Common ion effect : If to the solution of a weak acid or weak base, an electrolyte having a common ion is added, the ionization of the weak acid or the weak base is further suppressed e.g. if CH3COONa is added to CH3COOH, ionization of CH3COOH decreases. Similarly if NH4Cl is added to NH4OH, ionization of NH4OH decreases.
Solubility product : Solubility product of an electrolyte at a particular temperature is defined as the product of the molar concentrations of its ions in a saturated solution, each concentration raised to the power equal to the number of ions produced on dissociation of one molecule of the substance i.e. for the electrolyte
Ax By « xA+ + yB- , Ksp = [A]x [B]y
(1) Ionic product of the electrolyte AxBy is also equal to[A]x [B]y but it is applicable to all types of solutions, may be saturated or unsaturated.
(2) For AgCl, if solubility is s mole L-1, then as
Ag Cl ® Ag+ + Cl- ,
Ksp = [Ag][Cl] =s x s = s2
For PbCl2, if solubility is “s” mol L-1, then as
PbCl2 ® Pb2 + 2Cl- , Ksp = [Pb][Cl]2 = s x (2s)2 = 4s3
Applications of solubility product and common ion effect
(i) Purification of common salt – On mixing two solutions, if ionic product exceeds the solubility product, precipitate is formed and if ionic product is less than the solubility product, precipitate is not formed.
(ii) Precipitation of the sulphides of group (II) and (IV) – In II group of qualitative analysis, H2S is passed in the acidic solution (in HCl). This decreases the S2 ion concentration by common ion effect so that only radicals of group II are precipitated. In group IV, H2S is passed in the ammonical medium (in NH4OH). This increases the S2- ion concentration so that ionic product becomes large enough to exceed the solubility product of sulphides of group IV radicals.
(iii) Salting out of soap
(iv) Precipitation of the hydroxides of group (III)
(v) Using excess of precipitating agent in gravimetric estimations
(vi) Predicting precipitation in ionic reactions.
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