Surface Chemistry Class 12 NCERT

What is Surface Chemistry? Adsorption vs. Absorption

Surface chemistry is the branch of chemistry that deals with phenomena occurring at the interfaces or surfaces of phases. These interfaces can exist between a solid and a liquid, a solid and a gas, a liquid and a gas, or even two immiscible liquids. Unlike bulk phenomena, surface phenomena are driven by the imbalance of forces at the surface, where molecules are not surrounded by similar molecules on all sides, leading to residual attractive forces.

Adsorption

Adsorption is the phenomenon of accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed on the surface is called the adsorbate, and the solid or liquid surface on which adsorption occurs is called the adsorbent. For example, when charcoal adsorbs gases like hydrogen or nitrogen, the gas is the adsorbate and charcoal is the adsorbent. This process is generally exothermic, meaning heat is released during adsorption.

Absorption

In contrast to adsorption, absorption is a phenomenon in which a substance is uniformly distributed throughout the bulk of another substance. For example, when a sponge soaks up water, the water is absorbed by the sponge, distributing itself throughout the sponge's volume. While adsorption is a surface phenomenon, absorption is a bulk phenomenon. When both adsorption and absorption occur simultaneously, it is termed sorption.

Types of Adsorption: Physisorption and Chemisorption

Adsorption can be broadly classified into two main types based on the nature of forces existing between the adsorbate and the adsorbent:

1. Physisorption (Physical Adsorption): This type of adsorption is caused by weak van der Waals forces between the adsorbate and the adsorbent. It is similar to the condensation of gases into liquids. Key characteristics include:

• Non-specific: Any gas can be adsorbed on any solid, usually at low temperatures.
• Reversible: Adsorption can be reversed by increasing temperature or decreasing pressure.
• Low enthalpy of adsorption: Typically 20-40 kJ/mol.
• Multilayer formation: Adsorbate forms multiple layers on the adsorbent surface.
• Favoured by low temperature and high pressure.

1. Chemisorption (Chemical Adsorption): This type involves the formation of a chemical bond (covalent or ionic) between the adsorbate and the adsorbent surface. It's more specific and often irreversible. Key characteristics include:

• Highly specific: Only occurs if there is a possibility of chemical bond formation.
• Irreversible: Forms compounds on the surface; desorption requires higher energy.
• High enthalpy of adsorption: Typically 80-240 kJ/mol, similar to chemical bond energies.
• Unilayer formation: Adsorbate forms a single layer (monolayer) on the adsorbent surface.
• Favoured by high temperature (initially increases, then decreases) and high pressure.
• Requires activation energy, similar to chemical reactions.

Factors Affecting Adsorption and Adsorption Isotherms

Several factors play a crucial role in determining the extent of adsorption. Understanding these helps in predicting and controlling surface phenomena.

Factors Affecting Adsorption:

1. Nature of Adsorbate: Gases that are easily liquefiable (e.g., SO₂, NH₃, Cl₂) are more readily adsorbed because their intermolecular forces are stronger, making them easier to condense onto a surface.
2. Nature of Adsorbent: Porous and finely divided solids (e.g., charcoal, silica gel) provide a larger surface area, hence adsorbing more. The specific surface properties also play a role in chemisorption.
3. Surface Area of Adsorbent: The extent of adsorption is directly proportional to the surface area of the adsorbent. The more surface available, the more sites for adsorbate molecules to attach.
4. Temperature:

• Physisorption: Being an exothermic process, it decreases with an increase in temperature (Le Chatelier's principle).
• Chemisorption: It also tends to decrease with an increase in temperature at higher temperatures. However, initially, for some chemisorption processes, an increase in temperature can increase the rate of adsorption by providing the necessary activation energy for chemical bond formation, reaching an optimum before decreasing.

1. Pressure:

• Gases: For physisorption of gases, increasing pressure increases the extent of adsorption as more gas molecules are forced onto the surface. For chemisorption, the effect is similar but often reaches saturation faster due to monolayer formation.

