Surface Chemistry

Surface Chemistry

Question

(i) Differentiate between adsorbtion and absorption.

(ii)Out of MgCl2 and AlCl3, which one is more effective in causing coagulation of negatively charged sol and why?

(iii)Out of sulphur sol and proteins, which one form multimolecular colloids?

Answer

(i) Adsorption is a surface phenomenon that causes the accumulation of molecules of a substance at the surface of a solid or liquid rather than in the bulk. In adsorption, the substance gets concentrated at the surface only. It does not penetrate through the surface to the bulk of the solid or liquid. For example, when a chalk stick is dipped into an ink solution, only its surface becomes coloured.

On the other hand, the process of absorption is a bulk phenomenon. In absorption, the absorbed substance gets uniformly distributed throughout the bulk of the solid or liquid. For example, when a sponge is dipped in water the whole sponge gets wets.

(ii)According to the Schulze-Hardy rule, the effectiveness of the salt causing flocculation depends on the charge on the ion of opposite sign to the charge on the sol particles. The greater the magnitude of the opposite charge, the higher the ability of a salt to coagulate the sol.
Thus, trivalent salt AlCl3 is more effective in causing the coagulation of a negatively charged sol than divalent salt MgCl2.

(iii)Out of sulphur sol and proteins, sulphur sol forms multimolecular colloids. Sulphur sol consists of particles containing a thousand or more S8 molecules. On the other hand, proteins form macromolecular colloids.

 

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