Permanganate reactions

Manganese exists in many different oxidation states, each with a characteristic colour including: Mn VII: MnO 4 – purple Mn VI: MnO 4 2– deep green Mn IV MnO 2 dark brown Mn 2+ Mn 2+ colourless or very pale pink The many states of manganese

Fresh hydrogen peroxide solution is acidified with sulfuric acid. We don’t use HCl because the Cl - ion can act as a reducing agent. Permanganate in acid conditions

Purple permanganate solution turns colourless as it is added to the acidified peroxide. The MnO 4 – /H + has been reduced to Mn 2+.

Oxygen gas is again produced, but is not visible in this very dilute solution. MnO H + + 5e - → Mn H 2 O H 2 O 2 → O 2 + 2H + + 2e - 5H 2 O 2 + 2MnO H + → 5O 2 + 2Mn H 2 O

Hydrogen peroxide is neutral. Permanganate in neutral conditions

When potassium permanganate solution is added, the purple solution turns brown. The MnO 4 – has been reduced to MnO 2 is formed.

If you look closely, you can see bubbles of a colourless gas being formed.

Leave the beaker to settle for 10 minutes and a brown solid is clearly visible.

Neutral permanganate is reduced to MnO 2 (s): MnO H + + 3e - → MnO 2 (s) + 2H 2 O purple brown H 2 O 2 → O 2 (g) + 2H + + 2e - 2MnO H + + 3H 2 O 2 → 2MnO 2 + 4H 2 O + 3O 2

In the HSO 3 - ion, sulfur is in the +4 oxidation state. Sodium hydrogen sulfite is a white solid which dissolves in water to form a colourless solution. Permanganate in alkaline conditions

Sodium hydroxide solution is added to make the solution alkaline.

Purple potassium permanganate solution is added. The purple solution turns green.

MnO e - → MnO 4 2-

The green solution turns brown, then orange, and finally colourless as the excess HSO 3 - further reduces the green MnO 4 2- to brown MnO 2 and then colourless Mn 2+. The alkaline solution is acidified by adding hydrochloric acid.

Finally we add barium chloride solution, which reacts to form a white precipitate. This confirms that the HSO 3 - has been oxidised to SO The solution had to be acidified first, because Ba(OH) 2 is only slightly soluble.

MnO e - → MnO 4 2- purple green HSO H 2 O → SO H + + 2e - 2MnO HSO H 2 O → 2MnO SO H +

In acidic conditions, purple MnO 4 – is reduced to colourless Mn 2 +(aq): 5H 2 O 2 + 2MnO H + → 5O 2 + 2Mn H 2 O In neutral conditions, purple MnO 4 – is reduced to dark brown MnO 2 (s): 2MnO H + + 3H 2 O 2 → 2MnO 2 + 4H 2 O + 3O 2 In alkaline conditions, purple MnO 4 – is reduced to dark green MnO 4 2– (aq) : 2MnO HSO H 2 O → 2MnO SO H + Summary