Question

Write the observations, and balanced equations for the following reactions:
(i) Sodium hydroxide is added dropwise to a solution of zinc sulphate, till it is in excess.
(ii) Ammonium hydroxide is added first in a small quantity, and then in excess, to a solution of copper sulphate.
(iii) Excess of ammonium hydroxide is added to a substance, obtained by adding hydrochloric acid to silver nitrate solution.
(iv) Moist starch iodide paper is placed at the mouth of a test-tube containing chlorine gas.
(v) A paper dipped in potassium permanganate solution is placed at the mouth of a test-tube, containing sulphur dioxide gas.

Answer 1

(i) When sodium hydroxide is added, in drops, to a solution of zinc sulphate, white gelatinous precipitate of zinc hydroxide is formed, which forms a soluble complex sodium zincate in excess of sodium hydroxide.

$$\begin{gathered} {\mathrm{ZnSO}}_4+2\mathrm{NaOH}\stackrel{}{\to }\mathrm{Zn}(\mathrm{OH}{)}_2+{\mathrm{Na}}_2{\mathrm{SO}}_4 \\ \mathrm{Zinc}\mathrm{sulphate}\mathrm{Sodium}\mathrm{Zinc}\mathrm{hydroxide}\mathrm{Sodium}\mathrm{sulphate} \\ \mathrm{hydroxide}(\mathrm{White}\mathrm{ppt}.)(\mathrm{colourless}\left) \\ \mathrm{Zn}\right(\mathrm{OH}{)}_2+2\mathrm{NaOH}\left(\mathrm{excess}\right)\stackrel{}{\to }{\mathrm{Na}}_2{\mathrm{ZnO}}_2+2{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Zinc}\mathrm{hydroxide}\mathrm{Sodium}\mathrm{zincate} \\ \left(\mathrm{colourless}\right) \end{gathered}$$

(ii) When ammonium hydroxide is added in a small quantity to copper sulphate solution, pale blue precipitate of cupric hydroxide is formed. By adding ammonium hydroxide in excess, the formed precipitate dissolves to form deep blue solution of tetraamine copper (II) sulphate.

$$\begin{gathered} {\mathrm{CuSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\stackrel{}{\to }\mathrm{Cu}(\mathrm{OH}{)}_2+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4 \\ \mathrm{Copper}\mathrm{sulphate}\mathrm{Ammonium}\mathrm{Cupric}\mathrm{hydroxide} \\ \left(\mathrm{Blue}\right)\mathrm{hydroxide}(\mathrm{Pale}\mathrm{blue}\mathrm{ppt}) \end{gathered}$$

$$\begin{gathered} \mathrm{Cu}(\mathrm{OH}{)}_2+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\stackrel{}{\to }\left[\mathrm{Cu}\right({\mathrm{NH}}_3{)}_4]{\mathrm{SO}}_4+4{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Cupric}\mathrm{hydroxide}(\mathrm{In}\mathrm{excess})\mathrm{Tetrammine}\mathrm{copper}(\mathrm{II}) \\ (\mathrm{Pale}\mathrm{blue}\mathrm{ppt})\mathrm{sulphate}(\mathrm{deep}\mathrm{blue}) \end{gathered}$$

(iii) Silver nitrate on reaction with hydrochloric acid forms a white precipitate of silver chloride. When excess of ammonium hydroxide is added to silver chloride, it forms soluble diammine silver complex.

$$\begin{gathered} {\mathrm{AgNO}}_3+\mathrm{HCl}\stackrel{}{\to }\mathrm{AgCl}+{\mathrm{HNO}}_3 \\ \mathrm{Silver}\mathrm{nitrate}\mathrm{Hydrochloric}\mathrm{Silver}\mathrm{chloride} \\ \mathrm{acid}(\mathrm{White}\mathrm{solid}) \\ \mathrm{AgCl}+2{\mathrm{NH}}_4\mathrm{OH}\stackrel{}{\to }\left[\mathrm{Ag}\right({\mathrm{NH}}_3{)}_2]\mathrm{Cl}+2{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Silver}\mathrm{chloride}\mathrm{excess}\mathrm{Diammine}\mathrm{silver}\mathrm{complex} \\ (\mathrm{white}\mathrm{solid})(\mathrm{soluble}) \end{gathered}$$

(iv) The iodide paper turns blue because chlorine gas oxidises some of the iodide ions on the starch paper to iodine molecules. Further, these iodine molecules react with starch to form a complex, which gives blue colour.

$$\begin{gathered} {\mathrm{Cl}}_2+2\mathrm{KI}\stackrel{}{\to }{\mathrm{I}}_2+2\mathrm{KCl} \\ \mathrm{Chlorine}\mathrm{Potassium}\mathrm{Iodide}\mathrm{Iodine}\mathrm{Potassium}\mathrm{chloride} \\ \mathrm{gas}(\mathrm{present}\mathrm{on}\mathrm{starch}\mathrm{paper}) \end{gathered}$$

(v) The purple colour of the paper dipped in KMnO₄ gets de-colourised when sulphur dioxide gas comes in contact with it, because SO₂ is a reducing agent and reduces potassium permanganate.

$$\begin{gathered} 2{\mathrm{KMnO}}_4+5{\mathrm{SO}}_2+2{\mathrm{H}}_2\mathrm{O}\stackrel{}{\to }2{\mathrm{MnSO}}_4+{\mathrm{K}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2{\mathrm{SO}}_4 \\ \mathrm{Potassium}\mathrm{Sulphur}\mathrm{Manganese} \\ \mathrm{permangamate}\mathrm{dioxide}\mathrm{sulphate} \\ (\mathrm{purple}\mathrm{colour})\left(\mathrm{colourless}\right) \end{gathered}$$