Question

Why is granulated zinc and not the pure zinc used for the preparation of hydrogen gas in laboratory?

Answer 1

Laboratory Preparation of Hydrogen:

Hydrogen is prepared in the laboratory by the action of the dilute hydrochloric acid or dilute sulphuric acid on granulated zinc.

Chemical reaction:

$$\begin{gathered} \underset{\mathrm{Zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{\mathrm{dil}.\mathrm{Hydrochloric}\mathrm{acid}}{2\mathrm{HCl}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Zinc}\mathrm{chloride}}{{\mathrm{ZnCl}}_2\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{{\mathrm{H}}_2\left(\mathrm{g}\right)} \\ \underset{\mathrm{Zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}+\underset{\mathrm{dil}.\mathrm{Sulphuric}\mathrm{acid}}{{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}\to \underset{\mathrm{Zinc}\mathrm{chloride}}{{\mathrm{ZnSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{{\mathrm{H}}_2\left(\mathrm{g}\right)} \end{gathered}$$

Reasons for using granulated zinc:

• Granulated zinc always contains copper as an impurity. The copper acts as a catalyst and speeds up the reaction.
• Furthermore, it provides a large surface area, which speeds up the reaction.
• Zinc is preferred to other metals because $\mathrm{K},\mathrm{Na}$ are expensive and reacts violently.$\mathrm{Ca},\mathrm{Mg}$ are expensive and the rate of evolution of hydrogen is so fast that it is difficult to collect.