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Manganese Dioxide

What is manganese dioxide?

Manganese dioxide is a slightly brownish black solid with the chemical formula MnO2. It is the main source of manganese compounds and occurs in the earth’s crust as pyrolusite. Manganese dioxide is widely used in dry cells and in the synthesis of organic compounds.

Manganese Dioxide (MnO2) structure

It is insoluble in water and inert to cold dilute acids. It oxidizes concentrated hydrochloric acid and decomposes concentrated sulfuric acid and nitric acid on strong heating.

MnO2 + 4 HCl → MnCl2 + 2 H2O + Cl2
2 MnO2 + 2 H2SO4 → 2 MnSO4 + 2 H2O + O2

Structure of MnO2

MnO2 has a rutile-type structure at high temperatures but tends to be nonstoichiometric at ordinary temperatures when it becomes polymorphic.

Chemical Properties

Manganese dioxide is amphoteric in nature. It forms oxomanganate (IV) with fused alkali which have varied compositions.
MnO2 + CaO → CaMnO3

Chemical properties manganese dioxide
Chemical formula MnO2
Molar mass 86.9368 g/mol
Density 5.03 g/cm3
Melting point 535 °C
Appearance brown black solid
Odour odourless
Solubility in water insoluble in water
Oxidation number +4

MnO2 is a strong oxidant but it further oxidizes to Mn (VI) or Mn (VII) with strong oxidizing agents.
3 MnO2 + KClO3 + 6 KOH → 3 K2MnO4 + KCl + 3 H2O
2 MnO2 + 3 PbO2 + 6 HNO3 → 2 HMnO4 + 3 Pb(NO3)2 + 2 H2O

It is a stronger oxidant in an acid medium than in an alkaline medium. It readily reacts with hot HCl by liberating chlorine gas.

Careful heating MnO2 with other metal oxides gives manganites. It has different types of formulas, the simplest formula is MnO2MIIO or MIIMnO3.

Manganese dioxide uses

  • Manganese dioxide is widely used as a dipolar in dry cells. The common dry cell contains a zinc metal acting as a cathode and a central carbon rod as anode with a paste of carbon dust, ammonium chloride, and manganese dioxide.
    Zn → Zn+2 + 2e
    Zn+2 + 2 NH4Cl + 2 OH → [ZnCl2(NH3)2] + 2H2O
    2 MnO2 + 2 H2O + 2e → 2 [MnO(OH)] + 2 OH
  • MnO2 is used as an oxidant in the chemical industry.
  • It is used in the glass industry to decolorize glass and in the manufacture of matches.