Adsorption Isotherms:

An adsorption isotherm is a curve that describes the relationship between the quantity of adsorbate adsorbed on the surface of an adsorbent and the equilibrium pressure (for gases) or concentration (for liquids) at a constant temperature.

1. Freundlich Adsorption Isotherm: This is an empirical relationship proposed by Freundlich in 1909. It can be expressed as:

x/m = kP^(1/n) (for gases), or x/m = kC^(1/n) (for solutions)
Where:

• x = mass of adsorbate
• m = mass of adsorbent
• P = equilibrium pressure (or C = equilibrium concentration)
• k and n = constants (n > 1)

Taking log on both sides: log(x/m) = log k + (1/n)log P. This gives a straight line plot of log(x/m) vs log P.

Limitations: The Freundlich isotherm fails at high pressures and does not have a theoretical foundation.

1. Langmuir Adsorption Isotherm: Proposed by Langmuir in 1916, this isotherm is based on certain assumptions, including that adsorption occurs at specific sites on the surface, forming a monolayer, and there is no interaction between adsorbed molecules. The equation is:

x/m = (aP) / (1 + bP)
Where:

• a and b = Langmuir constants
• P = pressure

At very low pressures, x/m = aP. At very high pressures, x/m = a/b (saturation). The Langmuir model provides a good fit for chemisorption and some physisorption processes.

Catalysis: Speeding Up Reactions and Colloids: The Hidden World

Catalysis

Catalysis is the phenomenon where a substance, called a catalyst, alters the rate of a chemical reaction without itself being consumed in the process. Catalysts generally speed up reactions by providing an alternative reaction pathway with a lower activation energy. A promoter enhances the activity of a catalyst, while a catalytic poison reduces or destroys it.

Types of Catalysis:

1. Homogeneous Catalysis: The reactants and the catalyst are in the same phase (e.g., all liquid or all gas). Example: Oxidation of SO₂ to SO₃ in the lead chamber process using NO gas as a catalyst.
2. Heterogeneous Catalysis: The reactants and the catalyst are in different phases, typically a solid catalyst with gaseous or liquid reactants. Example: Haber's process for ammonia synthesis (N₂ + 3H₂ → 2NH₃) using finely divided iron as a catalyst.

Mechanism of Heterogeneous Catalysis: Involves five steps: diffusion of reactants to the catalyst surface, adsorption of reactant molecules, chemical reaction on the surface, desorption of products, and diffusion of products away from the surface.

1. Enzyme Catalysis: Enzymes are complex nitrogenous organic compounds produced by living organisms, acting as highly efficient biochemical catalysts. They are highly specific, acting on particular substrates (e.g., invertase hydrolyses sucrose). Their efficiency is maximal at an optimum temperature (298-310 K) and pH (5-7).

Colloids

Colloids represent an intermediate state between true solutions and suspensions. In a colloidal system, one substance is dispersed uniformly in another, forming a dispersed phase (the substance dispersed) and a dispersion medium (the substance in which it is dispersed). The size of dispersed particles in colloids typically ranges from 1 nm to 1000 nm.

Classification of Colloids:

1. Based on Physical State of Dispersed Phase and Medium: Examples include sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid), aerosol (solid/liquid in gas), etc.
2. Based on Nature of Interaction:

• Lyophilic Colloids (solvent-loving): Easily formed by mixing the substance with the dispersion medium. They are stable and reversible (e.g., starch, gum in water).
• Lyophobic Colloids (solvent-hating): Cannot be formed by simple mixing; require special methods. They are less stable and irreversible (e.g., metal sols, metal hydroxides).

1. Based on Type of Particles:

• Multimolecular Colloids: Particles consist of aggregates of many atoms or small molecules (e.g., gold sol).
• Macromolecular Colloids: Particles are large molecules themselves (e.g., starch, proteins).
• Associated Colloids (Micelles): Substances that behave as normal electrolytes at low concentrations but form aggregates (micelles) at higher concentrations (above Critical Micelle Concentration, CMC) due to having both lyophilic and lyophobic parts (e.g., soaps and detergents).

