CBSE Class 12 Chemistry Surface Chemistry Notes and Questions

The branch of Chemistry which deals with the study of surface phenomena is called surface chemistry.

POINTS TO BE REMEMBERED:

1. Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

2. Desorption: Removal of adsorbate from the surface of an adsorbent is known as desorption.

3. Sorption: When adsorption and absorption both take place simultaneously.

4. Types of adsorption: On the basis of interaction between adsorbate and adsorbent, adsorption is of two types:
(i) Physical adsorption / physisorption: When weak van der Waals interactions are involved between adsorbate and adsorbent.
(ii) Chemical adsorption / chemisorption: When chemical bonds are formed between adsorbate and adsorbent.

5. Adsorption isotherm: The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.

6. Applications of adsorption:
(a) Removal of colouring matter from solution using animal charcoal.
(b) Chromatographic analysis is based on adsorption.

7. Freundlich adsorption isotherm: It is a graph which shows the relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.

8. Factors affecting adsorption:
(i) Surface area: Adsorption increases with an increase in the surface area of the adsorbent.
(ii) Nature of adsorbate: Easily liquefiable gases are readily adsorbed.
(iii) Temperature: Low temperature is favorable for physical adsorption and high temperature is favorable for chemisorption.
(iv) Pressure: As pressure increases, adsorption increases.

9. CATALYSIS: Substances which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalysts, and the phenomenon is known as catalysis.

10. PROMOTERS AND POISONS: Promoters are substances that enhance the activity of a catalyst, while poisons decrease the activity of a catalyst.

11. Homogenous catalyst: When reactants and catalyst are in the same phase.
Example: \(2SO_{2(g)} + O_{2(g)} \xrightarrow{NO_{(g)}} 2SO_{3(g)}\)

12. Heterogeneous catalyst: The catalytic process in which the reactants and catalyst are in different phases.
Example: \(SO_2 + O_2 \xrightarrow{Pt} 2SO_{3(g)}\)

13. Adsorption theory of heterogeneous catalysis: It explains the mechanism of heterogeneous catalysis.
The mechanism involves 5 steps:
a. Diffusion of reactants to the surface of the catalyst.
b. Adsorption of reactant molecules on the surface of the catalyst.
c. Chemical reaction on the catalyst surface through the formation of an intermediate.
d. Desorption of reaction products from the catalyst surface.
e. Diffusion of reaction products away from the catalyst surface.

14. IMPORTANT FEATURES OF SOLID CATALYSTS:
Activity: The activity of a catalyst depends on the strength of chemisorption. Catalytic activity increases from group 5 to group 11 elements of the periodic table.
Example: \(2H_2 + O_2 \xrightarrow{Pt} 2H_2O\)
Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product.
Examples:
1. \(CO + 3H_2 \xrightarrow{Ni} CH_4 + H_2O\)
2. \(CO + H_2 \xrightarrow{Cu} HCHO\)

15. SHAPE SELECTIVE CATALYSIS: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis.
Example: Zeolites are good shape-selective catalysts.

16. ENZYME CATALYSIS: Enzymes are protein molecules of high molecular mass which catalyse biochemical reactions.
Example: Inversion of cane sugar by invertase enzyme.

17. Characteristics of enzyme catalysis:
a. Enzymes are specific to substrates.
b. Enzymes are highly active under optimum temperature.
c. Enzymes are specific to pH (e.g., Pepsin acts in an acidic medium).
d. Enzymes are inhibited by the presence of certain substances.
Mechanism of enzyme catalysis:
1. Binding of enzyme to substrate to form an activated complex: \(E + S \rightarrow ES^*\)
2. Decomposition of the activated complex to form product: \(ES^* \rightarrow E + P\)

18. Colloid: A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) in another substance called the dispersion medium, and the size of the dispersed phase is from \(1\text{ nm}\) to \(1000\text{ nm}\).

