(a) List the possible sources of energy required in decomposition reactions. Illustrate any one with a suitable example.

OR

(b) What is observed when hydrated ferrous sulphate crystals are heated in a dry boiling tube? Give balanced chemical equation(s) of the reactions(s) that occur(s).

(a) As explained in Section 1.2.2 of the chapter, decomposition reactions require energy to break a single reactant into two or more simpler products; this energy may be supplied in three possible forms:

1. Heat — reaction is called thermal decomposition, e.g. $CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)$ (decomposition of limestone into quick lime and carbon dioxide, used in the cement industry).
2. Light — e.g. $2AgCl(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Cl_2(g)$ (white silver chloride turns grey in sunlight; used in black and white photography).
3. Electricity — e.g. electrolysis of acidified water, which breaks down into hydrogen and oxygen gases at the two electrodes.

Illustration (any one is sufficient): On strongly heating limestone ($CaCO_3$), it decomposes to give quick lime ($CaO$) and carbon dioxide gas:

$$CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)$$

Since energy (heat) is absorbed to bring about this decomposition, it is an example of an endothermic reaction.

(b) This corresponds to Activity 1.5 of the chapter. On heating light green hydrated ferrous sulphate ($FeSO_4\cdot 7H_2O$) crystals in a dry boiling tube:

• The green colour of the crystals fades/changes as the crystals lose their water of crystallisation.
• On further heating, a characteristic smell of burning sulphur (due to $SO_2$ and $SO_3$ gases) is observed, and reddish-brown ferric oxide ($Fe_2O_3$) is left behind as a solid.

The balanced chemical equation for the reaction is:

$$2FeSO_4(s) \xrightarrow{\text{Heat}} Fe_2O_3(s) + SO_2(g) + SO_3(g)$$

This is a thermal decomposition reaction — a single reactant (ferrous sulphate) breaks down into simpler products (ferric oxide, sulphur dioxide, sulphur trioxide) on heating.