Chemistry 12 Chapter 5 Exemplar Solutions Surface Chemistry Part 5

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III. Short Answer Type

Question 38:

Why is it important to have clean surface in surface studies?

Answer:

It is important to have clean surface as it facilitates the adsorption of desired species.

Question 39:

Why is chemisorption referred to as activated adsorption?

Answer:

Chemisorption involves formation of bond between gaseous molecules/ atoms and the solid surface for which high activation energy is required. Thus it is referred to as activated adsorption.

Question 40:

What types of solutions are formed on dissolving different concentrations of soap in water?

Answer:

At lower concentration soap forms a normal electrolytic solution with water. After a certain concentration called critical micelle concentration, colloidal solution is formed.

Question 41:

What happens when gelatin is mixed with gold sol?

Answer:

Gold sol is a lyophobic sol. Addition of gelatin stabilises the sol.

Question 42:

How does it become possible to cause artificial rain by spraying silver iodide on the clouds?

Answer:

Clouds are colloidal in nature and carry charge. Spray of silver iodide, an electrolyte, results in coagulation leading to rain.

Question 43:

Gelatin which is a peptide is added in icecreams. What can be its role?

Answer:

Icecreams are emulsions which get stabilised by emulsifying agents like gelatin.

Question 44:

What is collodion?

Answer:

It is a solution of nitrocellulose in a mixture of alcohol and ether.

Question 45:

Why do we add alum to purify water?

Answer:

The colloidal impurities present in water get coagulated by added alum, thus making water potable.

Question 46:

What happens when electric field is applied to colloidal solution?

Answer:

The charged colloidal particles start moving towards oppositely charged electrodes.

Question 47:

What causes Brownian motion in colloidal dispersion?

Answer:

Unbalanced bombardment of the particles of dispersed phase by molecules of dispersion medium causes Brownian motion. This stabilises the sol.

Question 48:

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

Answer:

Positively charged sol of hydrated ferric oxide is formed and on adding excess of , negatively charged chloride ions coagulate the positively charged sol of hydrated ferric oxide.

Question 49:

How do emulsifying agents stabilise the emulsion?

Answer:

The emulsifying agent forms an interfacial layer between suspended particles and the dispersion medium thereby stabilizing the emulsion.

Question 50:

Why are some medicines more effective in the colloidal form?

Answer:

Medicines are more effective in the colloidal form because of large surface area and are easily assimilated in this form.

Question 51:

Why does leather get hardened after tanning?

Answer:

Animal hide is colloidal in nature and has positively charged particles. When it is soaked in tannin which has negatively charged colloidal particles, it results in mutual coagulation taking place.

Question 52:

How does the precipitation of colloidal smoke take place in Cottrell precipitator?

Answer:

In Cottrell precipitator, charged smoke particles are passed through a chamber containing plates with charge opposite to the smoke particles. Smoke particles lose their charge on the plates and get precipitated.

Question 53:

How will you distinguish between dispersed phase and dispersion medium in an emulsion?

Answer:

On adding dispersion medium, emulsions can be diluted to any extent. The dispersed phase forms a separate layer if added in excess.

Question 54:

On the basis of Hardy-Schulze rule explain why the coagulating power of phosphate is higher than chloride.

Answer:

Minimum quantity of an electrolyte required to cause precipitation of a sol is called its coagulating value. Greater the charge on flocculating ion and smaller is the amount of electrolyte required for precipitation, higher is the coagulating power of coagulating ion (Hardy-Schulze rule).

Question 55:

Why does bleeding stop by rubbing moist alum?

Answer:

Moist alum coagulates the blood and so formed blood clot stops bleeding.

Question 56:

Why is colloid positively charged, when prepared by adding to hot water?

Answer:

The adsorption of positively charged ions by the sol of hydrated ferric oxide results in positively charged colloid.

Question 57:

Why do physisorption and chemisorption behave differently with rise in temperature?

Answer:

Physisorption involves weak van der Waals forces which weaken with rise in temperature. The chemisorption involves formation of chemical bond involving activation energy and like any other chemical reaction is favoured by rise in temperature.

Question 58:

What happens when dialysis is prolonged?

Answer:

Due to excessive dialysis, traces of electrolyte which stabilises the colloids is removed completely, making the colloid unstable. As a result coagulation takes place.

Question 59:

Why does the white precipitate of silver halide become coloured in the presence of dye eosin.

Answer:

Eosin is adsorbed on the surface of silver halide precipitate making it coloured.

Question 60:

What is the role of activated charcoal in gas mask used in coal mines?

Answer:

Activated charcoal acts as an adsorbent for various poisonous gases present in the coal mines.

Question 61:

How does a delta form at the meeting place of sea and river water?

Answer:

River water is a colloidal solution of clay and sea water contains lot of electrolytes. The point at which river and sea meet is the site for coagulation. Deposition of coagulated clay results in delta formation.

Question 62:

Give an example where physisorption changes to chemisorption with rise in temperature. Explain the reason for change.

Answer:

The process of physisorption for example that of on finely divided nickel, involves weak van der Waals’ forces. With increase in temperature, hydrogen molecules dissociate into hydrogen atoms which are held on the surface by chemisorption.

Question 63:

Why is desorption important for a substance to act as good catalyst?

Answer:

After the reaction is over between adsorbed reactants, the process of desorption is important to remove products and further create space for the other reactant molecules to approach the surface and react.

Question 64:

What is the role of diffusion in heterogenous catalysis?

Answer:

After the reaction is over between adsorbed reactants, the process of desorption is important to remove products and further create space for the other reactant molecules to approach the surface and react.

Question 65:

How does a solid catalyst enhance the rate of combination of gaseous molecules?

Answer:

When gaseous molecules come in contact with the surface of a solid catalyst, a weak chemical combination takes place between the surface of the catalyst and the gaseous molecules, which increases the concentration of reactants on the surface. Different molecules adsorbed side by side have better chance to react and form new molecules. This enhances the rate of reaction. Also, adsorption is an exothermic process. The heat released in the process of adsorption is utilised in enhancing the reaction rate.

Question 66:

Do the vital functions of the body such as digestion get affected during fever? Explain your answer.

Answer:

Hint: The optimum temperature range for enzymatic activity is 298- 310 K. On either side of this temperature range, enzymatic activity gets affected. Thus, during fever, when temperature rises above 310 K, the activity of enzymes may be affected.

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