ROUTERA

Chapter 5 Surface Chemistry

Class 12th Chemistry Chapter Case Study

Case Study 1: Adsorption and Its Types

Case: Adsorption is the process by which molecules or atoms from a gas or liquid phase accumulate on the surface of a solid or liquid. It is a surface phenomenon and plays a significant role in various processes like catalysis, chromatography, and environmental cleanup. Adsorption is classified into two main types: physical adsorption and chemical adsorption.

  • Physical adsorption is due to van der Waals forces and is a reversible process. The amount of adsorbate increases with increasing pressure or decreasing temperature.
  • Chemical adsorption involves the formation of chemical bonds between the adsorbate and the surface, and it is usually irreversible.

Questions:

  1. The main difference between physical and chemical adsorption is that:

    • A) Physical adsorption involves the formation of chemical bonds, while chemical adsorption involves van der Waals forces.
    • B) Physical adsorption is irreversible, while chemical adsorption is reversible.
    • C) Physical adsorption is due to van der Waals forces, while chemical adsorption involves chemical bonds.
    • D) Chemical adsorption occurs at low temperatures, and physical adsorption occurs at high temperatures.
    • Answer: C) Physical adsorption is due to van der Waals forces, while chemical adsorption involves chemical bonds.

  2. The adsorption of a gas on a solid is an example of a:

    • A) Solution process
    • B) Chemical reaction
    • C) Surface phenomenon
    • D) Phase transition
    • Answer: C) Surface phenomenon

  3. Which of the following factors increases the adsorption of a gas on a solid surface?

    • A) Decreasing the temperature
    • B) Decreasing the pressure
    • C) Increasing the temperature
    • D) Increasing the surface area of the adsorbent
    • Answer: D) Increasing the surface area of the adsorbent

  4. In chemical adsorption, the adsorbate:

    • A) Forms weak van der Waals forces with the surface
    • B) Is easily desorbed with a decrease in temperature
    • C) Forms strong covalent or ionic bonds with the surface
    • D) Has no effect on the chemical composition of the surface
    • Answer: C) Forms strong covalent or ionic bonds with the surface

Case Study 2: Catalysis and Its Types

Case: Catalysis is the process in which a substance (catalyst) increases the rate of a chemical reaction without being consumed in the process. Catalysts are classified into two main types: homogeneous catalysts and heterogeneous catalysts.

  • Homogeneous catalysis occurs when the catalyst and reactants are in the same phase, usually liquid or gas.
  • Heterogeneous catalysis involves a catalyst that is in a different phase from the reactants, typically solid catalysts in reactions involving gases or liquids.

Catalysts play a crucial role in processes such as the production of ammonia through the Haber process and the catalytic converters used in automobiles.

Questions:

  1. A catalyst that is in the same phase as the reactants is known as a:

    • A) Homogeneous catalyst
    • B) Heterogeneous catalyst
    • C) Surface catalyst
    • D) Chemical catalyst
    • Answer: A) Homogeneous catalyst

  2. Which of the following is an example of heterogeneous catalysis?

    • A) Catalysis of the decomposition of hydrogen peroxide by iodide ions
    • B) Catalysis of the hydrogenation of alkenes by nickel
    • C) Catalysis of esterification by sulfuric acid
    • D) Catalysis of the formation of ozone from oxygen
    • Answer: B) Catalysis of the hydrogenation of alkenes by nickel

  3. In the presence of a catalyst, the activation energy of the reaction:

    • A) Increases
    • B) Decreases
    • C) Remains the same
    • D) Becomes zero
    • Answer: B) Decreases

  4. A catalyst:

    • A) Changes the equilibrium constant of the reaction
    • B) Is consumed in the reaction
    • C) Increases the activation energy of the reaction
    • D) Provides an alternative reaction pathway with lower activation energy
    • Answer: D) Provides an alternative reaction pathway with lower activation energy

Case Study 3: Adsorption Isotherms

Case: The adsorption of a gas on a solid surface can be described by adsorption isotherms, which show the relationship between the amount of adsorbate (gas) adsorbed and the pressure at a constant temperature. The two main types of adsorption isotherms are Langmuir isotherm and Freundlich isotherm.

  • The Langmuir isotherm assumes that the adsorption sites are equivalent and that only one molecule can occupy each site.
  • The Freundlich isotherm is an empirical equation that describes adsorption on heterogeneous surfaces and is valid for situations where the adsorbent surface is not uniform.

