Question

(a) What is decomposition reaction?

(b) Give two balanced chemical reactions for -

1. Heat
2. Electricity
3. Sunlight

Answer 1

(a) Decomposition reaction -

• A reaction in which a compound breaks into its constituent elements Such that these products of decomposition do not recombine to form the original compound is called a decomposition reaction.
• Decomposition may occur in the presence of heat, light or by the passage of an electric current.
• Example - $$\begin{gathered} 2\mathrm{HgO}\left(\mathrm{s}\right)\to 2\mathrm{Hg}\left(\mathrm{l}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \mathrm{Mercury}\mathrm{oxide}\mathrm{Mercury}\mathrm{Oxygen} \end{gathered}$$

(b) Decomposition

1. By heat - Decomposition of calcium carbonate on heating to produce calcium oxide and carbon dioxide.

$$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\underset{1000°\mathrm{C}}{\overset{\mathrm{Heat}}{\to }}\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Calcium}\mathrm{carbonate}\mathrm{Calcium}\mathrm{oxide}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

2. By light - Decomposition of silver chloride in presence of sunlight to produce silver and chlorine gas.

$$\begin{gathered} 2\mathrm{AgCl}\left(\mathrm{s}\right)\stackrel{\mathrm{sunlight}}{\to }2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{l}\right) \\ \mathrm{Silver}\mathrm{chloride}\mathrm{Silver}\mathrm{Chlorine} \end{gathered}$$

3. By electricity - Decomposition of water by the passage of electric current to produce hydrogen and oxygen gas.

$$\begin{gathered} 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\mathrm{electrolysis}}{\to }2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right) \\ \mathrm{Water}\mathrm{Hydrogen}\mathrm{Oxygen} \end{gathered}$$