Question

A crystalline pale green compound $\mathrm{A}$ when heated, gives the characteristic odour of burning sulphur and another residue $\mathrm{B}$ is obtained which is brown in colour. Write a balanced equation for the reaction and identify $\mathrm{A}$ and $\mathrm{B}$.

Answer 1

1. Types of reaction

Combination reaction: Reaction in which two or more reactants combine to form a single product. $\mathrm{A}+\mathrm{B}\to \mathrm{C}$

Decomposition reaction: Reaction in which reactant is broken down into two or more products in the presence of heat, light or electricity. $\mathrm{A}\to \mathrm{B}+\mathrm{C}$

Displacement reaction: Reaction in which one of the higher reactive element displaces a less reactive element from its solution. $\mathrm{A}+\mathrm{BC}\to \mathrm{AC}+\mathrm{B}$ ( Here A is more reactive than B)

Double displacement reaction: Reaction in which ions of both the reactants interchange their place and form new compounds. $\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}+\mathrm{CB}$

2. Reaction

${\mathrm{FeSO}}_4.7{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\stackrel{\mathrm{heat}}{\to }{\mathrm{FeSO}}_4\left(\mathrm{s}\right)+7{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$
(Ferrous sulphate hydrated) (Ferrous sulphate) (Water)

Green

$2{\mathrm{FeSO}}_4\left(\mathrm{s}\right)\stackrel{\mathrm{heat}}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{SO}}_3\left(\mathrm{g}\right)$

(Ferrous sulphate) (Ferric oxide) (Sulphur dioxide) (Sulphur trioxide)

Reddish-brown

On heating green colour hydrated ferrous sulphate crystals water molecules get evaporated and on further heating, they decompose into a ferric oxide which is reddish-brown in colour and two gases are released sulphur dioxide and sulphur trioxide. The choking odour of sulphur is of ${\mathrm{SO}}_2\left(\mathrm{g}\right)$.

So, this is a decomposition reaction since heating ${\mathrm{FeSO}}_4$ is getting decomposed into several products.
Since decomposition is taking place in presence of heat so it is known as thermal decomposition.

Here compound $\mathrm{A}$ which is green in colour is ${\mathrm{FeSO}}_4.7{\mathrm{H}}_2\mathrm{O}$ and compound $\mathrm{B}$ which is reddish-brown in colour is ${\mathrm{Fe}}_2{\mathrm{O}}_3$.