Surface Chemistry
Welcome to HSLC Guru! This chapter explains the chemistry that occurs at the boundary between two phases. Surface Chemistry is one of the most application-rich units of ASSEB Class 12 Chemistry — it underlies catalysis in industry, the action of detergents, the working of enzymes, the stability of milk and fog, the operation of catalytic converters in cars and the purification of water. Concepts such as adsorption, colloids, emulsions and catalysis form the backbone of this unit and frequently appear in board examinations.
Chapter Summary
Adsorption and its types: Adsorption is the accumulation of molecules of a substance (adsorbate) on the surface of another substance (adsorbent). It differs from absorption, in which the substance penetrates uniformly into the bulk. Two main kinds are recognised. Physisorption (physical adsorption) is caused by weak van der Waals forces, has low enthalpy (20–40 kJ/mol), is reversible, occurs at low temperature and is multilayered with no specificity. Chemisorption (chemical adsorption) involves chemical bond formation between adsorbate and adsorbent, has high enthalpy (80–240 kJ/mol), is generally irreversible, occurs at high temperature and is monolayered with high specificity. The extent of adsorption increases with surface area, so finely divided metals, charcoal, silica gel and zeolites are excellent adsorbents.
Adsorption isotherms and applications: An adsorption isotherm shows the variation of extent of adsorption (x/m) with pressure of the gas at constant temperature. The Freundlich isotherm explains the behaviour at low and intermediate pressures but fails at high pressure. The Langmuir isotherm assumes monolayer coverage and dynamic equilibrium between adsorbed and free molecules. Practical applications of adsorption include gas masks (activated charcoal), removal of moisture (silica gel), heterogeneous catalysis, chromatography, dyeing, froth flotation in metallurgy, removal of colouring matter (decolourisation of sugar), and curing of diseases (drugs adsorbed at infection sites).
Catalysis: A catalyst alters the rate of a chemical reaction without itself being consumed. Homogeneous catalysis is when reactants and catalyst are in the same phase (e.g. lead chamber process for H2SO4, ester hydrolysis by H+). Heterogeneous catalysis is when they are in different phases (e.g. Haber process for NH3 with iron, contact process for SO3 with V2O5, hydrogenation of oils with nickel). Promoters increase the activity of a catalyst (Mo for Fe in Haber process), while poisons decrease it (As in contact process). Enzyme catalysis is highly efficient and specific — biological catalysts like maltase, urease and zymase work at body temperature and physiological pH. Zeolites are shape-selective catalysts (e.g. ZSM-5 in petroleum cracking) whose porous honeycomb structure admits only molecules of a particular size.
Colloids, emulsions and gels: A colloid is a heterogeneous mixture in which the dispersed phase has particle sizes between 1 nm and 1000 nm. True solutions have particles below 1 nm, suspensions above 1000 nm. Colloids are classified by physical state (sol, gel, aerosol, emulsion, foam), by interaction (lyophilic — solvent-loving and stable; lyophobic — solvent-hating and unstable) and by molecular type (multimolecular, macromolecular, associated/micelles). Preparation methods include condensation (oxidation, reduction, hydrolysis, exchange of solvent) and dispersion (Bredig’s arc, peptisation, mechanical milling). Purification involves dialysis, electrodialysis and ultrafiltration. Characteristic properties include the Tyndall effect (scattering of light), Brownian motion (random zig-zag movement), electrical charge, electrophoresis (movement of particles in an electric field) and coagulation. The Hardy–Schulze rule states that the coagulating power of an ion increases with its valency. Emulsions (liquid-in-liquid colloids) are of two types — oil-in-water (milk) and water-in-oil (butter, cold cream) — and are stabilised by emulsifiers like soaps and proteins. Gels are jelly-like elastic solids (e.g. cheese, jellies, silica gel). Applications appear in medicine, food, cosmetics, sewage disposal (alum), rubber industry, photography and smoke precipitation by Cottrell precipitator.
Important Adsorption Equations
$\frac{x}{m} = k \cdot P^{1/n}$ (Freundlich adsorption isotherm)
$\log\frac{x}{m} = \log k + \frac{1}{n}\log P$
$\frac{x}{m} = \frac{aP}{1 + bP}$ (Langmuir adsorption isotherm)
Very Short Answer Questions (1 Mark)
Q1. What is adsorption?
