Manganese(IV) oxide (MnO₂) is a chemical compound also known as manganese dioxide or manganese oxide. It has a blackish or brown color. It occurs naturally as the mineral pyrolusite which is the main source for extracting the metal manganese. It is also present in manganese nodules. The principal use for MnO₂ is for dry-cell batteries, such as the alkaline_battery and the zinc-carbon_battery ; in 1976 this accounted for half a million tonnes per year of pyrolusite.^[1] It can also be used for production of manganates , both MnO₄^2- and MnO₄^-. It is used extensively as an oxidising agent in organic chemistry, for example, for the oxidation of allylic alcohols.

A green substance, potassium manganate , is obtained after about 10 minutes when melting manganese(IV) oxide with an alkaline salt and adding oxidizing agents like saltpeter (potassium nitrate KNO₃) or potassium perchlorate (KClO₄). Potassium manganate (K₂MnO₄) can be purified by vacuum distilling with a metal "finish" to give dark green crystals (rhombic prisms). Potassium manganate is soluble in alkali ONLY, otherwise it will disproportionate into potassium permanganate (KMnO₄^-) and manganese(IV) oxide (MnO₂).

Potassium manganate is converted into potassium permanganate in aqueous acidic solution:

3MnO₄^2- + 4H⁺ → 2MnO₄^- + MnO₂(s) + 2H₂O

MnO₄^- has a purple color

MnO₄^2- has a green color

MnO₂ is used as a catalyst in the laboratory preparation of oxygen from potassium chlorate, one of the classical experiments of elementary chemistry classes. The procedure consists of merely heating potassium chlorate and manganese dioxide together in a hard glass container and collecting the gas over water. See oxygen, manganese and potassium chlorate for precautions. Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water.

2 H₂O₂(aq) → O₂(g) + 2 H₂O(l)

This is the type of reaction that drives the famous Jet-packs.

Eighteenth-century British chemists referred to MnO₂ simply as manganese. Elemental manganese was known as reglus of manganese.

Ancient cave painters sometimes used MnO₂ as black paint.

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984.
2. Oosterhoeks Encyclopedie (Dutch)