19. TYPES OF COLLOIDS:
(1) On the basis of nature of interaction between dispersed phase and dispersion medium:
(a) Lyophobic colloid: Solvent-hating colloid. These colloids cannot be prepared by simply mixing the dispersed phase into the dispersion medium.
Example: Metallic sols.
(b) Lyophilic colloid: Solvent-loving colloid. These colloids can be prepared by simply mixing the dispersed phase into the dispersion medium.
Example: Starch sol.
(2) On the basis of types of particles of the dispersed phase:
(a) Multimolecular colloids: On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range. The species thus formed are called multimolecular colloids.
Example: Sulphur sol.
(b) Macromolecular colloids: Macromolecules in suitable solvents form solutions in which the size of the particles is in the colloidal range.
Example: Starch sol.
(c) Associated colloids (micelles): Some substances at low concentration behave as normal strong electrolytes, but at higher concentration exhibit colloidal behavior due to the formation of aggregates. The aggregated particles are called micelles and are also known as associated colloids.
(3) Kraft temperature: Temperature above which the formation of micelles takes place.
(4) Critical micelle concentration (CMC): Concentration above which micelle formation takes place.
(5) PREPARATION OF COLLOIDS:
(a) Chemical methods: By double decomposition, oxidation reaction, or hydrolysis.
Oxidation example: \(SO_2 + 2H_2S \rightarrow 3S \text{ (sol)} + 2H_2O\)
Hydrolysis example: \(FeCl_3 + 3H_2O \rightarrow Fe(OH)_3 \text{ (sol)} + 3HCl\)
(b) Bredig's arc method: For the preparation of metallic sol. It involves dispersion as well as condensation.
(c) Peptization: Process of converting a precipitate into a colloidal sol by shaking it with a dispersion medium in the presence of a small amount of electrolyte.
(6) PURIFICATION OF COLLOIDAL SOLUTIONS:
(a) Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of a semipermeable membrane.
(b) Electro-dialysis: When dialysis is carried out with an electric field applied around the membrane.
(c) Ultrafiltration: Use of special filters which are permeable to all ionic substances except colloidal particles.
(7) PROPERTIES OF COLLOIDAL SOLUTIONS:
1. They show colligative properties.
2. Brownian movement: Zig-zag motion of colloidal particles.
3. Tyndall effect: Scattering of light by colloidal particles by which the path of a light beam becomes clearly visible.
4. Charge on colloidal particles: Colloidal particles carry an electric charge, and the nature of the charge is the same on all particles in a given sol.
5. Electrophoresis: Movement of colloidal particles towards the opposite electrode in the presence of an external electric field.
6. Coagulation: The process of settling of colloidal particles is called coagulation of the sol.
7. Hardy-Schulze Law: The coagulating power of a coagulating ion is directly proportional to the fourth power of the valency of the ion.
Example: For negatively charged sols: \(Na^+ < Ca^{2+} < Al^{3+}\)
For positively charged sols: \(Cl^- < SO_4^{2-} < PO_4^{3-} < [Fe(CN)_6]^{4-}\)
8. Emulsion: Liquid-liquid colloidal system. There are two types:
a) O/W type - Oil dispersed in water. Example: Milk, vanishing cream.
b) W/O type - Water dispersed in oil. Example: Butter, cream.
9. Emulsifying agent: The substance which stabilizes an emulsion.

VERY SHORT ANSWER TYPE QUESTIONS (1 Mark)

Question 1. What are the physical states of dispersed phase and dispersion medium of froth?
Answer: Dispersed phase is gas, and dispersion medium is liquid.
In simple words: Froth is formed by trapping gas bubbles inside a liquid.

Exam Tip: Remember that froth is a type of gas-in-liquid colloidal system.

Question 2. What is the cause of Brownian movement among colloidal particles?
Answer: Due to unbalanced collisions between the molecules of the dispersion medium and the colloidal particles.
In simple words: Liquid molecules constantly hit colloidal particles from all sides, making them move in a zig-zag path.

Exam Tip: Mention unsymmetrical bombardment by dispersion medium molecules for full marks.

Question 3. Arrange the solutions: True solution, colloidal solution, suspension in decreasing order of their particles size?
Answer: Suspension > Colloidal solution > True solution.
In simple words: Suspensions have the largest particles, while true solutions have the smallest.

Exam Tip: Particle size ranges are: True solution (\(< 1\text{ nm}\)), Colloid (\(1\text{ nm} - 1000\text{ nm}\)), Suspension (\(> 1000\text{ nm}\)).

