Solution of $\ce{KMnO4}$ is decolorised by $\ce{SO2}$. Is this due to the reducing nature of $\ce{SO2}$ or acidic nature of $\ce{SO2}$ ?

I encountered following two explanations. While first one appears to be reasonable, I could not understand the second one.

Reducing nature of $\ce{SO2}$: $$\ce{2KMnO4 +5SO2 +2H2O -> K2SO4 + 2MnSO4 +2H2SO4}$$

Acidic nature of $\ce{SO2}$:

$$ \begin{align} \ce{SO2 + 2H2O &-> H2SO4 + 2H}\\ \ce{2H + O &-> H2O}\\ \ce{2KMnO4 +3H2SO4 &-> K2SO4 + 2MnSO4 +3H2O + 5[O]}\\ \end{align} $$

Also, it is said that $\ce{H2SO4}$ is a moderately strong oxidizing agent. How can $\ce{H2SO4}$ reduce $\ce{KMnO4}$ to $\ce{MnSO4}$ (last equation)? Is the second explanation correct?

  • 1
    $\begingroup$ Your first equation under "Acidic nature" is incorrect. $\ce{SO2 + H2O -> H2SO3 + H2O}$ Since now there's no nascent hydrogen atoms, the second reaction won't occur, and hence the entire "Acidic nature" reasoning has failed. $\endgroup$ – Gaurang Tandon Mar 1 '18 at 8:35

Potassium permanganate is a very strong oxidising agent (stronger than sulphuric acid). In acidic medium, $\ce{Mn}$ gets reduced from its +7 oxidation state to +2 oxidation state. (n-factor is 5).
$$\ce{2KMnO4 + 3H2SO4 -> K2SO4 + 2MnSO4 + 3H2O + 5[O]}$$

Since $\ce{SO2}$ is a strong reducing agent, it reduces $\ce{Mn}$ as denoted by the equation $$\ce{5SO2 + 2KMnO4 + 2H2O -> 2H2SO4 + 2MnSO4 + K2SO4}$$

Both the reactions are viable. Because of the formation of $ \ce{MnSO4}$ and the disappearance of $ \ce{KMnO4}$, decolorisation occurs.

But sulphur dioxide reacts with water to produce sulphurous acid and not sulphuric acid. $$ \ce{ SO2 + H2O —> H2SO3} $$ Hence, the acidic nature reasoning doesn’t hold good.

  • $\begingroup$ Thanks. Few clarifications: (1) Is the first equation redox? Mn gets reduced from +7 to +2. Which species is getting oxidised? (2) How is H2SO4 produced if SO2 gives H2SO3? (3) What is the conclusion? SO2 decolorises KMnO4 due to both acidic nature as well as reducing nature? $\endgroup$ – user59556 Mar 1 '18 at 10:23
  • $\begingroup$ (1) You can say oxygen is getting oxidised from -2 to 0 because of the formation of nascent oxygen (Although I am not sure about it) (2) $\ce{H2SO4} $is formed by contact process. (en.m.wikipedia.org/wiki/Contact_process) and also by reacting$ \ce{SO3}$ with water but the latter is highly exothermic. (3) Read gaurang’s comment on your question. That was why I said it can form only $\ce{H2SO}$ in my answer. I ll add that also. $\endgroup$ – MollyCooL Mar 1 '18 at 10:34
  • $\begingroup$ Reg. (2), my question was not on how H2SO4 is manufactured. Rather, it was in the given context of SO2 reacting with KMnO4 in acidic medium. So, based on your inputs, I understand that SO2 with H2O gives H2SO3, and not H2SO4. So 'acidic nature' explanation does not hold. Thanks. $\endgroup$ – user59556 Mar 2 '18 at 7:15

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