Question

What happens when acidic solution of potassium permanganate is allowed to stand for sometime ? Give the equation involved. What is this type of reaction called ?

Hint:

This is the reason why standard \(KMnO_4\) solutions are stored in dark (amber-coloured) bottles to protect them from light-induced decomposition.

Answer 1

Step 1: Understanding the Concept:
Acidic permanganate solutions are thermodynamically unstable and tend to decompose over time.
Step 2: Detailed Explanation:
When left to stand, acidic \(KMnO_4\) undergoes a slow decomposition reaction, liberating oxygen gas. This process is often catalyzed by sunlight or the presence of manganese(II) ions (\(Mn^{2+}\)) produced during the reaction itself (autocatalysis).
Equation:
\[ 4MnO_4^- + 12H^+ \rightarrow 4Mn^{2+} + 5O_2 + 6H_2O \]
This reaction is a redox decomposition reaction. In laboratory settings, it is called the self-decomposition of permanganate.
Step 3: Final Answer:
The solution decomposes to liberate oxygen. It is a decomposition reaction.