Peptisation | Breaking up a precipitate into colloidal particles using a peptising agent |
I. Choose the Best Answer (MCQs)
1. Adsorption is a surface phenomenon. The term that includes both adsorption and absorption is: (a) Desorption (b) Sorption (c) Occlusion (d) Absorption
Answer(b) Sorption Sorption is the general term covering both adsorption (surface) and absorption (bulk).
2. Physisorption differs from chemisorption in that: (a) It involves chemical bond formation (b) It is irreversible (c) It is multilayer and reversible (d) It has high enthalpy of adsorption
Answer(c) It is multilayer and reversible Physisorption: weak van der Waals, multilayer, reversible, low ΔH (20–40 kJ/mol), favoured at low temperature. Chemisorption: strong bonds, monolayer, usually irreversible, high ΔH (80–240 kJ/mol).
3. Which of the following phenomena can distinguish a colloidal solution from a true solution? (a) Osmosis (b) Brownian motion (c) Tyndall effect (d) Diffusion
Answer(c) Tyndall effect Colloidal particles scatter light (Tyndall effect). True solutions have particles too small to scatter visible light — they are transparent with no visible beam.
4. The process of adding an electrolyte to coagulate a colloid is known as: (a) Peptisation (b) Dialysis (c) Coagulation (flocculation) (d) Emulsification
Answer(c) Coagulation Adding an electrolyte neutralises the charge on colloidal particles → they come together (coagulate) and settle. Higher valency of the added ion = greater coagulating power (Hardy-Schulze law).
5. An emulsifying agent helps to stabilise an emulsion by: (a) Reducing the surface tension (b) Forming an interfacial film around droplets (c) Making the emulsion acidic (d) Increasing the viscosity
Answer(b) Forming an interfacial film around droplets Emulsifiers (like soap, gum arabic) adsorb at the interface between oil and water, forming a protective film around droplets that prevents them from coalescing.
6. The Freundlich adsorption isotherm gives a straight line when: (a) x/m is plotted vs p (b) log(x/m) is plotted vs log p (c) x/m is plotted vs 1/p (d) log(x/m) is plotted vs p
Answer(b) log(x/m) is plotted vs log p Freundlich: x/m = k·p^(1/n). Taking log: log(x/m) = log k + (1/n) log p → straight line with slope 1/n and intercept log k.
7. Which of the following is a lyophilic colloid? (a) Gold sol (b) Sulphur sol (c) Starch (d) Fe(OH)₃ sol
Answer(c) Starch Lyophilic colloids are formed by substances that have high affinity for the dispersion medium. Starch, proteins (gelatin, albumin), gum, and rubber in appropriate solvents are lyophilic. Gold sol, Fe(OH)₃, and sulphur sols are lyophobic.
II. Short Answer Questions
Q1. Distinguish between physisorption and chemisorption.
Answer
| Property | Physisorption | Chemisorption |
|---|
| Forces involved | Weak van der Waals forces | Strong chemical bonds |
| Enthalpy (ΔH) | Low: 20–40 kJ/mol | High: 80–240 kJ/mol |
| Reversibility | Reversible | Usually irreversible |
| Layers | Multilayer | Monolayer only |
| Temperature dependence | Decreases with increasing T | Increases with T initially, then decreases |
| Specificity | Not specific | Highly specific |
| Activation energy | Not required | Requires activation energy |
Q2. Explain the cleansing action of soaps.
Answer
Soap molecules (sodium/potassium salts of long-chain fatty acids) have two parts:
- Hydrophilic (polar) head: −COONa — attracted to water.
- Hydrophobic (non-polar) tail: long carbon chain — attracted to oil/grease.
Action: 1. Soap molecules form micelles in water — non-polar tails point inward (towards oil), polar heads point outward (towards water). 2. The micelle encapsulates the oil/grease droplet inside. 3. The charged outer surface of the micelle repels other micelles — oil droplets are suspended. 4. When rinsed, the micelles (with trapped grease) are washed away.
Q3. What is the Tyndall effect? What information does it give about a colloid?
Answer
The Tyndall effect is the scattering of a beam of light by colloidal particles, making the path of light visible as a bright cone (Tyndall cone).
Information it provides: 1. Confirms the mixture is a colloid (particles between 1–1000 nm). 2. True solutions do not show this — particles are too small to scatter visible light. 3. Colloids scatter light because their particles have dimensions comparable to the wavelength of visible light.
