Question

Complete and balance the following equations :
(i) \( 4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \longrightarrow \)
(ii) \( 5\text{Fe}^{2+} + \text{MnO}_4^- + 8\text{H}^+ \longrightarrow \)

Hint:

For redox reactions like (ii), remember the half-reaction for permanganate in acidic medium: \( \text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \). Because it takes 5 electrons, it requires 5 \( \text{Fe}^{2+} \) ions to balance the charge.

Answer 1

Concept: These are standard preparations and redox reactions involving d-block transition metal compounds (Chromates and Permanganates).

• Equation (i) represents the roasting of chromite ore, which is the primary industrial method for preparing sodium chromate.

• Equation (ii) is the standard redox titration reaction in acidic medium where permanganate acts as a strong oxidizing agent, oxidizing Iron(II) to Iron(III).
Step 1: Completing equation (i).
When chromite ore (\( \text{FeCr}_2\text{O}_4 \)) is fused with sodium carbonate in the presence of excess air (oxygen), it oxidizes to form yellow-colored sodium chromate (\(\text{Na}_2\text{CrO}_4\)), iron(III) oxide (\(\text{Fe}_2\text{O}_3\)), and carbon dioxide gas. Balanced Equation: \[ 4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \longrightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2 \]

Step 2: Completing equation (ii).
In an acidic medium, the purple permanganate ion (\(\text{MnO}_4^-\)) is reduced to the nearly colorless Manganese(II) ion (\(\text{Mn}^{2+}\)). Concurrently, it oxidizes ferrous ions (\(\text{Fe}^{2+}\)) to ferric ions (\(\text{Fe}^{3+}\)). Water is formed from the hydrogen ions. Balanced Equation: \[ 5\text{Fe}^{2+} + \text{MnO}_4^- + 8\text{H}^+ \longrightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O} \] Final Answer: (i) \( \longrightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2 \)
(ii) \( \longrightarrow 5\text{Fe}^{3+} + \text{Mn}^{2+} + 4\text{H}_2\text{O} \)