Question

Which of the following gases turns lead acetate paper black?

• A. ${\mathrm{SO}}_2$
• B. ${\mathrm{SO}}_3$
• C. ${\mathrm{H}}_2\mathrm{S}$
• D. ${\mathrm{H}}_2{\mathrm{SO}}_4$

Answer 1

The correct option is C

${\mathrm{H}}_2\mathrm{S}$

Explanation for correct option:

C. ${\mathrm{H}}_2\mathrm{S}$

Reaction of Hydrogen disulfide with Lead acetate:

• When Lead acetate$\left(\mathrm{Pb}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\right)$ reacts with Hydrogen disulfide$\left({\mathrm{H}}_2\mathrm{S}\right)$, it forms Lead sulfide$\left(\mathrm{PbS}\right)$ and Acetic acid$\left({\mathrm{CH}}_3\mathrm{COOH}\right)$.
• The lead sulfide so formed is black in color.
• The chemical reaction can be depicted as:

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)\to \mathrm{PbS}\downarrow +2{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right) \\ (\mathrm{Lead}\mathrm{acetate})(\mathrm{Hydrogen}\mathrm{disulfide})(\mathrm{Lead}\mathrm{sulfide})(\mathrm{Acetic}\mathrm{acid}) \end{gathered}$$

Explanation for incorrect options:

A. ${\mathrm{SO}}_2$

• When Lead acetate reacts with Sulfur dioxide to form Lead (IV) hyposulfite and Acetate ion.
• The chemical reaction can be depicted as:

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\left(\mathrm{aq}\right)+2{\mathrm{SO}}_2\left(\mathrm{g}\right)\to \mathrm{Pb}({\mathrm{SO}}_2{)}_2\left(\mathrm{aq}\right)+{{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2}^{-}(\mathrm{aq}\left) \\ \right(\mathrm{Lead}\mathrm{acetate})(\mathrm{Sulfur}\mathrm{dioxide})(\mathrm{Lead}\left(\mathrm{IV}\right)\mathrm{hyposulfite})(\mathrm{Acetate}\mathrm{ion}) \end{gathered}$$

B. ${\mathrm{SO}}_3$

• When ${\mathrm{Pb}}^{2+}$ salt reacts with ${\mathrm{SO}}_3$, it forms solid Lead sulfate.

D. ${\mathrm{H}}_2{\mathrm{SO}}_4$

• Lead acetate on reacting with Sulfuric acid and gives solid Lead sulfate and Acetic acid.
• The chemical reaction can be depicted as:

$$\begin{gathered} \mathrm{Pb}{\left({\mathrm{OCOCH}}_3\right)}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to \mathrm{Pb}\left({\mathrm{SO}}_4\right)\left(\mathrm{s}\right)+2{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right) \\ (\mathrm{Lead}\mathrm{acetate})(\mathrm{Sulfuric}\mathrm{acid})(\mathrm{Lead}\mathrm{sulfate})\hspace{0.17em}(\mathrm{Acetic}\mathrm{acid}) \end{gathered}$$

Hence, the correct option is C i.e. ${\mathrm{H}}_2\mathrm{S}$.