Important questions based on NCERT syllabus for Chapter 5 - Surface Chemistry:
Question-1: Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
Solution: This phenomenon of attracting and retaining the molecules of a substance by a solid (or a liquid) on its surface resulting into a higher concentration of the molecules on the surface is known as adsorption.
Absorption is different from adsorption. In absorption, the substance is uniformly distributed throughout the body of a solid or a liquid.
NH3 gets adsorbed on the charcoal where as NH3 when comes in contact with H20 gets absorbed by forming NH4OH solution of uniform concentration.
Question-2: What role does adsorption play in heterogeneous catalysis?
Solution: In heterogenous catalysis, generally the reactants are gaseous whereas catalyst is a solid. The reactant molecules are adsorbed on the surface of the solid catalyst by physical adsorption or chemisorption. As a result, the concentration of the reactant molecules on the surface increases further leading to increase in rate of reaction. Alternatively, one of the reactant molecules undergoes fragmentation on the surface of the solid catalyst producing active species which react faster. The product molecules in either case have no affinity for the solid catalyst and are deadsorbed making the surface free for fresh adsorption.
Question-3: What are emulsions? What are their different types? Give example of each type.
Solution: Emulsions: It is a colloidal system in which both the dispersed phase and the dispersion medium are liquids, e.g., milk consists of small drop’s of liquid fat dispersed in water.
Types of emulsions:
(a) Oil-in-water type in which small droplets of an oil are dispersed in water, e.g., milk, cod-liver oil, etc.
(b) Water-in-oil type in which water droplets are dispersed in an oil medium, e.g., butter.