Question

Distinguish between the following pairs of compound using the test given within brackets:

1. Manganese dioxide and copper (II) oxide (using concentrated HCl)

Answer 1

Manganese dioxide $\left({\mathrm{MnO}}_2\right)$ and copper $\left(\mathrm{II}\right)$oxide $\left(\mathrm{CuO}\right)$ can be distinguished by using the concentrated $\left(\mathrm{HCl}\right)$:

1. When Manganese Dioxide $\left({\mathrm{MnO}}_2\right)$ is heated with Concentrated Hydrochloric acid $\left(\mathrm{HCl}\right)$ then it produces Magnesium Chloride, water and the chlorine gas is evolved with a pungent smell.

$$\begin{gathered} {\mathrm{MnO}}_2\left(\mathrm{s}\right)+\mathrm{Conc}.4\mathrm{HCl}(\mathrm{aq}.)\to {\mathrm{MnCl}}_2(\mathrm{aq}.)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\uparrow \\ (\mathrm{manganese}(\mathrm{hydrochloric}(\mathrm{magnesium}(\mathrm{water})(\mathrm{chlorine} \\ \mathrm{dioxide})\mathrm{acid})\mathrm{chloride})\mathrm{gas}) \end{gathered}$$

The detection of chlorine gas can be done by using the starch paper. The chlorine gas can turn the moist starch iodide paper into blue-black colour. This is because the iodide ions on the paper are oxidized to iodine and then starch reacts to it to give the blue-black compound.

2. On the other hand, copper $\left(\mathrm{II}\right)$oxide when heated with concentrated $\left(\mathrm{HCl}\right)$, no such gas is evolved.

$$\begin{gathered} \mathrm{CuO}\left(\mathrm{s}\right)+2\mathrm{HCl}\left(\mathrm{aq}\right)\to {\mathrm{CuCl}}_2(\mathrm{aq}.)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ (\mathrm{copper}(\mathrm{hydrochloric}(\mathrm{copper}(\mathrm{water}) \\ \mathrm{oxide})\mathrm{acid})\mathrm{chloride}) \end{gathered}$$

When copper oxide reacts with concentrated hydrochloric acid then only copper chloride and water are being produced in this reaction, there is no such gas will be produced by this reaction.