Answer: Adsorption is the accumulation of molecular species at the surface of a solid or liquid resulting in a higher concentration at the surface than in the bulk.
Q2. Define adsorbent and adsorbate.
Answer: The substance on whose surface adsorption takes place is called the adsorbent (e.g. charcoal); the substance which gets adsorbed is called the adsorbate (e.g. a gas).
Q3. What is the Tyndall effect?
Answer: The scattering of light by colloidal particles, making the path of a beam of light visible through a colloidal solution, is called the Tyndall effect.
Q4. Define a catalyst.
Answer: A catalyst is a substance that alters the rate of a chemical reaction without itself being consumed at the end of the reaction.
Q5. What is a promoter? Give one example.
Answer: A promoter is a substance that increases the activity of a catalyst. Example: molybdenum (Mo) acts as a promoter for iron (Fe) in the Haber process.
Q6. What is Brownian motion?
Answer: The continuous, random zig-zag movement of colloidal particles in a dispersion medium, caused by collisions with molecules of the medium, is called Brownian motion.
Q7. What is meant by an emulsion?
Answer: An emulsion is a colloidal system in which both the dispersed phase and the dispersion medium are liquids (immiscible with each other). Milk and butter are common examples.
Q8. Name two methods to purify a colloidal solution.
Answer: Two methods are dialysis and ultrafiltration. Electrodialysis is a faster variant of dialysis.
Q9. What is meant by peptisation?
Answer: Peptisation is the process of converting a freshly prepared precipitate into a colloidal sol by shaking it with a small amount of an electrolyte called a peptising agent.
Q10. State the Hardy–Schulze rule.
Answer: Greater the valency of the flocculating ion (the ion having charge opposite to that of the colloidal particles), greater is its power to cause coagulation.
Short Answer Questions (2–3 Marks)
Q1. Distinguish between physisorption and chemisorption.
Answer: Physisorption involves weak van der Waals forces, low enthalpy of adsorption (20–40 kJ/mol), is reversible, takes place at low temperature, is non-specific and forms multilayers. Chemisorption involves chemical bond formation, high enthalpy (80–240 kJ/mol), is irreversible, occurs at high temperature, is highly specific and forms only a monolayer on the adsorbent surface.
Q2. Differentiate between true solution, colloidal solution and suspension.
Answer: A true solution has particles below 1 nm, is homogeneous, transparent, does not show the Tyndall effect and passes through filter paper and parchment. A colloidal solution has particles between 1 nm and 1000 nm, appears homogeneous but is heterogeneous, shows the Tyndall effect and passes through filter paper but not parchment. A suspension has particles larger than 1000 nm, is heterogeneous, opaque and does not pass through filter paper.
Q3. Explain homogeneous and heterogeneous catalysis with one example each.
Answer: In homogeneous catalysis, reactants and catalyst exist in the same phase. Example: oxidation of SO2 to SO3 by NO in the lead chamber process (all gases). In heterogeneous catalysis, reactants and catalyst exist in different phases. Example: synthesis of NH3 from N2 and H2 in the presence of finely divided iron in the Haber process (gas–solid).
Q4. What are lyophilic and lyophobic colloids? Give one example of each.
Answer: Lyophilic colloids (“solvent-loving”) have strong attraction between the dispersed phase and dispersion medium; they are reversible and stable. Example: gum, gelatin in water. Lyophobic colloids (“solvent-hating”) have little attraction between phases; they are irreversible and require stabilising agents. Example: sols of metals like Au and Ag, As2S3, Fe(OH)3 in water.
Q5. What is dialysis? How is it improved by electrodialysis?
Answer: Dialysis is the process of removing dissolved substances (electrolyte impurities) from a colloidal solution by diffusion through a parchment or cellophane membrane. Pure water is kept on one side of the membrane and the impure colloid on the other; only ions and small molecules pass through. In electrodialysis, an electric field is applied across the membrane, which speeds up the migration of ions and so makes the purification much faster.
Q6. Why are catalysts in industry highly significant? List four uses.
Answer: Catalysts increase reaction rate, reduce energy consumption and allow selective formation of products. (i) Iron in Haber process for NH3. (ii) V2O5 in contact process for H2SO4. (iii) Ni in hydrogenation of vegetable oils to vanaspati. (iv) Zeolites (ZSM-5) in cracking of hydrocarbons in the petroleum industry.
