Aamc 5 cbt # 24

[typicalindian]

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

The explanation provided on the test doesn't really make it clear to me. Any help would be appreciated. Thanks

---

Members don't see this ad.

 

[MedPR]

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

The explanation provided on the test doesn't really make it clear to me. Any help would be appreciated. Thanks

I can't really tell you why C and D are wrong, but B is wrong because there's no Carbon anywhere to make CO2. A is right because Copper is readily oxidized, thus HNO3 is going to be reduced.

I remember this question and I answered (basically guessed) using past experience with redox questions. H2 and O3 gas aren't common gaseous products.

 

[typicalindian]

I can't really tell you why C and D are wrong, but B is wrong because there's no Carbon anywhere to make CO2. A is right because Copper is readily oxidized, thus HNO3 is going to be reduced.

I remember this question and I answered (basically guessed) using past experience with redox questions. H2 and O3 gas aren't common gaseous products.

Hmm I still dont get it, I put H2 because I just figured it was a common gas but it was a 100% guess. I have no idea how oxidation states play a role in this

 

[Temperature101]

Hmm I still dont get it, I put H2 because I just figured it was a common gas but it was a 100% guess. I have no idea how oxidation states play a role in this

I picked H2 as well. I think this question is weird...

 

[typicalindian]

I picked H2 as well. I think this question is weird...

what was your reasoning for picking H2? Mine was purely a guess.

 

[Temperature101]

what was your reasoning for picking H2? Mine was purely a guess.

I wrote the equation as Cu + 2HNO3---------> Cu(NO3)2 + H2

 

[MedPR]

How would H2 be formed? If something pulls a hydrogen off of nitric acid or water, it's going to keep that hydrogen. You'd need to form a couple of free hydrogens to get H2 gas.

 

[MedPR]

How would H2 be formed? If something pulls a hydrogen off of nitric acid or water, it's going to keep that hydrogen. You'd need to form a couple of free hydrogens to get H2 gas.

I wrote the equation as Cu + 2HNO3---------> Cu(NO3)2 + H2

Well, ignore what I said since this seems plausible. My thinking was that any acid base reaction would occur between HNO3 and water and any redox would occur between HNO3 and Cu. If your reaction occured, the hydrogens would come off one at a time and probably form H3O+, not H2.

 

[Temperature101]

How would H2 be formed? If something pulls a hydrogen off of nitric acid or water, it's going to keep that hydrogen. You'd need to form a couple of free hydrogens to get H2 gas.[/QUOTE:

Ok......................

 

[dsoz]

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

The explanation provided on the test doesn't really make it clear to me. Any help would be appreciated. Thanks

Try A.

http://www.angelo.edu/faculty/kboudrea/demos/copper_HNO3/Cu_HNO3.htm

In concentrated nitric acid it is:
Cu + 4 HNO3 --> Cu(NO3)2 + 2 NO2 + 2 H2O

In dilute nitric acid it is:
3Cu + 8 HNO3 --> Cu(NO3)3 + 2 NO + 4 H2O

There is a redox reaction that oxidizes Cu to Cu2+
and N in HNO3 is reduced from a +5 oxidation state to a +2 oxidation state in NO.

NO2 is the brown noxious gas found in smog.

NO is nitric oxide (from A&P class)

If you get a chance, add nitric acid to copper. Lots of FUN! Just hold your breath.


H2 cannot be made because Cu is considered a "less reactive metal" than hydrogen. (table of redox potentials). Copper oxidation is a -0.34V and hydrogen reduction from H+ is 0.00V. There is nothing that will drive the reaction to completion (not energetically favorable).

I wrote the equation as Cu + 2HNO3---------> Cu(NO3)2 + H2

This would be a good guess if it were an ACTIVE metal like magnesium, sodium, etc. But copper is notorious for being unreactive (member of the coinage metal group). Whe know it is unreactive because we make things out of it that are supposed to last a long time (statues, coins, etc.).


dsoz

 

[Temperature101]

MCAT come up with sh..ty stuff sometimes.....

 

[MedPR]

Alternatively you can see that the answer probably has something to do with HNO3 since there was no rxn without HNO3.

 

[typicalindian]

so am I thinking of this right?
Since Cu is reduced more easily than Hydrogen, the H2 gas would not be produced because that would imply that hydrogen was reduced right? (+1 -> 0)

is the fact that Cu is reduced more easily than hydrogen one of the random facts we're supposed to know? or is there some sort of trend that I'm forgetting?