Preparation and Purification of Colloids: Colloids can be prepared by various methods, including chemical reactions, electrical disintegration (Bredig's Arc method), and peptization (converting precipitate into colloidal sol). Purification methods include dialysis, electrodialysis, and ultrafiltration.

Properties of Colloids: Colloids exhibit unique properties like the Tyndall effect (scattering of light), Brownian movement (zig-zag motion of particles), electrophoresis (movement of charged particles in an electric field), and coagulation/flocculation (precipitation of colloidal particles, explained by Hardy-Schulze rule).

Worked Examples on Surface Chemistry

• Example 1: Distinguishing Adsorption Types A student observed that when a gas was adsorbed on charcoal, the process released heat, and the adsorption increased significantly at lower temperatures. When the temperature was raised, the adsorbed gas could be easily removed. What type of adsorption is this, and why? Solution: Step 1: Analyze the given characteristics. "Released heat": Adsorption is exothermic. "Increased significantly at lower temperatures": This is characteristic of physisorption. "Easily removed when temperature was raised": This indicates reversibility. Step 2: Relate characteristics to types of adsorption. Exothermic nature and increase at lower temperatures strongly suggest physisorption. * Reversibility is another key characteristic of physisorption. Final Answer: This is physisorption (physical adsorption). It is characterized by weak van der Waals forces, low enthalpy of adsorption, reversibility, and a preference for low temperatures and high pressures.
• Example 2: Applying Hardy-Schulze Rule Arrange the following electrolytes in increasing order of their coagulating power for an arsenious sulphide sol (As₂S₃ sol): NaCl, MgCl₂, AlCl₃ Solution: Step 1: Identify the nature of the colloidal sol. Arsenious sulphide sol (As₂S₃ sol) is a negatively charged sol due to the adsorption of S²⁻ ions. Step 2: Recall the Hardy-Schulze rule. The Hardy-Schulze rule states that the coagulating power of an electrolyte for a colloidal sol increases with the increase in the valency of the oppositely charged ion (the active ion). Step 3: Determine the active ions and their valencies for the given electrolytes. For NaCl, the active ion is Na⁺ (valency +1). For MgCl₂, the active ion is Mg²⁺ (valency +2). For AlCl₃, the active ion is Al³⁺ (valency +3). Step 4: Arrange in increasing order of coagulating power. Based on valency, Na⁺ < Mg²⁺ < Al³⁺. Final Answer: The increasing order of coagulating power is: NaCl < MgCl₂ < AlCl₃.
• Example 3: Explaining Micelle Formation Why do soaps form micelles in water above a certain concentration? Solution: Step 1: Understand the structure of a soap molecule. A soap molecule (e.g., sodium stearate) has a long hydrocarbon chain (non-polar, hydrophobic or water-repelling 'tail') and a polar ionic group (e.g., -COO⁻Na⁺, hydrophilic or water-loving 'head'). Step 2: Describe behavior at low concentration. At low concentrations, soap molecules exist as individual ions or molecules dispersed in water, with their polar heads interacting with water. Step 3: Explain what happens above Critical Micelle Concentration (CMC). Above the Critical Micelle Concentration (CMC), the hydrophobic tails start to cluster together to minimize contact with water. Simultaneously, the hydrophilic heads remain exposed to the aqueous environment. Step 4: Describe the resulting structure. This aggregation leads to the formation of spherical structures called micelles, where the hydrocarbon tails are in the interior, and the polar heads are on the surface, facing outwards into the water. Final Answer: Soaps form micelles in water above CMC because their amphiphilic nature (hydrophobic tail, hydrophilic head) drives the non-polar tails to aggregate away from water, while the polar heads remain solvated on the micelle's surface.

