Aamc 5 #24

[kasho11]

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For those who have done it was there anything in the passage that would help indicate this?

When a strip of Cu is placed into H2O, no change is observed. However when a strip of Cu is placed into a solution of HNO3(aq), a gas evolves. What is the most likely identity of the gas?

A. NO(g)
B. CO2(g)
C. H2(g)
D. O3(g)

Now I narrowed it down to NO and H2. My reasoning was the HNO3 would dissociate into NO3- and interact with the Cu. I wasn't thinking about oxidation/reduction at all. But I guess Cu would obviously have to be oxidized for it to be positive and interact with a negative NO3-.

For questions like this, "a strip of metal is placed in solution, a gas evolves" is it always an oxidation/reduction thing? I can understand that NO is more electronegative than H and would more readily undergo reduction, but is this always the case? Like in cases where HCl is used with other reagents, is the more electronegative atom going to get reduced and bubble out of solution if it can?

 

[MedChad]

I think it will be the case most of the time unless they throw another twist into it. But like you said, the metal, in this case cu, will most readily be oxidized, and the more electronegative molecule will be reduced.

 

[pfaction]

We had a huge discussion on this, it boiled down to this:

BR says that: Pd, Pt, Au, Ag, Cu do not like to be oxidized. They want to be reduced. Hydrogen's reduction potential is way less than Cu. But NO- rips away those electrons.

It's a combination of outside knowledge, because on other tests, H2 and CO2 were evolved. I had a TPR passage just like this where H2SO4 was added, but H2 was evolved, not SO2 or SO3. It's facockta.

I on this test came up with: Cu + HNO3 [H+ + NO3-] -> Cu[NO3]2 + H2, which isn't right.

 

[MedPR]

The fact that the reaction requires HNO3 hints that the answer will involve HNO3.

 

[chiddler]

The fact that the reaction requires HNO3 hints that the answer will involve HNO3.

yup. if it involved H2, then it wouldn't need the acid to do that because this would indicate that H+ (that is already present in water) is a strong enough oxidizing agent to carry out the reaction.

alas, NO3(-) is needed therefore the reaction occurs with NO3(-).

 

[chiddler]

We had a huge discussion on this, it boiled down to this:

BR says that: Pd, Pt, Au, Ag, Cu do not like to be oxidized. They want to be reduced. Hydrogen's reduction potential is way less than Cu. But NO- rips away those electrons.

It's a combination of outside knowledge, because on other tests, H2 and CO2 were evolved. I had a TPR passage just like this where H2SO4 was added, but H2 was evolved, not SO2 or SO3. It's facockta.

I on this test came up with: Cu + HNO3 [H+ + NO3-] -> Cu[NO3]2 + H2, which isn't right.

i don't think this is proper reasoning. you do not necessarily require outside knowledge of this.

 

[SaintJude]

yup. if it involved H2, then it wouldn't need the acid to do that because this would indicate that H+ (that is already present in water) is a strong enough oxidizing agent to carry out the reaction.

alas, NO3(-) is needed therefore the reaction occurs with NO3(-).

Very good point.

 

[rgrell]

This is why I came here. An explanation like this, pointing out information given in the question itself that i was not able to apply. I literally just tossed that aside when I did the question, thinking it was erroneous. i I know this is an old post, and you may never see it, but your reasoning helped me out big time, thank you.