Questions:

  1. The Langmuir adsorption isotherm assumes that:

    • A) Adsorption sites are non-identical
    • B) Adsorption sites are equivalent
    • C) Adsorbate molecules interact with each other
    • D) Adsorption is not reversible
    • Answer: B) Adsorption sites are equivalent

  2. The Freundlich adsorption isotherm is applicable to:

    • A) Adsorption on a homogeneous surface
    • B) Adsorption on a surface with varying energies of adsorption
    • C) Chemical adsorption only
    • D) Physical adsorption only
    • Answer: B) Adsorption on a surface with varying energies of adsorption

  3. In the Langmuir adsorption isotherm, the maximum adsorption capacity qmq_m is reached when:

    • A) All adsorption sites are occupied
    • B) The temperature is zero
    • C) The surface is half-covered
    • D) The pressure becomes infinite
    • Answer: A) All adsorption sites are occupied

  4. The Freundlich isotherm equation is given by:

    • A) q=qm1+Kpq = \frac{q_m}{1 + Kp}
    • B) q=Kp1/nq = Kp^{1/n}
    • C) q=Kpq = Kp
    • D) q=1Kpq = \frac{1}{Kp}
    • Answer: B) q=Kp1/nq = Kp^{1/n}

Case Study 4: Colloids and Their Properties

Case: Colloids are mixtures in which one substance (the dispersed phase) is dispersed in another substance (the continuous phase). The dispersed phase particles are smaller than those in suspensions but larger than those in true solutions. Colloids exhibit various unique properties such as Brownian motion, Tyndall effect, and electrophoresis.

  • Brownian motion refers to the random movement of colloidal particles due to collisions with solvent molecules.
  • The Tyndall effect occurs when light is scattered by colloidal particles, making the path of the light visible.
  • Electrophoresis is the movement of charged colloidal particles under the influence of an electric field.

Questions:

  1. The random movement of colloidal particles due to collisions with solvent molecules is called:

    • A) Diffusion
    • B) Brownian motion
    • C) Osmosis
    • D) Sedimentation
    • Answer: B) Brownian motion

  2. The Tyndall effect is observed when:

    • A) Colloidal particles scatter light
    • B) Colloidal particles settle down in a solvent
    • C) Colloidal particles move towards the anode
    • D) Colloidal particles dissolve in the solvent
    • Answer: A) Colloidal particles scatter light

  3. Which of the following properties is characteristic of colloidal solutions?

    • A) They are homogeneous mixtures at the microscopic level
    • B) They do not scatter light
    • C) The dispersed phase is visible to the naked eye
    • D) They exhibit the Tyndall effect
    • Answer: D) They exhibit the Tyndall effect

  4. Colloidal particles can be separated from the dispersion medium by:

    • A) Filtration
    • B) Dialysis
    • C) Centrifugation
    • D) Evaporation
    • Answer: B) Dialysis

Case Study 5: Emulsions and Their Applications

Case: Emulsions are colloidal systems where two immiscible liquids, such as oil and water, are dispersed in each other. Emulsions can be of two types: oil-in-water (O/W) and water-in-oil (W/O). In an oil-in-water emulsion, oil droplets are dispersed in water, while in a water-in-oil emulsion, water droplets are dispersed in oil.

Emulsifiers are substances that stabilize emulsions by reducing the interfacial tension between the two immiscible liquids. Common emulsifiers include soaps, detergents, and bile salts. Emulsions are widely used in industries such as food processing, pharmaceuticals, and cosmetics.

Questions:

  1. Which of the following is an example of an oil-in-water emulsion?

    • A) Butter
    • B) Milk
    • C) Hair cream
    • D) Mayonnaise
    • Answer: B) Milk

  2. Emulsifiers stabilize emulsions by:

    • A) Increasing the size of the droplets
    • B) Decreasing the interfacial tension between the two liquids
    • C) Making the liquids miscible
    • D) Increasing the temperature of the mixture
    • Answer: B) Decreasing the interfacial tension between the two liquids

  3. The Tyndall effect is visible in:

    • A) True solutions only
    • B) Suspensions only
    • C) Colloidal solutions only
    • D) All types of mixtures
    • Answer: C) Colloidal solutions only

  4. Which of the following is a property of emulsions?

    • A) They are homogeneous at the microscopic level
    • B) They are heterogeneous at the microscopic level
    • C) The dispersed phase is visible to the naked eye
    • D) They do not scatter light
    • Answer: B) They are heterogeneous at the microscopic level