Predicting a gas

[reising1]

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Question:
When a strip of Cu is placed into H2O(l), 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)
Answer: A

I narrowed it down to A or C. Why can't it be C?

 

[secretsunset29]

It's all about redox, since Cu must be oxidized and HNO3 is reduced, the only logical choice for HNO3 reduction would be NO(g). Just gotta think it through!

 

[Patassa]

Question:
When a strip of Cu is placed into H2O(l), 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)
Answer: A

I narrowed it down to A or C. Why can't it be C?

I asked for help on this problem too but eventually just found the answer in my textbook, what's going on is that to make H2 gas, that would mean that H2 oxidized Cu. But if you look at the activity chart, Cu is literally the element under H, so that reaction just won't happen without an external voltage source. Cu is just "precious" enough, for instance you probably know that oxidizing Ag isn't going to happen, same with copper in this case.

 

[reising1]

Why is H being oxidized when making H2 gas? It's going from +1 in HNO3 to H2, or 0. This means it's being reduced, I thought, if H2 is being made.

 

[reising1]

It's all about redox, since Cu must be oxidized and HNO3 is reduced, the only logical choice for HNO3 reduction would be NO(g). Just gotta think it through!

Why *must* Cu be oxidized?

 

[Patassa]

Why is H being oxidized when making H2 gas? It's going from +1 in HNO3 to H2, or 0. This means it's being reduced, I thought, if H2 is being made.

I said "H2 oxidized Cu" aka H+ would have been reduced. Doesn't happen because Cu has just slightly higher electron affinity than H according to the chart.