Researchgate – Inorganic Chemistry

How does this reaction occur? We get the oxygen moiety of the manganese dioxide suddenly oxidized to gaseous oxygen, while the manganese gets reduced from +4 to +2.

The sulfuric acid is apparently unperturbed: its proton $ce{H+}$ and its $ce{SO4^{2-}}$ parts remain the same in terms of their oxidation states. What role does the sulfuric acid play here at all?

I am afraid I cannot provide the exact mechanism. Nevertheless, here is what I know.

Đang xem: Researchgate

The reaction is most certainly more complex than the equation

$$ce{2 MnO2_{(s)} + 2 H2SO4_{(aq)} -> 2 MnSO4_{(aq?)} + 3 O2_{(g)} + H2O_{(l,g)}}$$

suggests. One might say it is even misleading, but it is definitely not the whole story.

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A purple smoke

After adding concentrated sulfuric acid to manganese(IV) oxide, a purplish smoke manifests. This suggests to the species $ce{MnO4-}$. Yet, what is the balancing particle? Is it $ce{Mn^2+}$, or rather $ce{H}$? I have no clue.$$ce{2 Mn_{(aq)}^2+ + 5 MnO2_{(s)} + 6 H3O+_{(aq)} -> 2 HMnO4_{(aq)} + 5 Mn^2+_{(aq)} + 8 H2O_{(l,g)}}$$

A green to brown mixture

The initial mixture is green. This points to either $ce{MnO4^2-}$ or $ce{Mn2O7}$, even $ce{MnO4^3-}$ could be it. Again, which one is it? Maybe it is all of the above? Upon standing, we observe a brown precipitate. The manganese(IV) oxide appears to reform … Speculation (more in the next chapter):

$$ce{MnO4_{(aq)}^2- + H2SO4_{(aq)} -> H2MnO4_{(aq)} + SO4_{(aq)}^2-}\ce{3 H2MnO4_{(aq)} -> 2 HMnO4_{(aq)} + MnO2_{(s)} + 2 H2O_{(l,g)}}$$

$$ce{3 MnO4^2- + 2 H2O_{(l)} -> 2 MnO4- + MnO2_{(s)} + OH_{(aq)}-}$$

Oxygen, where”s that from?

I did the classic match experiment; it did indeed burn more vigorously near the reaction mixture.

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$$ce{HMnO4_{(aq)} -> MnO2_{(s)} + 3 O2_{(g)} + 2 H2O_{(l,g)}}$$$$ce{2 MnO4^2- -> MnO4^3- + MnO2_{(s)} + O2_{(g)}}$$$$ce{2 Mn2O7_{(l)} -> 4 MnO2_{(s)} + 3 O2_{(g)}}$$

I think you get the idea

Sorry I could not be of more assistance. The mixture is indeed complex with multiple processes happening in parallel. (This is actually the case with every reaction.) It should at least point you in some direction.

Extra

I was a bit loosey–goosey with the ionic equations; you should not do that ;). In my opinion, it made the process more clear.One might also wish to consider $ce{Mn2MnO4}$ or $ce{Mn3O4}$ which, I believe, are also brown.