Question

When $\mathrm{Zn}$ granules are added to concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$, which of the following reactions does not takes place?

• A. $\mathrm{Zn}$ gets oxidized
• B. $\mathrm{S}$ gets reduced
• C. ${\mathrm{H}}_2$ is evolved
• D. ${\mathrm{H}}_2\mathrm{O}$ is formed

Answer 1

The correct option is C

${\mathrm{H}}_2$ is evolved

Explanation for the correct option:

1. When $\mathrm{Zn}$ granules reacts with Conc.${\mathrm{H}}_2{\mathrm{SO}}_4$, Zinc sulphate$\left({\mathrm{ZnSO}}_4\right)$, Sulphur dioxide$\left({\mathrm{SO}}_2\right)$ and Water$\left({\mathrm{H}}_2\mathrm{O}\right)$ is formed.
2. Reaction: $$\begin{gathered} \mathrm{Zn}\left(\mathrm{s}\right)+2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)\uparrow \\ \left(\mathrm{Zinc}\right)(\mathrm{Sulphuric}\mathrm{acid})(\mathrm{Zinc}\mathrm{sulphate})\left(\mathrm{Water}\right)(\mathrm{Sulphur}\mathrm{dioxide}) \end{gathered}$$
3. In the above reaction Hydrogen gas $\left({\mathrm{H}}_2\right)$ is not evolving.

Explanation for the incorrect option:

1. From the above reaction we could see, Zinc $\left(\mathrm{Zn}\right)$ is getting oxidized, as its oxidation state is getting increased from $0\to \left(+2\right)$ i.e., ${\mathrm{Zn}}^0\to {\mathrm{Zn}}^{+2}$, $\mathrm{Zn}$ gets oxidized to ${\mathrm{ZnSO}}_4$, so option (A) is incorrect.
2. Again from the above reaction we could observe that Sulphur $\mathrm{S}$ is getting reduced as its oxidation state is getting decreased from $\left(+6\right)\mathrm{to}\left(+4\right),$ i.e., ${\mathrm{S}}^{+6}\to {\mathrm{S}}^{+4}$,$\mathrm{S}$(Sulphur) gets reduced to ${\mathrm{SO}}_2$( Sulphur dioxide) ,so option (B) is incorrect.
3. In the above reaction, ${\mathrm{H}}_2\mathrm{O}$ is formed, so option (D) is incorrect.

Hence, option (C) is correct. i.e., ${\mathrm{H}}_2$ gas is not evolved, When $\mathrm{Zn}$ granules are added to concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$.