Question 4. Give an example of micelles system?
Answer: Sodium stearate (\(C_{17}H_{35}COO^-Na^+\)).
In simple words: Soap in water forms micelles at higher concentrations.

Exam Tip: Soap and synthetic detergents are classic examples of associated colloids (micelles).

Question 5. Why is it necessary to remove CO when ammonia is obtained by Haber's process?
Answer: CO acts as a poison for the iron catalyst used in Haber's process, thereby lowering its activity. Therefore, it is necessary to remove CO before passing the gases into the catalytic chamber.
In simple words: Carbon monoxide deactivates the catalyst, stopping the reaction from producing ammonia efficiently.

Exam Tip: Use the keyword catalyst poison when explaining catalyst deactivation.

Question 6. How is adsorption of a gas related to its critical temperature?
Answer: Higher the critical temperature of the gas, greater is the ease of liquefaction, and consequently, greater is the adsorption of the gas.
In simple words: Gases that liquefy easily get adsorbed more readily because of stronger van der Waals forces.

Exam Tip: Link critical temperature directly with intermolecular forces of attraction.

Question 7. What is meant by Shape Selective Catalyst?
Answer: A catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis.
In simple words: It is a catalyst that acts like a sieve, allowing only molecules of a specific size to enter and react.

Exam Tip: Zeolites are the most common example of shape-selective catalysts.

Question 8. Of the physisorption and chemisorption, which type of adsorption has higher enthalpy of adsorption?
Answer: Chemisorption.
In simple words: Chemical bonds release more energy when formed compared to weak physical forces.

Exam Tip: Physisorption enthalpy is \(20-40\text{ kJ mol}^{-1}\), whereas chemisorption enthalpy is \(80-240\text{ kJ mol}^{-1}\).

Question 9. Write down the example of positive sol?
Answer: Hydrated ferric oxide sol (\(Fe(OH)_3\)).
In simple words: Ferric oxide particles carry a positive charge in this colloidal solution.

Exam Tip: Metal hydroxides are generally positively charged sols.

Question 10. Write down the example of negative sol?
Answer: Arsenic sulphide sol (\(As_2S_3\)).
In simple words: Arsenic sulphide particles carry a negative charge in this colloidal solution.

Exam Tip: Metal sulphides are generally negatively charged sols.

SHORT ANSWER TYPE QUESTIONS (2 Marks)

Question 1. Differentiate between physical and chemical adsorption?
Answer:

In simple words: Physical adsorption is weak and temporary, while chemical adsorption is strong and permanent.

Exam Tip: Mention reversibility and activation energy differences for complete scoring.

Question 2. Differentiate between lyophobic and lyophilic colloids?
Answer:

In simple words: Lyophilic colloids love the liquid medium and stay mixed easily, whereas lyophobic colloids hate it and need stabilizers.

Exam Tip: Give starch as an example of lyophilic and metal sol as lyophobic.

Question 3. Differentiate between multimolecular, macromolecular, and associated colloids?
Answer:

In simple words: Multimolecular colloids build up from tiny particles, macromolecular ones are naturally huge molecules, and associated ones cluster together only when concentrated.

Exam Tip: Focus on particle formation mechanisms for each type.

Question 4. What is the difference between sol and gel?
Answer: Both are colloidal solutions. A sol has a solid as the dispersed phase and a liquid as the dispersion medium (solid-in-liquid). A gel has a liquid as the dispersed phase and a solid as the dispersion medium (liquid-in-solid).
In simple words: In a sol, solid floats in liquid; in a gel, liquid is trapped inside a solid network.

Exam Tip: Paint is a sol, while hair gel or cheese is a gel.

Question 5. Action of soap is due to emulsification and micelle formation. Comment.
Answer: Soaps are sodium or potassium salts of higher fatty acids (\(C_{17}H_{35}COONa\)). The hydrocarbon chain dissolves in oil or grease, while the polar carboxylate group remains dissolved in water. This forms a micelle structure that emulsifies the greasy dirt, allowing it to be washed away with water.
In simple words: Soap acts as a bridge between greasy dirt and water, trapping grease inside micelles so it can be rinsed off.

