Chemistry

Common Ion Effect Applications, problems and examples

Common Ion Effect can be described as“The lowering of the degree of discussion of weak electrolytes by adding a strong electrolyte having a common ion ”.

Or

“The decrease in the solubility of the salt in a solution that already contains an ion common to that salt is called common ion effect”.

For Example

Consider equilibrium state of sparingly soluble electrolyte AgCl in a saturated solution.

AgCl  →   Ag+  + Cl

Now add NaCl to this solution, which is complete ionized.

NaCl  →   Na+  + Cl

Then there will be common ion i.e. chlorine ion (Cl).

AgCl →    Ag+  + Cl

NaCl  →   Na+  + Cl

Consequently, equilibrium will be shifted to the left in accordance with Le-Chatlier’s principle AgCl will be precipitated. In other words, the ionization of AgCl is suppressed due to common ion, chlorine (Cl) and it forms the precipitate.

Application of Common Ion Effect

Common ion effect finds a useful application in a qualitative salt analysis.

Precipitation of Sulphides of Group II

In order to precipitate the Sulphides of the group II, H2S is passed through the original solution (O.S) in presence of HCl.

H2S  →   2H+  + S2-

HCl furnishes H+ as common ions, which shift the above equilibrium to left according to Le-Chatlier’s principle. In other words, an addition of HCl suppresses the ionization of H2S thereby lowering the concentration of sulphide ions (S2-), which is however just enough to exceed the solubility product of the group II Sulphides. In this way group, II cations are precipitated such as CuS, PbS, CdS etc.

Precipitation of Sulphides in group III

Group III cations are precipitated as hydroxides’ by the NH4OH in the presence of NH4Cl. The common ion, in this case, is NH4+, which suppresses the ionization of NH4OH.

NH4OH      →    NH4+   + OH

Thus with the addition of the common ion (NH4+) The equilibrium is shifted towards left and the concentration of OH decreases. Under these conditions, the solubility products of hydroxides of Al, Fe and Cr is only exceeded due to which they are precipitated.

The hydroxides of the other cations such as Zn, Ni, Co etc. are not precipitated due to their higher solubility product.

Precipitation of Sulphides of Group IV

Group IV Sulphides having higher solubility product are precipitated by H2S in the presence of NH4OH. The OH of NH4OH combines with the H+ of H2S to form H2O.

H+   + OH     →   H2O

The removal of H+ from the product side shift the equilibrium to right.

H2S       →      2H+  + S2-

In this way, the concentration of the sulphide ion (S2-) increases which the enough to exceed the solubility product for the precipitation of Sulphides, e.g. CoS, NiS, ZnS.

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