Surface Chemistry Class 12 Notes

Surface Chemistry Class 12 Notes

Surface Chemistry


Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
Answer: The phenomenon of accumulation of the molecules of a substance on a solid or liquid surface resulting in the increased concentration of the molecules on the surface is called adsorption. In absorption, the substance is uniformly distributed throughout the bulk of the solution. A distinction can be made by taking an example of water vapours. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel.

Give reason why a finely divided substance is more effective as an adsorbent.
Answer: The extent of adsorption increases with increase in surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents.

What are the factors which influence the adsorption of a gas on a solid?
Answer: Factors affecting adsorption of a gas on solids are:
Nature of the adsorbent: The same gas is adsorbed to different extents by different solids at the same temperature. Also, greater the surface area of the adsorbent, more is the gas adsorbed.
Nature of the adsorbate: Different gases are adsorbed to different extents by different solids at the same temperature. Higher the critical temperature of the gas, greater is its amount adsorbed.
Temperature: Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.
Specific area of the adsorbent: Surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. Greater the surface area, higher would be the adsorption therefore, porous or powdered adsorbents are used.
Pressure: At constant temperature, the adsorption of gas increases with pressure.
Activation of adsorbent: It means increasing the adsorbing power of anadsorbent by increasing its surface area. It is done by:
a. making the adsorbent’s surface rough
b. removing gases already adsorbed
c. subdividing the adsorbent into smaller pi

Why is adsorption always exothermic ?
Answer: During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat. Adsorption therefore, is invariably an exothermic process. In other words, ΔH of adsorption is always negative to keep the value of ΔG negative for the reaction to be spontaneous as ΔS decreases during adsorption.

How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium ?
Answer: Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.
classification of colloidal solution

Discuss the effect of pressure and temperature on the adsorption of gases on solids.
Answer: Adsorption decreases with an increase in temperature because it is an exothermic process and according to Le Chatelier’s principle the reaction will proceed in backward direction with increase in temperature. However at a constant temperature, adsorption increases with pressure.

What are lyophilic and lyophobic sols ? Give one example of each type. Why are hydrophobic sols easily coagulated ?
Answer: There are two types of colloidal sols:
a. Lyophilic sols: The word lyophilic means solvent loving. They are obtained by direct mixing the dispersed phase and the dispersion medium, e.g., sols of gum, gelatin, starch, etc. They are solvent attracting hence quite stable and cannot be coagulated easily.
b. Lyophobic sols: They cannot be prepared by direct mixing the dispersed phase and dispersion medium but are prepared by special methods, e.g., sols of metals. They are solvent repelling. Hydrophobic sols are easily coagulated due to repulsion between water and dispersed phase.

What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids different from these two types of colloids ?
Answer: Depending upon the type of particles of the dispersed phase, colloids are classified as : multimolecular, macromolecular and associated colloids.
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 (diameter < 1nm). The species thus formed are called multimolecular colloids. For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur molecules.
b. Macromolecular colloids: Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples of naturally occurring macromolecules are starch, cellulose, proteins and enzymes; and those of man-made macromolecules are polythene, nylon, polystyrene, synthetic rubber, etc.
c. Associated colloids (Micelles): There are some substances which at low concentrations behave as normal strong electrolytes but, at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10–4 to 10–3 mol L–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Why is it essential to wash the precipitate with water before estimating it quantitatively ?
Answer: Few impurities which are soluble in water and are adsorbed on the surface of the precipitate are removed by washing them with water.

Comment on the statement that ‘colloid is not a substance but a state of substance’.
Answer: Colloid is not a substance, but a state of substance because the same substance may exist as a colloid or crystalloid under different conditions e.g., sulphur. Colloidal solution of sulphur consists of sulphur molecules dispersed in water. In this state, sulphur atoms combine to form multimolecules whose size lies between 1 nm to 1000 nm and form colloidal state. Sulphur forms true solution in carbon disulphide. Similarly soap is a solution at low concentration but a colloid at higher concentration.

Explain the terms with suitable examples: Alcosol, Aerosol and Hydrosol
Alcosol: The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.
Aerosol: The sol in which dispersion medium is gas and dispersed phase is either solid or liquid, the colloidal system is called aerosol e.g., fog, insecticides, sprays, etc.
Hydrosol: The sol in which dispersion medium is water is called hydrosol e.g., starch sol. a. Alcosol : The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.