Gas evolved problems?

[axp107]

Advertisement - Members don't see this ad

How do I approach a problem like this?

When a strip of Cu is placed in to H20, no change is observed. However, when a strip of Cu is placed in a solution of HNO3, a gas evolves. What is the most likely identity of the gas?

a) NO
b) CO2
c) H2
d) O3

I ruled out b) b/c theres no Carbon anywhere

Answer is A)

All I know is that Cu gets oxidized, transferring electrons to HNO3... It can't be O3, b/c why would Oxygen be oxidized...
so it was between H2 and NO.. I picked H2.. just b/c it "sounded" right.

How're you supposed to predict what happens on questions like these... any tips on the type of logic you use to do these types of problems?

 

[Summer Sweet]

If Cu + H2O produced a gas,
you'd have to eliminate A and B because there is no N or C in solution.

If Cu + HNO3 produced a gas,
you'd have to eliminate B because there is no C in solution.

But the first reaction did NOT produce a gas, so H2 and O3 probably aren't it. The difference between the two reactions is that the second one contains N, so I'd pick A.

Also, think about what would result from the reaction. If a gas evolves, the remaining substances probably react in some way.

So this is what I think might happen:
4 HNO3 --> 2 H2O + 3 O2 + 4 NO

That would be my reasoning...but don't take my word for it and look it up if you want to be sure.

 

[MundaneMD]

How're you supposed to predict what happens on questions like these... any tips on the type of logic you use to do these types of problems?

I'm guessing that the H2 "sounded right" because you were thinking that an acid + a metal gives you H2 gas. That is the case with most acids.

But, HNO3 does not "act" like an acid in this case. If you notice, it has a lot of Oxygens, so it has the potential to oxidize things. Cu, a metal, is good at reducing things.

The redox reactions (in my experience, correct me if I'm wrong) seem to take preference over other reactions when dealing with acids and metals.

Always keep an eye out for redox reactions.

 

[axp107]

I'm guessing that the H2 "sounded right" because you were thinking that an acid + a metal gives you H2 gas. That is the case with most acids.

But, HNO3 does not "act" like an acid in this case. If you notice, it has a lot of Oxygens, so it has the potential to oxidize things. Cu, a metal, is good at reducing things.

The redox reactions (in my experience, correct me if I'm wrong) seem to take preference over other reactions when dealing with acids and metals.

Always keep an eye out for redox reactions.

I understand its a redox equation, but why would N get oxidized.. and not H? just b/c N is more electronegative? That was the reasoning behind the answer... I would have never have been able to come up with that!

 

[MundaneMD]

I understand its a redox equation, but why would N get oxidized.. and not H? just b/c N is more electronegative? That was the reasoning behind the answer... I would have never have been able to come up with that!

I think you meant to ask why N would get reduced. The reason is because the HNO3 is a strong oxidizing agent. This means that it strongly favors being reduced.

I am not sure about the electronegativity being relevant in this case. It's actually the reduction potentials, which dictate the reaction.

NO3(-) + 4H(+) + 3e- ==> NO(g) + 2H2O = +0.96V
Cu(2+) + 2e- ==> Cu(s) = +0.34V
2H(+) + 2e- ==> H2(g) = +0.0V

The general trend is that reactions with higher reduction potentials are stronger oxidizing agents.

And don't worry, I'm sure I wouldn't have even thought about the redox stuff, either. I only thought about it because you said H2 is not the right answer. 😎

 

[mozdef]

Yeah, I thought it was H2 too. For me it seemed obvious that Cu metal would give up electrons to the H+. But I guess the question was trying to tell you something when it said Cu with H2O didn't produce a gas. Although I took it as since H2O barely contains any H+ the reaction is so far to the left. Tough question.