CBSE Class 12 Chemistry Chapter 5 Notes: Surface Chemistry

Key Definitions

Adsorption

The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The substance accumulating is the adsorbate, and the surface on which it accumulates is the adsorbent.

Desorption

The process of removing an adsorbed substance from a surface.

Catalysis

The phenomenon of increasing the rate of a chemical reaction by adding a substance (catalyst) which is not consumed in the reaction.

Critical Micelle Concentration (CMC)

The minimum concentration of surfactant above which micelle formation takes place.

Tyndall Effect

The scattering of light by colloidal particles when a beam of light is passed through a colloidal solution, making the path of light visible.

Electrophoresis

The movement of charged colloidal particles under the influence of an electric field towards the oppositely charged electrode.

Coagulation (Flocculation)

The process by which colloidal particles aggregate to form larger particles that settle down due to gravity.

Peptization

The process of converting a precipitate into a colloidal solution by adding a suitable electrolyte (peptizing agent).

Adsorption: Mechanism and Types

Adsorption is a surface phenomenon where molecules adhere to the surface of a solid or liquid. This occurs because the surface particles of the adsorbent have unbalanced residual attractive forces which can hold adsorbate molecules. It's an exothermic process because surface energy decreases during adsorption.

There are two main types of adsorption:

1. Physisorption (Physical Adsorption):

• Caused by weak van der Waals forces between adsorbate and adsorbent.
• Non-specific in nature (any gas can adsorb on any solid to some extent).
• Reversible: Adsorption increases with decreasing temperature and increasing pressure.
• Low enthalpy of adsorption (20-40 kJ/mol).
• Forms multilayered adsorption on the adsorbent surface.
• No significant activation energy required.

1. Chemisorption (Chemical Adsorption):

• Caused by strong chemical bonds (covalent or ionic) between adsorbate and adsorbent.
• Highly specific in nature (requires formation of chemical bonds).
• Irreversible: Forms a surface compound. Increases with temperature initially (due to activation energy) then decreases.
• High enthalpy of adsorption (80-240 kJ/mol).
• Forms a monolayer on the adsorbent surface.
• Requires significant activation energy.

Understanding the factors affecting adsorption, such as surface area of adsorbent, nature of adsorbate gas (critical temperature), and temperature/pressure, is vital. Freundlich adsorption isotherm (x/m = kP^(1/n)) and Langmuir adsorption isotherm (qualitative understanding) describe the relationship between the amount of gas adsorbed and pressure at a constant temperature.

Distinction Between Lyophilic and Lyophobic Colloids

Key Points to Remember

• Heterogeneous catalysis involves reactants and catalyst in different phases (e.g., solid catalyst, gaseous reactants).
• Homogeneous catalysis involves reactants and catalyst in the same phase (e.g., liquid catalyst, liquid reactants).
• Enzyme catalysis is highly specific and efficient, often showing lock and key mechanism.
• Shape-selective catalysis occurs in zeolites, where the pore structure dictates reactivity.
• Hardy-Schulze Rule states that the coagulating power of an electrolyte's active ion increases with the increase in the magnitude of its charge (valency).
• Protective colloids are lyophilic colloids added to lyophobic colloids to stabilize them against coagulation.
• Brownian movement is the continuous zig-zag motion of colloidal particles due to bombardment by dispersion medium molecules.
• Critical Temperature of a gas is important for adsorption: gases with higher critical temperature are more easily liquefied and thus more readily adsorbed.

Worked Examples

• {"title":"Example 1: Hardy-Schulze Rule Application","bodyMarkdown":"Explain which electrolyte would be more effective in coagulating a negatively charged arsenic sulphide sol: NaCl or CaCl₂.\n\nSolution: Arsenic sulphide sol (As₂S₃) is negatively charged. According to the Hardy-Schulze rule, the effectiveness of an electrolyte in coagulation depends on the charge of the oppositely charged ion (the cation here). CaCl₂ provides Ca²⁺ ions, while NaCl provides Na⁺ ions. Since Ca²⁺ has a higher positive charge than Na⁺, CaCl₂ will be more effective in coagulating the negatively charged arsenic sulphide sol."}
• {"title":"Example 2: Tyndall Effect","bodyMarkdown":"Why does a beam of light passing through a true solution not show the Tyndall effect, but a colloidal solution does?\n\nSolution: The Tyndall effect is the scattering of light by colloidal particles. True solutions have solute particles smaller than the wavelength of visible light, so they cannot scatter light. Colloidal particles, however, are larger (1 nm to 1000 nm), allowing them to effectively scatter light, making the path of the beam visible."}

Exam Tip: Distinguish Clearly!

Many questions in Surface Chemistry test your ability to differentiate between similar-sounding terms or concepts. Practice comparing and contrasting: adsorption vs. absorption, physisorption vs. chemisorption, lyophilic vs. lyophobic colloids, and homogeneous vs. heterogeneous catalysis. Use tables or bullet points in your answers to score maximum marks. Also, pay attention to the applications of adsorption, catalysis, and colloids, as direct questions on these are common.

Practice Questions with Solutions

• Q: What is the primary difference in the forces involved in physisorption and chemisorption? A: Physisorption involves weak van der Waals forces, while chemisorption involves strong chemical (covalent/ionic) bonds.
• Q: State the Hardy-Schulze rule. A: It states that the coagulating power of an electrolyte is directly proportional to the valency of the active ion responsible for coagulation (i.e., the ion carrying charge opposite to that on colloidal particles).
• Q: Give one example of a homogeneous catalyst and the reaction it catalyzes. A: NO gas acting as a catalyst in the lead chamber process for the oxidation of SO₂ to SO₃ (2SO₂(g) + O₂(g) --(NO(g))--> 2SO₃(g)).
• Q: What is the role of an emulsifying agent in an emulsion? A: An emulsifying agent stabilizes an emulsion by forming an interfacial film between the dispersed phase and the dispersion medium, reducing interfacial tension and preventing coalescence of droplets.

Frequently Asked Questions