question_answer

  (a) Heat of adsorption is greater for chemisorptions than physisorption. Why? (b) What is collodion ? (c) Differentiate between peptization and coagulation.                                                                                                 [3]

Answer:

  Physical adsorption is an exothermic process but its   enthalpy   of  adsorption   is   very   low (20-40 kJ ) and is smaller than that of chemisorption. In physical adsorption, attraction between gas molecules (adsorbate) and solid surface (adsorbent) is very weak due to van der Waals' forces. But in case of chemisorption, adsorption is due to new bond formation hence heat of adsorption is very high.                         [1] (b) Cellulose nitrate is peptised by organic solvents such as ethyl alcohol and the product is called colloidion. [1] (c)

Peptisation

Coagulation

Peptisation  involves  conversion of a freshly formed ppt (such as AgCI, , into sol by the addition of small amount of a suitable electrolyte. An electrolyte having an ion in common with the material to be dispersed is required for sol formation. The peptisation action is due     to     preferential adsorption of the common ion of the electrolyte and the precipitate to be dispersed Aal is oeotised bv Kl or

Coagulation involves Conversion of a colloidal sol into suspended (ppt) state. Coagulation can take place by addition of suitable electrolyte.   Greater   the charge on the ion (of electrolyte)   greater the coagulating   power   to coagulate  ppositely charged colloidal sol. is     coagulated     byandion and coagulating power is        in        order by Hardy-SchuIze rule.