Question

Ferrous and ferric ions in solution may be distinguished by using
(A) Silver nitrate solution
(B) Lead acetate solution
(C) Acidified solution of potassium permanganate
(D) Sodium chloride solution

Answer 1

Hint: Ferrous and ferric ions can be differentiated using solution if the two give different reactions with ferrous and ferric ions. Ferrous and ferric are different from one another in the oxidation state of iron. Ferrous has $ + 2$ oxidation state and ferric has $ + 3$ oxidation state.

Complete Step by Step Solution:
Option A: ferrous ions in form of ferrous sulphate react with silver nitrate solution to undergo double displacement reaction as follows and forms silver sulphate and ferrous nitrate.
\[FeS{O_4}{\text{ }} + {\text{ }}AgN{O_3}{\text{ }} \to {\text{ }}A{g_2}S{O_4}{\text{ }} + {\text{ }}Fe{\left( {N{O_3}} \right)_2}\]

A similar double displacement reaction is observed when ferric sulphate reacts with silver nitrate.
\[F{e_2}{(S{O_4}{\text{)}}_3}{\text{ }} + {\text{ }}AgN{O_3}{\text{ }} \to {\text{ }}A{g_2}S{O_4}{\text{ }} + {\text{ }}Fe{\left( {N{O_3}} \right)_3}\]
Hence silver nitrate can’t distinguish between the two as both form silver sulphate.

Option B: with lead acetate, both ferrous and ferric ions form ferrous acetate and ferric acetate respectively. Hence, lead acetate cannot be used to differentiate ferric and ferrous ions.

Option C: Acidified solution of potassium permanganate acts as an oxidising agent. It oxidises ferrous into ferric with reduction of permanganate ion. But ferric ions cannot be further oxidized as it is already in maximum oxidation state.
\[MnO_4^{2 - } + 5F{e^{2 + }} + 8{H^ + } \to M{n^{2 + }} + 5F{e^{3 + }} + 4{H_2}O\]. This is a redox reaction.

The reaction that has occurred or not can be identified using discoloration of the purple coloured solution of potassium permanganate. If the discoloration takes place then the solution contains ferrous ions and can be used to differentiate from ferric ions solution.

Option D: sodium chloride gives the same product with ferrous and ferric ions solution. So it cannot be used to differentiate them.


Hence, option C is correct.

Note: Potassium permanganate ion is reduced to different compounds depending on the type of medium. In potassium permanganate, manganese in its maximum oxidation state is $ + 7$. In acidic medium, permanganate is reduced to \[M{n^{2 + }}\]. In a strongly basic solution, permanganate is reduced to\[Mn{O^{2 - }}\]which is green in colour..In a neutral medium, it gets reduced to the brown coloured MnO₂.