JEE Chemistry 2023 Question with Solution

Given: The question asks why prolonged heating is avoided during the preparation of ferrous ammonium sulphate.

Find: The correct reason for avoiding prolonged heating.

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

$$(NH_4)_2Fe(SO_4)_2 \cdot 6H_2O$$

and contains iron in the $+2$ oxidation state as $Fe^{2+}$.

The solution explains that $Fe^{2+}$ is susceptible to oxidation in the presence of oxygen, especially on heating. The oxidation reaction is

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

So, prolonged heating accelerates the conversion of $Fe^{2+}$ to $Fe^{3+}$, which changes the composition of the salt.

Therefore, chemically the reason is to prevent oxidation.

However, the provided the solution labels the correct option as A, while the listed options show "Prevent oxidation" as option D. This is a source discrepancy.

Therefore, the most defensible choice from the given options is D because it matches the extracted solution working.

Given: Ferrous ammonium sulfate contains ferrous ion $Fe^{2+}$.

Find: Why prolonged heating should be avoided.

The extracted explanation states:

1. Ferrous ammonium sulfate contains iron in the ferrous state, $Fe^{2+}$.
2. On prolonged heating, oxidation by air becomes faster.
3. This converts $Fe^{2+}$ into $Fe^{3+}$.

The relevant reaction is

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

This produces ferric species and introduces impurity.

So, prolonged heating is avoided in order to stop oxidation of ferrous ion.

Therefore, the correct statement is Prevent oxidation, which corresponds to option D in the provided option list.