Spot on. The sulfate is not essential because any anion that doesn't precipitate Mn2+ would work. The mecanism is bit more complex because
it involves Mn+2 and Mn+3, and possibly HOO- and O2-. The simplified version is the following:
the surface of the manganese dioxide is reduced to Mn2+ with liberation of oxigen:
MnO2 + H2O2 +2H+ => Mn2 + 2H2O + O2.
In contact with water it forms Mn(OH)2, which is insoluble (and forms a layer over the MnO2 underneath), and H+:
Finally, the Mn(OH)2 is oxidized by the peroxide to regenerate the MnO2 that you had in the beginning.
If the solution is acidic, the equilibrium in the first equation is shifted to the right. The Mn2+ is solubilized and the cycle is broken
(either because the Mn2+ goes directly into the solution or the Mn(OH)2 is formed and immediately attacked by the acid; maybe
both, who knows). And the funny part is that the peroxide acts both as reducing and oxidizing agent in the same solution.
The purple smoke may be residues of permanganate, a mist of permanganic acid (which is violet, if I'm not mistaken) made airborne by the reaction, or
both. |