Manganese(IV) oxide

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Manganese(IV) oxide
Image:Manganese(IV) oxide.jpg
General
Systematic name	Manganese dioxide Manganese(IV) oxide
Other names	Pyrolusite
Molecular formula	MnO₂
Molar mass	86.94 g/mol (varies)
Appearance	black solid
CAS number	[1313-13-9]
Properties
Density and phase	5.026 g/cm³, solid
Solubility in water	Insoluble
Melting point	535 °C decomp.
Thermodynamic data
Standard enthalpy of formation Δ_fH°_solid	−520.9 kJ/mol
Standard molar entropy S°_solid	53.1 J.K⁻¹.mol⁻¹
Hazards
EU classification	Harmful (Xn)
R-phrases	R20/22
S-phrases	S2, S25
NFPA 704
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
Related compounds
Other anions	Manganese disulfide
Other cations	Technetium dioxide Rhenium dioxide
Related compounds	Manganese(II) oxide Hausmannite Manganese(III) oxide Manganese trioxide Manganese heptoxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Manganese(IV) oxide (MnO₂) is a chemical compound also known as manganese dioxide. It has a blackish or brown colour. 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 K N O₃) or potassium perchlorate (K Cl O₄).

2MnO₂ + 4OH^- + O₂ → 2MnO₄^2- + 2H₂O

Potassium manganate (K₂MnO₄) can be purified by vacuum distilling with a metal "finish" to give dark green crystals (rhombic prisms). Potassium manganate is only soluble in alkali; in non-alkali conditions 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 colour.

MnO₄^2- has a green colour.

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.

The MnO₂ was used for production of chlorine, in eighteen century, before the eletrolysis methods became more profitable. The Mn recovery for process is called Weldon process

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