Long Answer Questions (5–7 Marks)
Q1. Discuss the Freundlich and Langmuir adsorption isotherms. Mention their assumptions and limitations.
Answer: The Freundlich isotherm is given by x/m = k·P^(1/n), where x is the mass of gas adsorbed on mass m of adsorbent at pressure P, and k and n are constants depending on adsorbent and gas (n > 1). Taking logarithms gives log(x/m) = log k + (1/n) log P, which is a straight line of slope 1/n and intercept log k. At very low pressure x/m varies linearly with P (n = 1); at very high pressure x/m becomes independent of P (1/n = 0); at intermediate pressure the equation holds. Limitation: the Freundlich isotherm is purely empirical and fails at high pressure.
The Langmuir isotherm, x/m = aP/(1 + bP), is based on the assumptions: (i) adsorption is monolayer, (ii) all adsorption sites are equivalent, (iii) there is dynamic equilibrium between adsorbed gas and free gas, and (iv) molecules adsorbed do not interact. At low pressure (bP << 1), x/m ≈ aP (linear); at high pressure (bP >> 1), x/m ≈ a/b (constant, saturation). It applies well to chemisorption but not to multilayer physisorption.
Q2. Discuss enzyme catalysis. Mention its characteristics and four examples of biocatalytic reactions.
Answer: Enzymes are complex nitrogenous organic compounds (proteins) produced by living organisms that catalyse biochemical reactions. They show a number of unique features: (i) they are highly efficient — a tiny amount catalyses a huge number of substrate molecules; (ii) they are highly specific — each enzyme catalyses only one type of reaction; (iii) they are most active at an optimum temperature (around 37 °C in humans) and optimum pH (usually 5–7); (iv) their activity increases in the presence of activators (e.g. Mn^2+, Mg^2+) and decreases by inhibitors and poisons. Mechanism follows the lock-and-key model — the substrate fits into the active site of the enzyme to form an enzyme–substrate complex, which then breaks into product and free enzyme.
Examples: (i) Inversion of cane sugar — invertase converts sucrose into glucose and fructose. (ii) Conversion of glucose into ethanol — zymase. (iii) Conversion of urea into ammonia — urease. (iv) Conversion of starch into maltose — diastase, then maltose into glucose by maltase. (v) Digestion of proteins in the stomach by pepsin.
Q3. Describe in detail the optical, kinetic, electrical and coagulation properties of colloidal solutions.
Answer: Optical (Tyndall effect): When a beam of light is passed through a colloidal solution, the colloidal particles scatter light, making the path of light visible (Tyndall cone). This happens because the colloidal particle size is comparable to the wavelength of visible light. True solutions do not show this effect.
Kinetic (Brownian motion): Colloidal particles move in a random, ceaseless zig-zag manner due to unbalanced bombardment by molecules of the dispersion medium. Brownian motion prevents settling of particles and contributes to colloid stability.
Electrical (charge and electrophoresis): Colloidal particles carry a definite electric charge — positive (e.g. Fe(OH)3 sol, hydrated metal oxides) or negative (e.g. As2S3 sol, gold sol, clay). Charged particles move under an electric field — this migration toward an oppositely charged electrode is electrophoresis. The arrangement of charges produces an electrical double layer at the surface of the particle, giving rise to the zeta potential.
Coagulation: The process of settling of colloidal particles into a precipitate is called coagulation or flocculation. It can be brought about by addition of an electrolyte, mutual mixing of oppositely charged sols, prolonged dialysis, boiling or by passing electric current. The Hardy–Schulze rule states that the coagulating power of the flocculating ion increases with its valency: Al^3+ > Ba^2+ > Na^+ for a negatively charged sol.
Q4. Explain the methods of preparation of colloidal solutions in detail.
Answer: Lyophilic sols (gum, gelatin, starch) are prepared by simply mixing them with the dispersion medium and warming. Lyophobic sols are prepared by two general approaches.
Condensation methods involve aggregation of molecules or ions of true solutions into colloidal-sized particles: (i) Oxidation — H2S passed in SO2 gives sulphur sol. (ii) Reduction — AuCl3 with HCHO gives gold sol. (iii) Hydrolysis — FeCl3 + boiling water gives Fe(OH)3 sol. (iv) Double decomposition — As2O3 + H2S gives As2S3 sol. (v) Exchange of solvent — sulphur dissolved in alcohol when poured into water gives sulphur sol.
