Question

Prolonged heating of ferrous ammonium sulphate is avoided to prevent :

• A. Oxidation
• B. Reduction
• C. Hydrolysis
• D. Breaking

Answer 1

The Correct Option is A

Introduction to Mohr’s Salt: 

Mohr’s salt, also known as ferrous ammonium sulfate, has the chemical formula:

\[ (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O \]

It contains iron in the +2 oxidation state (\( \text{Fe}^{2+} \), ferrous ion).

Oxidation of Fe²⁺ to Fe³⁺:

Ferrous ions (\( \text{Fe}^{2+} \)) are susceptible to oxidation in the presence of oxygen, particularly at elevated temperatures. The oxidation reaction is:

\[ 4Fe^{2+} + O_2 + 4H^+ \rightarrow 4Fe^{3+} + 2H_2O \]

This leads to the formation of ferric ions (\( \text{Fe}^{3+} \)), which can create impurities in the Mohr’s salt and alter its composition and properties.

Reason for Avoiding Prolonged Heating:

Prolonged heating accelerates the oxidation of \( \text{Fe}^{2+} \) to \( \text{Fe}^{3+} \) by increasing the reaction rate with oxygen in the air. To preserve the desired composition of Mohr’s salt and prevent the formation of ferric compounds, prolonged heating is avoided.

Conclusion:

Prolonged heating of Mohr’s salt is avoided to minimize the oxidation of ferrous ions (\( \text{Fe}^{2+} \)) to ferric ions (\( \text{Fe}^{3+} \)).

Answer 2

Ferrous ammonium sulfate is a green crystalline solid commonly used as a reducing agent in various chemical reactions. It is sensitive to oxidation, particularly when subjected to prolonged heating.

Reason for Oxidation: 

• Heating provides energy that helps overcome the activation energy barrier for oxidation.
• When oxidized, ferrous ammonium sulfate forms ferric ammonium sulfate, altering its chemical composition and properties.

Chemical Reaction:

The oxidation reaction is represented as:

\[ 4FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O + O_2 \rightarrow 2Fe_2(SO_4)_3 \cdot (NH_4)_2SO_4 + 12H_2O \]

This reaction is exothermic, releasing a significant amount of heat, which further drives the reaction forward.

Conclusion:

To prevent oxidation and the formation of ferric ammonium sulfate, prolonged heating of ferrous ammonium sulfate is avoided.

Answer: A