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Surface Chemistry

Short Answer Type

201. How does chemical adsorption of a gas on a solid vary with temperature?

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202. How does the addition of alum purify water?

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203. State one difference between an emulsion and a sol.

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204. Give an example of heterogeneously catalysed reaction.

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205.

(a) How can a colloidal solution and true solution of the same colour be distinguished from each other ?
(b) List four applications of adsorption.

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206.

Explain the following observations:
(a) Lyophilic colloids are more stable than lyophobic colloid.
(b) Coagulation takes place when sodium chloride solution is added to a colloidal solution of ferric hydroxide.
(c) Sky appears blue in colour.

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207.

(a) In which of the following does adsorption take place and why?
(i) Silica gel placed in the atmosphere saturated with water.
(ii) Anhydrous CaCl₂ placed in the atmosphere saturated with water.
(b) How does BF₃ act as a catalyst in industrial process?
(c) Give an example of shape selective catalysis.

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208. What are micelles? How do they differ from ordinary colloidal particles ? Give two examples of micelles forming substances.

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209. State Hardy-Schulze rule.

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Long Answer Type

210.
(a) Of NH₃ and CO₂ which will be adsorbed more readily on the surface of charcoal and why?
(b) How are multimolecular colloids different from macro-molecular colloids?
(c) State the principle of electro-dialysis.

Answer:

a) NH₃ has higher critical temperature than CO₂ i.e. NH₃ has greater intermolecular forces of attraction hance will be adsorbed more readily.

b) In this type of colloids, colloidal particles are aggregates of atom or molecules each having size less than 1 nm e.g. sulphur sol. gold sol.
multimolecular colloids are generally lyophobic in nature.
In this type of colloids, colloidal particles are themselves large molecules of colloidal dimension e.g. strach protein.

c)The movement of colloidal particles towards oppositely charged electrode in an electric field is called electrophoresis.

When electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards one or the other electrode.

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