Question

(a) When sodium is burnt in chlorine, sodium chloride is formed, and when molten sodium chloride is electrolysed, then sodium and chlorine are formed. What type of reaction takes place:
(i) in the first case?
(ii) in the second case?
(b) When hydrogen is passed over heated copper oxide, copper and steam are formed. Write a balanced chemical equation for this reaction. State which of the chemicals is oxidised and which one is reduced?
​(c) What would you observe when zinc is added to a solution of iron sulphate? Write the chemical equation of the reaction which takes place. Why does this reaction occur?

Answer 1

(a)
(i) When sodium is burnt in chlorine, due to addition reaction sodium chloride is formed.
$2\mathrm{Na}+{\mathrm{Cl}}_2\to 2\mathrm{NaCl}$
(ii) When this sodium chloride is electrolyzed, sodium chloride is decomposed into its ionic state as sodium ions and chloride ions.
$$\begin{gathered} \mathrm{At}\mathrm{cathode}:\mathrm{Na}\to {\mathrm{Na}}^{+}+{\mathrm{e}}^{-} \\ \mathrm{At}\mathrm{anode}:\mathrm{Cl}+{\mathrm{e}}^{-}\to {\mathrm{Cl}}^{-} \\ \mathrm{Net}\mathrm{reaction}:\mathrm{Na}+\mathrm{Cl}\to \mathrm{NaCl} \end{gathered}$$
So, it is a decomposition reaction.

(b) When hydrogen gas is passed over heated copper oxide, then the black copper oxide is reduced and red-brown copper metal is obtained. The reaction involved can be shown as:
$\mathrm{CuO}+{\mathrm{H}}_2\to \mathrm{Cu}+{\mathrm{H}}_2\mathrm{O}$
As copper is losing oxygen and hydrogen is accepting oxygen, so copper is reduced and hydrogen is oxidized.

(c) When zinc is added to a solution of iron sulphate, then it would displace iron from the solution and form zinc sulphate.
$\mathrm{Zn}+{\mathrm{FeSO}}_4\to {\mathrm{ZnSO}}_4+\mathrm{Fe}$
This reaction is an example of a displacement reaction because zinc is more reactive than iron, zinc will displace iron from the solution.