Dispersion methods involve breaking down larger particles to colloidal size: (i) Mechanical dispersion — using a colloid mill to grind solid in a liquid (e.g. paint, ink). (ii) Bredig’s arc method — an electric arc is struck between two metal electrodes immersed in cold water (used for sols of Au, Ag, Pt). (iii) Peptisation — adding a small quantity of an electrolyte (peptising agent) to a fresh precipitate breaks it into colloidal particles (e.g. freshly precipitated Fe(OH)3 + FeCl3 → Fe(OH)3 sol). (iv) Ultrasonic dispersion — using high-frequency sound waves.
Q5. Mention five important applications of colloids in daily life and industry.
Answer: (i) Medicine: Many medicines are colloidal in nature for quicker assimilation — milk of magnesia, argyrol (silver sol used as eye antiseptic), colloidal antibiotics. (ii) Sewage and water purification: Dirty water contains negatively charged particles which are coagulated by adding alum (Al^3+, applying Hardy–Schulze rule). (iii) Smoke precipitation (Cottrell precipitator): Smoke is a colloidal sol of carbon particles in air; passing it through metallic plates at high voltage neutralises the charge and precipitates the particles. (iv) Rubber industry: Latex from rubber tree is a colloid; rubber is obtained by coagulation. (v) Cleansing action of soaps: Soaps form micelles that solubilise grease as an emulsion. (vi) Food and cosmetics: Milk, butter, ice-cream, creams, lotions are all colloids.
Multiple Choice Questions (MCQ)
Q1. Which of the following is an example of chemisorption?
(a) Adsorption of N2 on charcoal at 80 K
(b) Adsorption of H2 on Pt at high temperature
(c) Adsorption of dye on alumina
(d) Adsorption of CO2 on charcoal at 273 K
Answer: (b) Adsorption of H2 on Pt at high temperature.
Q2. The Freundlich adsorption isotherm is correctly given by
(a) x/m = kP
(b) x/m = kP^(1/n)
(c) x/m = aP/(1+bP)
(d) x/m = k log P
Answer: (b) x/m = kP^(1/n).
Q3. Which of the following is a homogeneous catalysis?
(a) Haber process for NH3
(b) Contact process for H2SO4
(c) Hydrolysis of ester by HCl
(d) Hydrogenation of oils with Ni
Answer: (c) Hydrolysis of ester by HCl.
Q4. The size range of colloidal particles is
(a) less than 1 nm
(b) between 1 nm and 1000 nm
(c) greater than 1000 nm
(d) exactly 1 nm
Answer: (b) between 1 nm and 1000 nm.
Q5. Milk is an example of
(a) Sol
(b) Gel
(c) Oil-in-water emulsion
(d) Water-in-oil emulsion
Answer: (c) Oil-in-water emulsion.
Q6. The Tyndall effect is observed in
(a) true solution
(b) suspension
(c) colloidal solution
(d) all of these
Answer: (c) colloidal solution.
Q7. Which of these has the highest coagulating power for a negative sol?
(a) Na+
(b) Mg2+
(c) Al3+
(d) NH4+
Answer: (c) Al3+ (Hardy–Schulze rule).
Q8. Bredig’s arc method is used for the preparation of sols of
(a) sulphur
(b) gold and silver
(c) Fe(OH)3
(d) starch
Answer: (b) gold and silver.
Q9. Zeolite (ZSM-5) is used as a catalyst in
(a) Haber process
(b) Contact process
(c) cracking of hydrocarbons
(d) hydrogenation of oils
Answer: (c) cracking of hydrocarbons.
Q10. The promoter used along with iron catalyst in the Haber process is
(a) Vanadium
(b) Molybdenum
(c) Platinum
(d) Nickel
Answer: (b) Molybdenum.
Fill in the Blanks
Q1. Adsorption of gases on solid surfaces is generally __________ (exothermic / endothermic).
Answer: exothermic.
Q2. The substance whose surface adsorbs the gas is called the __________.
Answer: adsorbent.
Q3. Movement of colloidal particles under an applied electric field is called __________.
Answer: electrophoresis.
Q4. Butter is an example of __________ type of emulsion.
Answer: water-in-oil.
Q5. The catalyst used in the contact process for manufacture of H2SO4 is __________.
Answer: V2O5 (vanadium pentoxide).
True or False
Q1. Physisorption is generally irreversible.
