Question

When ferrous sulphate is heated, a colourless gas is liberated and a residue is left in the test tube.

(a) Name the gas and the residue.

(b) Write a balanced chemical equation for the reaction.

Answer 1

Decomposition reactions

Reactions in which complex molecules are converted into simpler products by breaking the chemical bonds present in them are called decomposition reactions. These reactions require energy to break the covalent bonds, and the reactant can absorb the energy in the form of heat, light or electricity.

The decomposition reactions that occur by heat absorption are called thermal decomposition reactions.

Heating of ferrous sulphate

(a) When ferrous sulphate $\left({\mathrm{FeSO}}_4\right)$ is heated, it undergoes thermal decomposition to form ferric oxide $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$, sulphur dioxide $\left({\mathrm{SO}}_2\right)$ and sulphur trioxide $\left({\mathrm{SO}}_3\right)$. Of these, ferric oxide is a brown solid. Sulphur dioxide and sulphur trioxide are gases.
Therefore, the colourless gas liberated is a mixture of sulphur dioxide and sulphur trioxide. The residue left is ferric oxide.

(b) The balanced chemical equation for the decomposition reaction is:

$$\begin{gathered} 2{\mathrm{FeSO}}_4\left(\mathrm{s}\right)\stackrel{∆}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{SO}}_3\left(\mathrm{g}\right) \\ \left(\mathrm{Ferrous}\mathrm{sulphate}\right) \end{gathered}$$

Therefore, two molecules of ferrous sulphate decompose to form one molecule each of ferric oxide, sulphur dioxide and sulphur trioxide.