Applications of solubility product

Applications of Solubility product

source : en.wikipedia.org

Applications of solubility product-

i) Precipitation of  IInd and IV th group  metal sulphides –

The most important application of solubility product is the precipitation of  IInd and IV th group  metal sulphides

H2S is a weak electrolyte , it is used in the precipitation of  IInd and IV th group  metal sulphides  in qualitative analysis.

H2S    ⇌  2H+ + S – –

According to Law of mass action ,

K = [H+]2 [S – –] / [H2S]

In  II nd group , lower concentration of sulphide ions is required for the precipitation of metal sulphides  because Ksp of IInd group metal sulphides is low. In presence of HCl , concentration of  sulphide ion decreases due to common ion effect.

H2S    ⇌  2H+ + S – –

HCl ——> H +   + Cl

In presence of common ion i.e- hydrogen ion ( H+) , dissociation of weak electrolyte decreases thus concentration of  sulphide ions (S)decreases. Hence IInd group metal sulphides get precipitated in presence of HCl.

Hence,

Ionic product of [ M++] [S – –]   > Ksp of  IInd group metal sulphides

In  IV th  group , higher concentration of sulphide ions is  required for the precipitation of metal sulphides  . This is done by changing the medium from acidic to basic. In IV th group H2S gas is passed in presence of NH4OH.

NH4OH   ⇌  NH4+  +  OH

H2S    ⇌        2H+ + S – –

H+ + OH    ⇌  H2O

[H+]  of  H2S  and [OH ] of  NH4OH form H2O molecules then ionisation of H2S increases thus concentration of S (sulphide ions) increases . Ksp of IV th group metal sulphides is higher . Hence,

Ionic product of [ M++] [S – –]   > Ksp of  IVth group metal sulphides

Thus fourth group metal sulphides get precipitated.

ii) Predicting precipitation reactions –

An important conclusion is derived from the solubility product . No precipitation of the electrolyte occurs if the ionic product is less than the solubility product . No precipitation of the electrolyte occurs if the ionic product is less than the solubility product .

            ionic product   <   solubility product 

Then no precipitation occurs.

If,

           ionic product    >      solubility product 

Then  precipitation occurs.

iii) Precipitation of fifth group radicals-

The unique applications of solubility product is the precipitation of fifth group radicals.

The group reagent is ammonium carbonate [(NH4)2 CO3 ] in presence of  ammonium chloride [NH4Cl].

(NH4)2 CO3    ⇌   2NH4+   + CO3– –

weak electrolyte

NH4Cl ———–> NH4+    +   Cl

Strong electrolyte

Due to common ion effect ( common ion is NH4+), ionisation of weak electrolyte i . e- ammonium carbonate is suppressed and only fifth group metal carbonates having low solubility product  will be precipitated out.