Answer: False. Physisorption is reversible; it is chemisorption that is generally irreversible.
Q2. A catalyst changes the equilibrium constant of a reversible reaction.
Answer: False. A catalyst only changes the rate, not the position of equilibrium.
Q3. Lyophilic sols are more stable than lyophobic sols.
Answer: True.
Q4. Tyndall effect is observed in true solutions.
Answer: False. Tyndall effect is observed only in colloidal solutions and suspensions.
Q5. Enzyme catalysts are highly specific in nature.
Answer: True.
Glossary
| Term | Definition |
|---|---|
| Adsorption | Accumulation of molecular species at a surface, with higher concentration at the surface than in the bulk. |
| Absorption | Uniform distribution of a substance throughout the bulk of another. |
| Adsorbent | Solid (or liquid) on whose surface adsorption takes place. |
| Adsorbate | Substance that gets adsorbed on a surface. |
| Physisorption | Adsorption due to weak van der Waals forces; reversible and multilayered. |
| Chemisorption | Adsorption involving chemical bond formation; irreversible and monolayered. |
| Adsorption isotherm | Plot of x/m versus pressure of gas at constant temperature. |
| Catalyst | Substance that alters the rate of a reaction without itself undergoing change. |
| Promoter | Substance that enhances the activity of a catalyst. |
| Poison | Substance that decreases or destroys the activity of a catalyst. |
| Enzyme | Biological protein that catalyses biochemical reactions with high specificity. |
| Zeolite | Aluminosilicate with porous honeycomb structure; acts as a shape-selective catalyst. |
| Colloid | Heterogeneous mixture with dispersed particles between 1 nm and 1000 nm. |
| Lyophilic colloid | Solvent-loving, stable, reversible colloid (gum, gelatin). |
| Lyophobic colloid | Solvent-hating, less stable, irreversible colloid (Au sol, As2S3 sol). |
| Sol | Colloid with solid dispersed in liquid (paint). |
| Gel | Colloid with liquid dispersed in solid (jelly, cheese). |
| Aerosol | Colloid with solid or liquid dispersed in gas (smoke, fog). |
| Emulsion | Colloid with liquid dispersed in liquid (milk, butter). |
| Foam | Colloid with gas dispersed in liquid or solid (whipped cream, sponge). |
| Micelle | Aggregate of soap/detergent molecules above critical micelle concentration. |
| Tyndall effect | Scattering of a light beam by colloidal particles, making the path visible. |
| Brownian motion | Random zig-zag movement of colloidal particles due to molecular collisions. |
| Electrophoresis | Migration of charged colloidal particles in an electric field. |
| Coagulation | Settling of colloidal particles into a precipitate. |
| Hardy–Schulze rule | Greater the valency of the flocculating ion, greater is its coagulating power. |
| Dialysis | Removal of dissolved electrolyte impurities from a colloid through a membrane. |
| Peptisation | Conversion of a precipitate into a sol by addition of a peptising agent. |
| Cottrell precipitator | Device used to remove colloidal smoke particles from chimney gases. |
Formula Table
| Concept | Formula | Description |
|---|---|---|
| Freundlich isotherm | x/m = k P^(1/n) | Empirical relation between extent of adsorption and pressure. |
| Freundlich (log form) | log(x/m) = log k + (1/n) log P | Linear form to determine k and n from a graph. |
| Langmuir isotherm | x/m = aP / (1 + bP) | Theoretical relation assuming monolayer adsorption. |
| Low pressure limit | x/m ∝ P | x/m linear in P. |
| High pressure limit | x/m = constant | Saturation of surface sites. |
| Colloidal size range | 1 nm – 1000 nm | Particle size that defines a colloid. |
| Hardy–Schulze (charge) | Coagulating power ∝ |z| | Power increases with valency of opposite-charged ion. |
| Ester hydrolysis (homog.) | RCOOR’ + H2O → RCOOH + R’OH (H+ catalyst) | Example of homogeneous catalysis. |
| Haber process (heterog.) | N2 + 3H2 → 2NH3 (Fe / Mo) | Example of heterogeneous catalysis with promoter. |
| Contact process | 2SO2 + O2 → 2SO3 (V2O5) | Industrial example, As is a poison. |
End of Chapter 5 — Surface Chemistry. Continue practising on HSLC Guru for more ASSEB Class 12 Chemistry chapters.