Question

Which of the following is a redox reaction?

`Cr_2O_7^(2-)+2OH^(-) -> 2CrO_4^(2-)+H_2O`

`[Cu(H_2O)_6]^(2+)+4Cl^(-) -> [CuCl_4]^(2-)+6H_2O`

`4OH^(-) + 4MnO_4^(-) -> 4MnO_4^(2-)+2H_2O+O_2`

`[Fe(H_2O)_6]^(3+)+3OH^(-) -> [Fe(H_2O)_3(OH)_3]+3H_2O`

Is there a way to work this out without going through every single oxidation state? The question is only worth one mark, although I'd like to understand the concept behind this. Thank you

Answer 1

You need to learn something early on in chemistry, maybe in life.

It's not about how little work you can do. If you work hard now, this will be easier in an exam setting. I worked hard at this and this is trivial to me, now—I can do this in my head. Moving on!

In reduction-oxidation reactions, one species is reduced and another is oxidized. Usually this is straightforward, having calculated the oxidation states, one must see which atom's increases, decreases, or neither.

In (A) the chromium cation is being reduced, however nothing is being oxidized.

In (B) no measurable oxidation or reduction are occurring.

In (C) the manganese ion is being reduced and the oxide ion is being oxidized to oxygen gas.

In (D) not much is measurably happening.

Hence, `"C"` is the answer.