I have the equation:

$$\ce{2 Cu2O + Cu2S -> 6 Cu + SO2}$$

My doubt is that since copper is reduced (it goes from $+1$ to a $0$ O.S.), then which of the two reactants is reduced? Or why would it be fine to say that only copper is reduced and not an entire reactant? Because from my experience, I've seen that an entire reactant is considered to be reduced or oxidized. So is it right to tell only a part of the compound is oxidized?

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    $\begingroup$ Compounds are not reduced; elements are. To say that a compound is reduced is an abuse of language. Sometimes it is forgivable, sometimes it isn't. $\endgroup$ – Ivan Neretin Feb 4 '19 at 7:23
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    $\begingroup$ So is it more correct to refer to a reactant as a reducing/oxidizing agent rather than saying "a compound is reduced/oxidized"? $\endgroup$ – Apekshik Panigrahi Feb 4 '19 at 7:29
  • $\begingroup$ Exactly. But then again, is can so happen that one element in a compound is oxidized and another reduced; what would you call that? $\endgroup$ – Ivan Neretin Feb 4 '19 at 7:33
  • $\begingroup$ So that would then be referred to as a disproportionation reaction, I believe? And to determine which type of reagent it is, we would have to look at the particulars of the mole ratio and the individual changes in oxidation number of that particular reactant, right? $\endgroup$ – Apekshik Panigrahi Feb 4 '19 at 7:39
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    $\begingroup$ No and no. Disproportionation is a reaction where the same element reduces and oxidizes at the same time. As for the type of reagent, this is not something worth caring about. Call it a reducing and oxidizing agent at once, if you wish, or don't call it anything special at all. $\endgroup$ – Ivan Neretin Feb 4 '19 at 7:56

For accounting purposes, we can assign oxidation numbers to all the atoms. Oxygen could be -II in all species here, then sulfur would be +IV in $\ce{SO4}$ and -II in $\ce{Cu2S}$ (oxygen and sulfur are in the same group and both are more electronegative than copper, so assigning the same oxidation number in $\ce{Cu2S}$ and $\ce{Cu2O}$ makes sense). Oxidation numbers of atoms in elements are zero, and the copper in both compounds would be +I to balance out the -II of the chalcogens.

Once you are in agreement with your discussion partner on these oxidation states, you can make a statement about which atoms undergo oxidation or reduction: Sulfur gets oxidized, and copper in both copper compounds gets reduced.

In cases where only one atom in a compound changes oxidation states, or when all changes are in the same direction, we do talk about the compound being reduced or oxidized. For example, it is common to say that when you oxidize an aldehyde, you get a carboxylic acid (and when you reduce it, you get an alcohol or an alkane).

On the other hand, if you hydrate an alkene, some folks don't consider that a redox reaction.

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