Redox reaction

[rasen58]

Cu(s) + 2NO₃^-(aq) + 4H⁺(aq) -> Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)

Which of the following takes place during the reaction above?

A) Cu(s) is oxidized

B)Cu(s) is reduced

C)H+(aq) is oxidized

D)H+(aq) is reduced

E)NO3-(aq) is oxidized

 

The answer is A, and I got it right, but the solution also said that NO₃^- is reduced, but I don't see how that's possible.Wouldn't it be oxidized if it's going from NO3- to NO2?

 

 

[hypervalent_iodine]

No. You can't have two things being oxidised and nothing reduced in the same reaction like that. Remember that you aren't looking at the charge on the whole molecule as an indication of the oxidation state. You need to look at specifically which atom is changing in oxidation state and by how much. The charge on the whole molecule is the sum of the charges / oxidation states of everything within it, so you are able to work out the charge of an individual element by treating the whole thing algebraically. For NO₃^- you would write:

 

-1 = N + 3 x O

 

We know that O has an oxidation / charge of -2 in this case (unless it's in a peroxide or elemental form, it is -2 for oxygen), so:

 

-1 = N + 3 x -2

-1 + 6 = N

 

Thus, the nitrogen in NO₃^- has an oxidation state of +5. doing the same thing for NO₂, you should come to N = +4. The change in oxidation for nitrogen is then +5 --> +4, which is a gain of 1 electron and thus it has been reduced.

[rasen58]

Oh, so is it just convention to say that the NO₃^- is reduced when it's only the N part of it being reduced?

[hypervalent_iodine]

Either or is fine, I think. When you are talking about something being oxidised or reduced, you are only really talking about the electron movement to and from individual elements in a molecule rather than an entire molecule. By stating that NO₃^- is reduced there is an implied understanding that the N is the part that's actually changing oxidation state.

[Ayiah]

A is correct because Oxidation is lose of electron and cu was having no charge at the reactant side and at the product side it got 2+ ,so A is correct,