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Chemical Equilibrium, Law Of Mass-action : Solubility Product |
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If solid sodium chloride is in contact with its saturated solution we have two connected equilibria: NaCl (solid) <=> NaCl (dissd.) <=> Na۰ + Cl'. If the law of mass-action applied to the ions we should have: [Na۰] x [Cl'] = K[NaCl]. But [NaCl], the concentration of unionised salt, is zero in the modern theory of strong electrolytes. In Arrhenius's theory it was assumed that [NaCl] is constant at a given temperature if excess of solid is present. Hence, in equilibrium the product of the ionic concentrations is constant at a given temperature. This constant product, e.g., [Na۰] x [Cl'], is called the solubility product. When the product of the ionic concentrations is equal to the solubility product, the solution is in equilibrium with the solid, since then the concentration of unionised salt in solution must be that which is in equilibrium with solid. If the ionic product is less than the solubility product, the solution is unsaturated with respect to the solid and more of the latter dissolves. But if the ionic product is greater than the solubility product, the solution is supersaturated and precipitation of solid occurs, unless the solution remains supersaturated. The value of the ionic product, [Na۰] x [Cl'], may be increased by adding to the solution an electrolyte which has an ion in common with the substance in solution. Thus if hydrochloric acid is added to a saturated solution of common salt, the concentration of chloride ions is increased, and the ionic product, [Na۰] x [Cl' from NaCl + added Cl' from HCl], is increased above the value corresponding with the solubility product. Solid sodium chloride is precipitated until the ionic product becomes equal to the solubility product. The other ion of the added electrolyte, H', has no effect, as may be proved by adding a quantity of another chloride, e.g., LiCl, containing the same quantity of chloride ions as the acid, when the same weight of NaCl is precipitated as in the first experiment. If an equivalent amount of Na۰ ions, e.g., as NaClO3, had been added instead of Cl' ions, the effect is the same, as it should be, since the product [Na۰] x [Cl'] is affected to the same extent by equivalent amounts of Na۰ and Cl'. Although Arrhenius's theory of the solubility product is not in agreement with the modern theory of strong electrolytes, and does not agree with experiment in the case of salts so readily soluble as sodium chloride, it is in qualitative agreement with experiment in nearly all cases and hence it has been given here. Expt. 3. - Pass gaseous hydrogen chloride into a filtered saturated solution of common salt, which contains magnesium chloride as impurity using the apparatus shown in
A white crystalline powder of NaCl fails. This is filtered off in a Büchner funnel, dried on a porous plate, and heated carefully in a dish to drive off hydrochloric acid; it is then pure. Expt. 4. To a saturated solution of silver acetate add: (a) a concentrated solution of silver nitrate: (b) a saturated solution of sodium acetate. In each case silver acetate is precipitated. The effect of adding excess of a reagent in analytical chemistry is clear from the solubility product theory. If exactly equivalent amounts of silver nitrate and a soluble chloride are mixed in aqueous solution, sparingly soluble silver chloride is precipitated, but a small quantity (0.002 gm./lit.) remains in solution. For the dissolved part, the product [Ag۰] x [Cl'] is constant, hence if excess of either silver nitrate or the soluble chloride is added, more silver chloride is precipitated and the reaction is more complete.The solubility product rule may be disturbed when complex ions are formed. For example, if potassium cyanide solution is added to a solution of silver nitrate, a white precipitate of silver cyanide, AgCN, is first produced, since the solubility product [Ag۰] x [CN'] = 2 x 10-12 corresponding with the solubility of AgCN is exceeded. On further addition of potassium cyanide, however, the silver cyanide dissolves, since a complex ion is formed: AgCN + CN' = Ag(CN)2'. The silver is contained in the anion and practically no silver ions are present. The complex ion is, however, slightly dissociated: Ag(CN)2' <=> Ag۰ + 2CN', so that silver may be plated from the solution by electrolysis. No precipitate is formed with soluble chlorides, since the Ag۰ concentration is too small to reach the solubility product [Ag۰] x [Cl'], but silver sulphide is precipitated on adding sodium sulphide, since the solubility product [Ag۰]2 x [S''] is very small, about 10-50. The solubility products of some common precipitates at room temperature are:
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