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Chemical Equilibrium, Law Of Mass-action : Hydrolysis |
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Salts formed from weak acids, or weak bases, or both, are decomposed in aqueous solution with production of free acid and base. This reaction is called hydrolysis. The salts themselves are usually largely ionised in solution; the weak acid or base is only slightly ionised and its ionisation is also further repressed by the ion of the salt which is common to the acid or base. The hydrolysis is most simply represented as due to the withdrawal of H۰ or OH' ions from the water by the anion or cation of the salt, so forming the weak acid or base, the other ion of the water remaining free and exhibiting an alkaline or acid reaction. Although only traces of H۰ and OH' ions are present in water, further ionisation occurs when one ion is withdrawn, since the product of the concentrations is constant: [H۰] x [OH'] = Kw. Thus, when the H۰ ion is withdrawn to form a weak acid, an appreciable concentration of OH' ions results, and the solution shows an alkaline reaction, e.g., with potassium cyanide: K۰ + CN' + (H۰ + OH') <=> K۰ + OH' + HCN. The ionisation of the very weak hydrocyanic acid, HCN <=> H۰ + CN' is almost entirely suppressed by the action of the large excess of cyanide ion in the solution. The solution smells of hydrocyanic acid. A salt of a strong acid and weak base hydrolyses to the weak base by withdrawal of OH' ions, and shows an acid reaction in solution; in a solution of ferric chloride, ferric hydroxide is produced and exists in a state of colloidal solution showing a dark red colour, and the solution has an acid reaction: Fe۰۰۰ + 3Cl' + 3(H۰ + OH') <=> Fe(OH)3 + 3H۰ + 3Cl'. A solution of a salt of a weak acid and weak base is hydrolysed, e.g., ammonium carbonate: (NH4)2CO3 + 2H2O <=> 2NH4OH(<=>NH4۰ + OH') + H2CO3(<=> H۰ + HCO3') Since in this case the base is stronger than the acid, the solution reacts alkaline. In the case of ammonium acetate, where the acid and base are equally weak, the reaction is practically neutral. An application of the law of mass-action to hydrolysis: [acid] x [base] /[salt] x [water] = const. = Kh shows that the extent of hydrolysis increases with dilution, except in the case of a salt of a very weak base and very weak acid, when it is practically independent of dilution. The extent of hydrolysis increases with rise in temperature. Expt. 6. - Add phenolphthalein to a cold saturated solution of borax and acetic acid till the pink colour just disappears. Dilute the solution: the pink colour reappears: Na2B4O7 + 3H2O <=> 2NaBO2 + 2H3BO3; NaBO2 + 2H2O <=> NaOH + H3BO3. The boric acid also produced is so weak an acid that it has no action on the indicator; the caustic soda, a strong base, turns it pink. Expt. 7. - Pour 2 to 3 c.c. of 30 per cent, ferric chloride solution into 500 c.c. of boiling distilled water. A deep red solution of colloidal ferric hydroxide is formed: FeCl3 + 3H2O <=> Fe(OH)3 + 3HCl. Expt. 8. - Add phenolphthalein to a concentrated solution of sodium sulphite and heat. The pink liquid becomes colourless on cooling, but the colour reappears on heating (Raschig).
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