Class 12 (surface chemistry) Important questions

  

Part 2.

Q.21. Gelatin which is a peptide is added in ice-creams. What can be its role?

Ans. Ice-creams are emulsions. Gelatin is an emulsifying agent which stabilizes the emulsions.

Q.22. What is collodion?

Ans, It is a 4% solution of nitro-cellulose in a mixture of alcohol and ether.

Q.23. Why do we add alum to purify water?

Ans.Alum when added to impure muddy water coagulates the impurities which settle down and can be separated.

Q.24. What happens when electric field is applied to colloidal solution?

Ans. The charged colloidal particles start moving towards the oppositely charged electrodes and get deposited there.

Q.25 What causes Brownian motion in colloidal dispersion?

Ans.25 Brownian movement is due to the bombardment of the colloidal particles by the particles of the dispersion medium from different directions with different speeds. This phenomenon is responsible for the stability of sol.

Q.26. A colloid is formed by adding FeCl3 in excess of hot water. What will happen if excess sodium chloride is added to this colloid?

Ans. By adding ferric chloride in excess of hot water, a colloidal sol of hydrated ferric oxide is obtained which carries a positive charge due to the adsorption of Fe+ions on its surface, On adding NaCl, ions coagulate the positive charged sol.

Q.27. Why does leather get hardened after tanning?

Ans. Animal hides which are colloidal in nature carry positively charged particles. Tannin which is used for tanning leather contains negatively charged colloidal particles. Therefore, during the process(tanning, oppositely charged colloidal particles coagulate each other and leather becomes hard.

Q.28. Why does physisorption decrease with the increase of temperature ?

Ans. With increase in temperature, the strength of van der Waals" forces decreases. Hence, with increase of temperature, the physisorption which is because of van der Waals' forces also decreases.

Q.29. Give reason : Special type of filter papers are used for filtration of colloidal solutions.

Ans. The pores of an ordinary filter paper are sufficiently large to permit the colloidal particles as well as true solution particles to pass through it. Therefore, special type of ultrafilters with reduced sized pores are used to filter the colloidal solutions.

Q.30. Give reason: Bleeding from a fresh cut can be stopped by applying alum.

Ans. This is because alum coag ilates the blood and, therefore, the bleeding stops.

Q.31. What are emulsions ?

Ans. Emulsions: These are the colloids in which both dispersed phase and dispersion medium are liquids.

Emulsions are of two types

(i)Oil in water emulsions (ii) Water in oil emulsions.

Q.32. Define adsorpiton. Give one example.

Ans. The phenomenon of attracting and retaining any molecular species on the surface of a solid (or a liquid) giving rise to a higher concentration of the same species at the surface, than in the bulk of the solid (or liquid) is known as adsorption.

As a result of adsorption, there is a decrease of surface energy. The substance that attracts and retains onto its surface any molecular species is called the adsorbent, while the species, which is held to its (solid) surface is called adsorbate.For example, ammonia gas is adsorbed by charcoal.

Q.33. Define positive and negative catalysis.

Ans. Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is saia to be a positive catalyst and phenomenon as positive catalysis.

Negative catalysis: There are certain, substance which, when added to the reaction mixture, retain the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and phenomenon is known an negative catalysis.

SHORT ANSWER QUESTIONS (3 Marks Each)

Q.1. What are lyophobic colloids? Give one example.

Ans, Lyophille colloids are those substances which have a great affinity for disperstion

lyo-means liquid and philic-means loving, i.e, they are liquid-loving substances. When these substances are mixed wilh a suitable liquid as the dispersion medium, they are directly converted into a colloidal system which is named as lyophilic sol. If water is used as the dispersion medium the sol obtained is known as hydrophilic sol. Substances like gum, starch, gelatin, rubber, proteins, etc from such sols since their sols are obtained by simply treating them with water or some other suitable liquid. They are also called as intrinsic colloids. Lyophilic sols are reversible sols Since the dispersed phase,if separated by evaporating the dispersion medium, changes back into the colloidal sol simply when the dispersion medium is remixed with it. These sols are highly stable and cannot be precipitated easily. The stability of the lyophilic colloids can be attributed to large amount of its hydration in the solution.

Q.2. Write short note on Tyndall effect.

Ans, Ifa strong beam of light passes through a true solution taken in a beaker in darkroom, the path of the light remains invisible. But if the same beam of light is allowed to pass through a sol, placed in the same room, the path of the beam gets illuminated by a bluish light when observed from a direction at right angles to that of incident beam Thus, the phenomenon of scattering of light by colloidal particles as result of which the path of the beam becomes visible is called Tyndall effect.

Q.3. What do you understand by homogeneous catalysis and heterogeneous catalysis.

Ans. A catalytic reaction is said to be homogeneous if the catalyst is in the same phase as the reactants and the products

For example,

(a) Oxidation of sulphur dioxide by oxygen in presence of nitric oxide (Lead chamber process of manufacture of sulphuric acid).

NO (8)

2S0 (g)+O2 (g) -----> 2SO3 (g)

(b) Hydrolysis of an organic ester is a thoroughly studied example of homogeneous catalysis.

CH3COOC2H5(l) + H2O (l)---->CH3COOH(l) +C2H5OH(l)

A catalytic reaction is said to be heterogeneous if the catalyst is in a diferent phase than that of the

reactants.

In heterogeneous catalysis, a catalyst is in solid phase while the reactants are generally gases although sometimes liquid reactants are also involved. It is also known as surface catalysis as the reaction is considered to be taking place at the surface of the solid catalyst. The catalyst used in these reactions have large surface (or specific) area, i.e., between l to 500 m2/g for contact. Most of the reactions taking place on a metal surface such as decomposition of N2O on platinum and decomposition of HI on gold are zero order reactions because the rate determining steps takes place on the surface itself.

One of the most important example of heterogeneous catalysis is the catalytic hydrogenation, i.e.. addition of hydrogen on the C=Cbonds of organic compounds to form C-Cbonds. The petroleum plastics and food industries frequently use catalytic hydrogenation.