Exam Tip: Explain both the hydrophobic tail and hydrophilic head parts of the soap molecule.

SHORT ANSWER TYPE QUESTIONS (3 Marks)

Question 1. Discuss the effect of pressure and temperature on the adsorption of gases on solids?
Answer:
Effect of pressure:
At constant temperature, the extent of adsorption (\(x/m\)) increases with pressure. According to the Freundlich adsorption isotherm:
At low pressure: \(\frac{x}{m} \propto p^1\)
At high pressure: \(\frac{x}{m} \propto p^0\)
At intermediate pressure: \(\frac{x}{m} = k \cdot p^{1/n}\) (where \(1/n\) ranges between 0 and 1).
Taking logarithm: \(\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n} \log p\)
Effect of temperature:
Adsorption is generally exothermic. Therefore, physical adsorption decreases with an increase in temperature according to Le Chatelier's principle. In chemisorption, adsorption first increases with temperature (due to activation energy) and then decreases.
In simple words: Higher pressure increases adsorption, while higher temperature usually decreases physical adsorption.

Exam Tip: Draw the Freundlich adsorption isotherm graph (\(\log(x/m)\) versus \(\log p\)) with slope equal to \(1/n\).

Question 2. Explain what is observed when:
(i) An electrolyte (\(NaCl\)) is added to hydrated ferric oxide sol.
(ii) Electric current is passed through a colloidal sol.
(iii) When a beam of light is passed through a colloidal sol.
Answer:
(i) The positively charged colloidal particles of \(Fe(OH)_3\) get coagulated by the oppositely charged chloride (\(Cl^-\)) ions provided by \(NaCl\).
(ii) The colloidal particles migrate towards the oppositely charged electrode (electrophoresis), lose their charge, and get coagulated.
(iii) Scattering of light by the colloidal particles takes place, making the path of the light beam clearly visible (Tyndall effect).
In simple words: Electrolytes cause particles to clump and settle, electricity pulls particles to electrodes, and light scattering makes the beam visible.

Exam Tip: Clearly mention coagulation, electrophoresis, and Tyndall effect terms for the three sub-parts.

Question 3. Describe some features of catalysis by zeolites?
Answer:
1. Zeolites are hydrated aluminosilicates with a three-dimensional network structure containing interconnected pores and cavities.
2. When heated, water of hydration is lost, leaving vacant pores that act as molecular sieves.
3. The pore size varies from \(260\text{ pm}\) to \(740\text{ pm}\), allowing only molecules of suitable size to enter and undergo catalysis (shape-selective catalysis).
4. An important catalyst used in the petroleum industry is ZSM-5, which converts alcohols directly into petrol (gasoline) by dehydration.
In simple words: Zeolites act like microscopic sieves that only let molecules of the right size enter and react.

Exam Tip: Mention ZSM-5 and alcohol-to-petrol conversion as a key example.

Question 4. Comment on the statement that "colloid is not a substance but a state of a substance"?
Answer: The given statement is true. A substance can exist as a colloid under certain conditions and as a crystalloid under other conditions. For example, sodium chloride (\(NaCl\)) in water behaves as a crystalloid, whereas in benzene, it behaves as an associated colloid. It is the particle size (ranging between \(1\text{ nm}\) and \(1000\text{ nm}\)) that determines the colloidal state.
In simple words: Any substance can become a colloid if its particle size falls within the colloidal range.

Exam Tip: Emphasize that colloidal nature depends on particle size, not chemical composition.

Question 5. Write short notes on the following:
(a) Tyndall effect
(b) Brownian movement
(c) Hardy-Schulze rule
Answer:
(a) Tyndall effect: The scattering of a beam of light by colloidal particles, making the path of the light beam visible in a dark room.
(b) Brownian movement: The continuous, rapid, zig-zag motion of colloidal particles caused by unequal bombardment by molecules of the dispersion medium.
(c) Hardy-Schulze rule: The coagulating power of an active ion is directly proportional to the valency of the ion. Higher the valency, greater is its precipitating power.
In simple words: Tyndall effect scatters light, Brownian movement creates zig-zag motion, and Hardy-Schulze rule explains how ions cause coagulation.

Exam Tip: Keep definitions concise with appropriate scientific keywords for each short note.