Question

Describe all what you will observe and write chemical equations, when limited amount of ammonia gas is passed through the following aqueous solution:
(i) ZnCl₂
(ii) FeSO₄
(iii) FeCl₃
(iv) Pb(NO₃)₂
(v) CuSO₄
(vi) CrCl₃.

Answer 1

(i) When a small amount of ammonia gas is passed through zinc chloride, a white precipitate of zinc hydroxide is formed. The reaction is:
${\mathrm{ZnCl}}_2+2{\mathrm{NH}}_4\mathrm{OH}\to 2{\mathrm{NH}}_4{\mathrm{NO}}_3+\mathrm{Zn}{\left(\mathrm{OH}\right)}_2$

(ii) The reaction of ferrous sulphate with ammonia gas leads to the formation of iron hydroxide precipitate, which is dirty green in colour. The reaction is:
${\mathrm{FeSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4+\mathrm{Fe}{\left(\mathrm{OH}\right)}_2$

(iii) Iron chloride consists of iron in the trivalent state. Therefore, reaction with ammonia yields a reddish brown precipitate of Fe(OH)₃. The reaction is:
${\mathrm{FeCl}}_3+3{\mathrm{NH}}_4\mathrm{OH}\to 3{\mathrm{NH}}_4\mathrm{Cl}+\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$

(iv) Reaction of lead nitrate with small amounts of ammonia leads to the formation of white lead hydroxide precipitate. The reaction is:
$\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to 2{\mathrm{NH}}_4{\mathrm{NO}}_3+\mathrm{Pb}{\left(\mathrm{OH}\right)}_2$

(v) Copper sulphate reacts with a small amount of ammonium hydroxide to form copper hydroxide, which is bluish white in colour. The reaction is:
${\mathrm{CuSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4+\mathrm{Cu}{\left(\mathrm{OH}\right)}_2$

(vi) Chromium chloride reacts with the ammonium hydroxide obtained by passing ammonia through the aqueous solution to yield green-coloured chromium hydroxide. The reaction is:${\mathrm{CrCl}}_3+3{\mathrm{NH}}_4\mathrm{OH}\to 3{\mathrm{NH}}_4\mathrm{Cl}+\mathrm{Cr}{\left(\mathrm{OH}\right)}_3$