Question

When Lead nitrate is heated it decomposes with a cracking sound, a reddish brown gas is liberated and a solid residue is left in the test tube. Write the name and colour of the residue left.

Answer 1

1. Decomposition reaction

The formation of more than one smaller compound (X and Y) from a reactant (XY) is called a decomposition reaction. It can be represented as:

$\mathrm{XY}\to \mathrm{X}+\mathrm{Y}$

2. Heating of Lead nitrate

Lead nitrate undergoes a decomposition reaction to give Lead oxide (solid residue) and nitrogen oxide (reddish-brown gas).

The reaction for the decomposition of Lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$ is given below.

$$\begin{gathered} 2\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{s}\right)\to 2\mathrm{PbO}\left(\mathrm{s}\right)+2{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \left(\mathrm{Lead}\mathrm{nitrate}\right)\left(\mathrm{Lead}\mathrm{oxide}\right)\left(\mathrm{Nitrogen}\mathrm{dioxide}\right)\left(\mathrm{Oxygen}\right) \end{gathered}$$

The residue left in the test tube after the thermal decomposition of Lead nitrate $\left[\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\right]$ is Lead oxide $\left(\mathrm{PbO}\right)$. It is yellowish-orange in colour.