Question

A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

• A. ${\mathrm{KMnO}}_4$ is an oxidising agent, it oxidises ${\mathrm{FeSO}}_4$
• B. ${\mathrm{FeSO}}_4$ acts as an oxidising agent and oxidises ${\mathrm{KMnO}}_4$
• C. The colour disappears due to dilution; no reaction is involved
• D. ${\mathrm{KMnO}}_4$ is an unstable compound and decomposes in the presence of ${\mathrm{FeSO}}_4$ to a colourless compound.

Answer 1

The correct option is A

${\mathrm{KMnO}}_4$ is an oxidising agent, it oxidises ${\mathrm{FeSO}}_4$

The explanation for the correct answer:-

Option (A) ${\mathrm{KMnO}}_4$ is an oxidising agent, it oxidises ${\mathrm{FeSO}}_4$

The reaction of Ferrous sulphate with acidified Potassium permanganate:-

• When Ferrous sulphate ${\mathrm{FeSO}}_4$ is added to a beaker containing Potassium permanganate ${\mathrm{KMnO}}_4$, the products formed are Potassium sulphate${\mathrm{K}}_2{\mathrm{SO}}_4$, Manganese(II) sulphate${\mathrm{MnSO}}_4$, Ferric sulphate ${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3$and water.
• The Complete balanced reaction involved is:

$\underset{\left(\mathrm{Purple}\right)}{\underset{\mathrm{Potassium}\mathrm{permanganate}}{2{\mathrm{KMnO}}_4\left(\mathrm{aq}\right)}}+\underset{\mathrm{Ferrous}\mathrm{sulphate}}{10{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Sulphuric}\mathrm{acid}}{8{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)}\to \underset{\mathrm{Potassium}\mathrm{sulphate}}{{\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Manganese}\left(\mathrm{II}\right)\mathrm{sulphate}}{2{\mathrm{MnSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Ferric}\mathrm{sulphate}}{5{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{8{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

• The oxidation state of Iron is ${\mathrm{FeSO}}_4$is $+2$, but in ${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3$ the oxidation state of Iron is $+3$, thus it gets oxidised.
• This is because here ${\mathrm{KMnO}}_4$ acts as a strong oxidising agent, and it oxidises Iron in the presence of Sulphuric acid.
• The purple colour of the solution which was due to ${\mathrm{KMnO}}_4$, now slowed turn colourless as all the ${\mathrm{KMnO}}_4$ is utilized.
• Hence it is the correct option.

The explanation for incorrect answers:-

Option (B) ${\mathrm{FeSO}}_4$ acts as an oxidising agent and oxidises ${\mathrm{KMnO}}_4$

• In this above reaction ${\mathrm{KMnO}}_4$ acts as a strong oxidising agent and oxidises Iron.
• ${\mathrm{FeSO}}_4$does not act as a strong oxidising agent.
• Hence it is the incorrect option.

Option (C) The colour disappears due to dilution; no reaction is involved

• The colour disappears due to the complete utilization of ${\mathrm{KMnO}}_4$.
• The complete reaction is:

$\underset{\left(\mathrm{Purple}\right)}{\underset{\mathrm{Potassium}\mathrm{permanganate}}{2{\mathrm{KMnO}}_4\left(\mathrm{aq}\right)}}+\underset{\mathrm{Ferrous}\mathrm{sulphate}}{10{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Sulphuric}\mathrm{acid}}{8{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)}\to \underset{\mathrm{Potassium}\mathrm{sulphate}}{{\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Manganese}\left(\mathrm{II}\right)\mathrm{sulphate}}{2{\mathrm{MnSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Ferric}\mathrm{sulphate}}{5{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{8{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

• Hence it is the incorrect option.

Option (D) ${\mathrm{KMnO}}_4$ is an unstable compound and decomposes in the presence of ${\mathrm{FeSO}}_4$ to a colourless compound

• ${\mathrm{KMnO}}_4$ is a strong oxidising agent and a stable compound.
• It oxidises ${\mathrm{FeSO}}_4$to ${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3$.
• The purple colour disappears due to the complete utilization of ${\mathrm{KMnO}}_4$in this oxidising process.

Therefore, option (A) ${\mathrm{KMnO}}_4$ is an oxidising agent, it oxidises ${\mathrm{FeSO}}_4$ is the correct statement.