Question

Passing ${\mathrm{H}}_2\mathrm{S}$ gas into a mixture of ${\mathrm{Mn}}^{2+},{\mathrm{Ni}}^{2+},{\mathrm{Cu}}^{2+}\mathrm{and}{\mathrm{Hg}}^{2+}$ ions in an acidified aqueous solution precipitates?

Answer 1

Precipitation Reaction:

• The precipitation reaction is known as the chemical reaction which occurs in an aqueous solution by the combination of two ionic bonds, which results in the formation of an insoluble salt called as precipitates.
• Precipitation reactions are generally known as the type of double displacement reactions which involve the production of the precipitate.

The precipitates formed when ${\mathrm{H}}_2\mathrm{S}$ gas passed into the mixture:

• When ${\mathrm{H}}_2\mathrm{S}$ gas is passed through the given mixture in the presence of the acidic aqueous medium it basically tends to precipitate as Sulfide $\mathrm{Cu}\mathrm{and}\mathrm{Hg}$in the form of insoluble salts such as $\mathrm{CuS}\mathrm{and}\mathrm{HgS}.$
• For this the reaction can be given as :

$$\begin{gathered} {\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{Cu}}^{+2}\left(\mathrm{aq}\right)\rightleftharpoons \mathrm{CuS}\left(\mathrm{s}\right) \\ {\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{Hg}}^{+2}\left(\mathrm{aq}\right)\rightleftharpoons \mathrm{HgS}\left(\mathrm{s}\right) \end{gathered}$$

• Here ${\mathrm{H}}_2\mathrm{S}$ gets slightly ionized by giving small concentrations of Sulfide ions which react with the given ions to form the precipitates.
• But as the solubility product of ${\mathrm{Mn}}^{2+}\mathrm{and}{\mathrm{Ni}}^{2+}$ is higher than the other two ions, hence they tend to remain unaffected during precipitation.

Hence, Passing ${\mathrm{H}}_2\mathrm{S}$ gas into the given mixture in an acidified aqueous solution precipitates insoluble salts such as $\mathrm{CuS}\mathrm{and}\mathrm{HgS}.$