Mastering Surface Chemistry (Chemistry Class XII)

Distinction between adsorption and absorption

• Adsorption is a surface phenomenon of accumulation of molecules of a substance at the surface rather than in the bulk of a solid or liquid. It is an exothermic process.
• Absorption is a bulk phenomenon. In absorption, the absorbed substance gets uniformly distributed throughout the bulk of the solid or liquid.

Types of Adsorption

• Physical adsorption or physisorption – Here, the accumulation of gas on the surface of a solid occurs on account of weak van der Waal’s forces.
• Chemical adsorption or chemisorption − When the gas molecules or atoms are held to the surface by chemical bonds.

Freundlich adsorption isotherm

• Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a specific temperature.
• Graph is plotted between the extent of adsorption against the pressure (P) of the gas at constant temperature (T)
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• Adsorption from Solution Phase– It can be approximately described by Freundlich’s equation by using the concentration (C) of the solution instead of pressure
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Some common important terms:

• Catalysts alter the rate of a chemical reaction, and remain chemically and quantitatively unchanged after the reaction.
• Promoters enhance the activity of a catalyst.
• ·Poisons decrease the activity of a catalyst.
• ·Homogeneous catalysis – Here, the reactants and the catalyst are in the same phase
• ·Heterogeneous catalysis –Here, the reactants and the catalyst are in different phases
• ·Selectivity − Ability to direct a reaction to yield a particular product
• ·Enzymes − Complex nitrogenous compounds which are produced by living plants and animals
• ·Kraft temperature (T_k) – The temperature above which the formation of micelles takes place
• ·Critical micelle concentration (CMC) – The concentration above which the formation of micelles takes place.
• ·Peptisation − Process of converting a precipitate into colloidal sol by shaking it with dispersion medium in presence of small amount of electrolyte (peptising agent)
• ·Dialysis– The process of removing a dissolved substance by diffusion through a suitable membrane.
• ·Tyndall effect– The scattering of light in all directions due to colloidal particles
• ·Brownian movement– Zig-zag movement of colloidal particles
• ·Electrophoresis– Movement of colloidal particles under applied electric potential
• ·Electro osmosis– Movement of dispersion medium through the membrane, in an applied electric field
• ·Hardy−Schulze rule– The greater the valence of the flocculating ion added, the greater is its power to cause precipitation
• ·Smoke– A colloidal solution of solid particles (carbon, dust, arsenic compounds, etc.)
• ·Emulsifying agent: The third component used for stabilising an emulsion
• ·Emulsification: Process of making an emulsion

Colloid:

Emulsions: Colloidal systems in which both the dispersed phase and the dispersion medium are liquids.

Types of emulsions:

• Oil dispersed in water (O/W type)
• Water dispersed in oil (W/O type)

Questions which were asked previously:

Q. Define ‘peptization’. (Delhi 2012 Set 3)

Q. Write three distinct features of chemisorptions which are not found in physisorption. (Delhi 2012 Set 3)

Q. Define ‘electrophoresis’. (Delhi 2011 Set 1)

Q. Explain how the phenomenon of adsorption finds application in each of the following processes?(Delhi 2011 Set 1)

(a) Production of vacuum

(b) Heterogeneous catalysis

(c) Froth flotation process

Q. Define each of the following term: (Delhi 2011 Set 1)

(a) Micelles

(b) Peptization

(c) Desorption

Q. Give an example of ‘shape-selective catalyst’. (Delhi 2010 Set 3)

Q. How are the following colloids different from each other in respect of their dispersion medium and dispersed phase? Give one example of each. (Delhi 2010 Set 3)

1. Aerosol 2. Emulsion 3. Hydrosol

All the best,

Team Meritnation!