I learnt at school that when sulfur dioxide gas, $\ce{SO2}$, is combined with potassium permanganate, $\ce{KMnO4}$, the purple solution will become colourless.
What happens in the solution to cause this to happen?
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asked Nov 13, 2014 at 13:45
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1 Answer
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This is a redox reaction. The permanganate ion (purple) is reduced to manganese(II) ion which is a very pale pink, or almost colourless.
$$\ce{MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}$$
while sulfur dioxide is oxidized to the sulfate ion, which is colourless.
$$\ce{SO2 + 2H2O -> SO4^2- + 4H+ + 2e-}$$
So, the solution is decolourised. The overall equation is
$$\ce{2MnO4- + 5SO2 + 2H2O -> 2Mn^2+ + 5SO4^2- + 4 H+}$$
answered Nov 13, 2014 at 15:04
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$\begingroup$ But what happen to the potassium ion ? Does it combine with the sulfate ion or still remains as an ion? $\endgroup$ Nov 13, 2014 at 22:04
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$\begingroup$ Potassium ion is an spectator ion. It doesn't participate in the redox equation. Its role is to keep the solution electrically neutral. $\endgroup$ Nov 14, 2014 at 12:30