Question

A solution of KMnO₄ on reduction yields either a colourless solution or a brown precipitate or a green solution depending on pH of the solution. What different stages of the reduction do these represent and how are they carried out?

Answer 1

The different stages of KMnO4 reduction represent the formation of brown MnO2 precipitate in basic conditions, green MnO42- solution in neutral/slightly acidic conditions, and colorless Mn2+ ions in acidic conditions, which are influenced by reaction pH and other conditions.

Step-by-step explanation:

The reduction of KMnO4 (potassium permanganate) in aqueous solution can lead to different products depending on the pH of the solution. At higher pH levels, the reduction of KMnO4 is likely to form a brown precipitate of manganese dioxide, MnO2, a reaction often carried out in basic conditions, as shown in the given reaction:

2 MnO4- (aq) + 3 NO2- (aq) + H2O(l) → 2 MnO2 (s) + 3 NO3-(aq) + 2 OH-(aq)

Under acidic conditions, KMnO4 is typically reduced to the colorless Mn2+ ion:

4 MnO4- (aq) + 8 H+ (aq) + 5 e- → 4 Mn2+ (aq) + 4 H2O (l)

In neutral or slightly acidic conditions, a green solution containing MnO42- can form. Overall, redox reactions, the type of reactions where these changes occur, are influenced by factors such as pH, presence of catalysts, light, heat, and whether the solutions are acidic or basic.