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 the synthesis of organic compounds. 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 + 4HCl → MnCl2 + 2H2O + Cl2
2MnO2 + 2H2SO4 → 2MnSO4 + 2H2O + 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.
Manganese dioxide is amphoteric in nature. It forms oxomanganate (IV) with fused alkali which have varied compositions.
MnO2 + CaO → CaMnO3
|Chemical properties manganese dioxide|
|Molar mass||86.9368 g/mol|
|Melting point||535 °C|
|Appearance||brown black solid|
|Solubility in water||insoluble in water|
MnO2 is a strong oxidant but it further oxidizes to Mn (VI) or Mn (VII) with strong oxidizing agents.
3MnO2 + KClO3 + 6KOH → 3K2MnO4 + KCl + 3H2O
2MnO2 + 3PbO2 + 6HNO3 → 2HMnO4 + 3Pb(NO3)2 + 2H2O
It is a stronger oxidant in an acid medium than 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 + 2NH4Cl + 2OH– → [ZnCl2(NH3)2] + 2H2O
2MnO2 + 2H2O + 2e → 2[MnO(OH)] + 2OH–
- 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.