AAMC 5 Question 24

[ridethecliche]

The question asked which gas is evolved when Cu is placed in HNO3

I was deciding between H2 and NO, but in the passage the info showed Cu existing as Cu (NO3)2.

The answer is NO is evolved. I'm not sure why this is possible. The answer says that it's an redox rxn where N is reduced and Cu is oxidized.

I'm having trouble seeing what's really happening and why an NO is given off.

---

Members don't see this ad.

 

[Rabolisk]

The question asked which gas is evolved when Cu is placed in HNO3

I was deciding between H2 and NO, but in the passage the info showed Cu existing as Cu (NO3)2.

The answer is NO is evolved. I'm not sure why this is possible. The answer says that it's an redox rxn where N is reduced and Cu is oxidized.

I'm having trouble seeing what's really happening and why an NO is given off.

The info in the passage and this question are separate. Nowhere in the passage did Cu react with HNO3(aq). In the passage Cu(NO3)2 reacted with three possible solutions (Experiment 2). Those are not redox reactions like the one between neutral Cu and HNO3.

Nevertheless, this is a tough question, and I vaguely remember getting it wrong too a year ago. One clue is that a strongly reducing metal (e.g. Li) would have reacted with water to produce H2. Cu does not, suggesting that it is not a strong enough reducing agent to reduce water. It is plausible that Cu could reduce H+ to H2 (H+ is a stronger oxidizing agent than water), but it turns out that the nitrate ion is a stronger oxidizing agent.

3Cu + 2NO3- + 8H+ --> 3Cu+2 + 2NO + 4H2O

 

[ridethecliche]

Thanks man.

I'll chalk this up to a 'energy can be better spent learning something else' kind of thing.

I'll keep the question form in mind. This is one of those things that I truly didn't know. If I can cut down my 'stupid mistakes' in PS, I can give myself an extra 2 points easy...

Thanks for the help!

 

[chemtopper]

HNO3 is a very strong oxidizing agent and can oxidize even less reactive Cu metal .
Oxidation reaction -
Cu (s) -----> Cu^+2 + 2e
HNO3 + 3 e ------> NO(s) + H2O
So the reactions is due to strong oxidizing nature of HNO3
actually when you add copper turnings(thin pieces of wires) in HNO3 you can see brown gas evolving out it.
NO is a colorless gas so brown gas is due to this reaction
2NO + O2 (g) -------> 2 NO2 (g)
NO2 a gas of brown color.

 

[ridethecliche]

HNO3 is a very strong oxidizing agent and can oxidize even less reactive Cu metal .
Oxidation reaction -
Cu (s) -----> Cu^+2 + 2e
HNO3 + 3 e ------> NO(s) + H2O
So the reactions is due to strong oxidizing nature of HNO3
actually when you add copper turnings(thin pieces of wires) in HNO3 you can see brown gas evolving out it.
NO is a colorless gas so brown gas is due to this reaction
2NO + O2 (g) -------> 2 NO2 (g)
NO2 a gas of brown color.

Can you explain the reduction of HNO3?

NO3 by itself is negatively charged and H is positively charged. I just don't see it.

Thanks so much for the help!

 

[Rabolisk]

Nitrogen's oxidation state changes.

 

[ridethecliche]

Ah.

As N is the most likely thing to change oxidation state, that's what happens.

That makes more sense.

So oxygen is set, H isn't as likely to change as Nitrogen, so Nitrogen it is.

Thanks!

*lightbulb goes off by head*