Applications of solubility product-Part 1

Applications of solubility product

source : en.wikipedia.org

Applications of solubility product-

i) Precipitation of  II^nd and IV ^th group  metal sulphides –

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

H₂S is a weak electrolyte , it is used in the precipitation of  II^nd 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 K_sp of II^nd 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 II^nd group metal sulphides get precipitated in presence of HCl.

Hence,

Ionic product of [ M⁺⁺] [S ^{– –}]   > K_sp of  II^nd 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 . K_sp of IV ^th group metal sulphides is higher . Hence,

Ionic product of [ M⁺⁺] [S ^{– –}]   > K_sp of  IV^th 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 .

            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⁺), ionization of weak electrolyte i . e- ammonium carbonate is suppressed and only fifth group metal carbonates having low solubility product  will be precipitated out.