Real examples: Sunbeams visible through forest canopy; headlights in fog; blue colour of sky.
Q4. State Hardy-Schulze law with one example.
Answer
Hardy-Schulze law: The coagulating power of an electrolyte depends on the valency of the ion responsible for coagulation (the oppositely charged ion to the colloid).
- Higher the valency of the coagulating ion → greater the coagulating power.
- Coagulating power: Al³⁺ > Ba²⁺ > Na⁺ (for a negatively charged colloid).
Example: Fe(OH)₃ sol is positively charged. Coagulating anions: PO₄³⁻ >> SO₄²⁻ > Cl⁻ PO₄³⁻ (3−) coagulates Fe(OH)₃ sol more effectively than Cl⁻ (1−).
Q5. What is catalysis? Distinguish between homogeneous and heterogeneous catalysis.
Answer
Catalysis is the process of increasing the rate of a reaction using a substance (catalyst) that is not consumed in the reaction.
| Feature | Homogeneous | Heterogeneous |
|---|
| Phase | Same phase as reactants | Different phase from reactants |
| Example | SO₂ + [NO catalyst] in gas phase | Fe catalyst (solid) in Haber process |
| Contact | Molecular level | Adsorption on surface |
| Efficiency | Generally lower | Very high surface area |
Worked Examples
Example 1 — Freundlich adsorption isotherm
Givenx/m = 0.05 g/g when p = 1 atm. When p = 8 atm, x/m = 0.10 g/g. Find 1/n.
x/m = k · p^(1/n)
Taking ratio: 0.10/0.05 = (8/1)^(1/n)
2 = 8^(1/n)
Take log: log 2 = (1/n) log 8 = (1/n) × 0.903
0.301 = (1/n) × 0.903
1/n = 0.301/0.903 = 0.333 ≈ 1/3
Example 2 — Tyndall effect vs true solution
| Property | True Solution | Colloid | Suspension |
|---|
| Particle size | < 1 nm | 1–1000 nm | > 1000 nm |
| Tyndall effect | No | Yes | Yes (but settles) |
| Filtration | Passes through filter paper | Passes through filter | Does not pass |
| Dialysis | Cannot separate | Can separate | Can separate |
| Visibility | Transparent | May be turbid | Turbid |
Example 3 — Comparison: lyophilic vs lyophobic colloids
| Property | Lyophilic | Lyophobic |
|---|
| Affinity for solvent | High | Low |
| Stability | Very stable | Less stable |
| Example | Starch, gelatin, proteins | Gold sol, Fe(OH)₃, As₂S₃ |
| Reversibility | Can be reformed after precipitation | Cannot be reformed |
| Coagulation | Needs large amount of electrolyte | Coagulated easily |
| Hydration shell | Present | Absent |
Example 4 — Negative catalysis (inhibition)
Some substances slow down reactions — called negative catalysts or inhibitors.
Example:
- A small amount of H₃PO₄ slows down the decomposition of H₂O₂:
2H₂O₂ → 2H₂O + O₂ (decomposition is inhibited)
- Tetraethyl lead (TEL) acts as an anti-knock agent in petrol — it inhibits the
chain reaction of combustion.
Example 5 — Enzyme catalysis
Enzymes are biological catalysts (proteins) that catalyse biochemical reactions:
- Lock-and-key model: the substrate fits exactly into the enzyme active site.
- Extremely specific — each enzyme catalyses only one type of reaction.
- Highly efficient: 10⁶–10¹⁷ times faster than uncatalysed reactions.
- Sensitive to pH and temperature: denatured above ~50 °C.
- Inhibitors can block the active site and stop enzyme action.
Answer Pattern
1. Define the term precisely. 2. Write one distinguishing characteristic. 3. Give one example. 4. State one application if asked.
Common Mistakes
- Confusing adsorption (surface) with absorption (bulk).
- Saying catalyst changes equilibrium constant — it does not; only rate is changed.
- Mixing lyophilic (stable, solvent-loving) and lyophobic (unstable, solvent-hating).
- Not stating examples for colloid types.
- Writing Tyndall effect for true solutions — only colloids show it.
Exam Checklist
- Definitions are precise and concise.
- Comparison answers use table format.
- Examples correctly match the category.
- Applications are specific (not vague).
- Physisorption vs chemisorption differences all four rows correct.
