Surface Chemistry

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Ans: 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.

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Ans: The gaseous molecules diffuse on to the surface of the solid catalyst and get adsorbed. After the required chemical changes the products diffuse away from the surface of the catalyst leaving the surface free for more reactant molecules to get adsorbed and undergo reaction.

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Ans: 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.

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Ans: The process of physisorption for example that of H2 on finely divided nickel, involves weak van there Waals' forces. With increase in temperature, hydrogen molecules dissociate into hydrogen atoms which are held on the surface by chemisorption.

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Ans: 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.

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

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

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

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

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Ans: The adsorption of positively charged Fe³⁺ ions by the sol of hydrated ferric oxide results in positively charged colloid.