Exam Tips and Common Errors in Surface Chemistry

To excel in Surface Chemistry, focus on clear conceptual understanding and precise definitions. Here are some key tips and common pitfalls to avoid:

1. Differentiate Adsorption vs. Absorption: This is a frequently asked question. Remember, adsorption is a surface phenomenon (accumulation on the surface), while absorption is a bulk phenomenon (uniform distribution throughout).
2. Master Physisorption vs. Chemisorption: Understand their distinct characteristics (forces, enthalpy, reversibility, temperature/pressure effects, number of layers). Create a comparison table for quick revision. Don't confuse their temperature dependence: physisorption decreases with increasing T, while chemisorption first increases (due to activation energy) then decreases.
3. Hardy-Schulze Rule: Practice applying this rule correctly. Remember, the coagulating power depends on the valency of the oppositely charged ion. For a negative sol, look for the cation's valency; for a positive sol, look for the anion's valency.
4. Colloid Classification: Pay attention to the three main ways colloids are classified (physical state, nature of interaction, type of particles) and be able to give examples for each. The difference between lyophilic and lyophobic colloids, especially their stability and methods of preparation, is important.
5. Tyndall Effect and Brownian Movement: Understand why these phenomena occur in colloids but not in true solutions. The size of colloidal particles is key to both.
6. Adsorption Isotherms: While detailed derivations might not be asked, understand the equations and their graphical representation for Freundlich and Langmuir isotherms. Know their assumptions and limitations.

Practice Questions with Solutions

• Q: What is the main difference between physisorption and chemisorption in terms of the forces involved and enthalpy of adsorption? A: Step 1: Identify the primary forces for each type. Physisorption involves weak van der Waals forces. Chemisorption involves strong chemical bonds (covalent or ionic). Step 2: Relate the force strength to the enthalpy of adsorption. Due to weak forces, physisorption has a low enthalpy of adsorption (20-40 kJ/mol). Due to strong chemical bonds, chemisorption has a high enthalpy of adsorption (80-240 kJ/mol). Final answer: Physisorption involves weak van der Waals forces with low enthalpy of adsorption, while chemisorption involves strong chemical bonds with high enthalpy of adsorption.
• Q: Explain why a finely divided catalyst is generally more effective than a solid lump of the same mass. A: Step 1: Consider the role of surface area in catalysis. Catalysis, especially heterogeneous catalysis, is a surface phenomenon where reactant molecules adsorb onto the catalyst surface. Step 2: Compare surface area of finely divided vs. lump. A finely divided catalyst has a much larger surface area compared to a solid lump of the same mass. Step 3: Conclude the effect on reaction rate. A larger surface area provides more active sites for the adsorption of reactants, leading to a higher rate of reaction. Final answer: A finely divided catalyst is more effective because it offers a significantly larger surface area, providing more active sites for adsorption and reaction, thereby enhancing the rate of catalysis.
• Q: Give two examples of associated colloids (micelles) and explain what happens above their Critical Micelle Concentration (CMC). A: Step 1: Identify examples of associated colloids. Soaps (e.g., sodium stearate, C₁₇H₃₅COO⁻Na⁺) and detergents are common examples. Step 2: Explain behavior above CMC. Above the Critical Micelle Concentration (CMC), the amphiphilic molecules (with both hydrophobic and hydrophilic parts) aggregate to form micelles. In aqueous solution, the hydrophobic (non-polar) tails cluster together in the interior, while the hydrophilic (polar) heads arrange on the surface, facing the water. Final answer: Soaps and detergents are examples of associated colloids. Above their Critical Micelle Concentration (CMC), their molecules aggregate to form micelles, spherical structures where hydrophobic parts are shielded from water and hydrophilic parts interact with water.
• Q: What is the Tyndall effect, and why is it exhibited by colloidal solutions but not by true solutions? A: Step 1: Define the Tyndall effect. The Tyndall effect is the phenomenon of scattering of light by colloidal particles as the light beam passes through a colloidal dispersion, making the path of light visible. Step 2: Compare particle sizes in colloidal and true solutions. Colloidal particles (1 nm to 1000 nm) are large enough to scatter light, whereas particles in true solutions (less than 1 nm) are too small to cause significant scattering. Step 3: Conclude why true solutions don't show it. In true solutions, the particles are so small that they cannot scatter the incident light to a detectable extent. Final answer: The Tyndall effect is the scattering of light by colloidal particles. It is exhibited by colloidal solutions because their particle size (1-1000 nm) is large enough to scatter light, unlike true solutions where particles are too small to cause such scattering.

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