 

[MedPR]

so am I thinking of this right?
Since Cu is reduced more easily than Hydrogen, the H2 gas would not be produced because that would imply that hydrogen was reduced right? (+1 -> 0)

is the fact that Cu is reduced more easily than hydrogen one of the random facts we're supposed to know? or is there some sort of trend that I'm forgetting?

Copper is getting oxidized, thereby reducing HNO3 to NO. You probably should know that many of the common transition metals are readily oxidized and therefore are good reducing agents. Also, reduction potentials are measured against the standard hydrogen electrode, which has an emf of 0.

 

[pfaction]

So I didn't read anything here, just gonna hemorrhage what I know before.

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?

HNO3 is a strong acid. It disassociates and thus H+ and NO3- (stable). NO3 automatically is a spectator ion. H+ is evolved.

B and D are out since there's no carbon involved and ozone is highly unlikely as it is, since oxygen's not involved. So that leaves NO and H2.

...Well NO3 ain't going anywhere.

H+ is going to want those electrons bad, so Cu gives'em up. Where hydrogen picks it up, to become hydrogen gas.

And

Cu[0] + H+ + -NO3 -> CuNO3(2) + H2 gas

 

[folktale]

So I didn't read anything here, just gonna hemorrhage what I know before.

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?

HNO3 is a strong acid. It disassociates and thus H+ and NO3- (stable). NO3 automatically is a spectator ion. H+ is evolved.

B and D are out since there's no carbon involved and ozone is highly unlikely as it is, since oxygen's not involved. So that leaves NO and H2.

...Well NO3 ain't going anywhere.

H+ is going to want those electrons bad, so Cu gives'em up. Where hydrogen picks it up, to become hydrogen gas.

And

Cu[0] + H+ + -NO3 -> CuNO3(2) + H2 gas

But the answer is NO.

 

[pfaction]

dafuq, did I get it wrong? Lemme check my test. Wat.

Okay, that's weird, I got it right. I have no idea.

 

[folktale]

dafuq, did I get it wrong? Lemme check my test. Wat.

Okay, that's weird, I got it right. I have no idea.

lol.

but yeah I got it right too, because I just remembered seeing that reaction in BR or EK1001. It also came down to H2 and NO for me.

 

[typicalindian]

So I didn't read anything here, just gonna hemorrhage what I know before.

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?

HNO3 is a strong acid. It disassociates and thus H+ and NO3- (stable). NO3 automatically is a spectator ion. H+ is evolved.

B and D are out since there's no carbon involved and ozone is highly unlikely as it is, since oxygen's not involved. So that leaves NO and H2.

...Well NO3 ain't going anywhere.

H+ is going to want those electrons bad, so Cu gives'em up. Where hydrogen picks it up, to become hydrogen gas.

And

Cu[0] + H+ + -NO3 -> CuNO3(2) + H2 gas

sorry but I lol'd when I saw you put H2 thanks for trying though

 

[SaintJude]

Kaplan writes their own explanations for AAMC tests.

It looks like we all agree it's either NO or H2. I quote:

C.) Distortion. Nitrogen is more electronegative than hydrogen, making it more likely to be reduced

Interesting...

Edit: And "When a strip of Cu is placed into H2O (l), no change is observed." Is it a useless filler or hint?

 

[pfaction]

sorry but I lol'd when I saw you put H2 thanks for trying though

It's so bull****, though, because it's supposed to be a spectator ion.

 

[MrNeuro]

yeah still dont get why NO is better (i get the whole electronegative thing) but how do we determine what gases are formed for other rxns?

 

[pfaction]

There's something I don't understand.

Let's say you reconstructed this with SO4. You put Cu and H2SO4 in.

Am I to understand that something like SO3 or SO2 evolves, instead of H2?

 

[folktale]

There's something I don't understand.

Let's say you reconstructed this with SO4. You put Cu and H2SO4 in.

Am I to understand that something like SO3 or SO2 evolves, instead of H2?

Well, since H2SO4 is a strong acid, it will readily dissociate, and I would think the SO4^2- anion would react with Cu. So the H2 gas evolved is more likely in this case.

But then HNO3 is also a strong acid, so I don't know.

 

[fas376]

This thread should be renamed the 04/28 thread since we're all here.

And I swear, it's ridiculous how similar we are. I'd like to add that I also put H2 for this annoying question. Once you guys take AAMC 8, you'll notice there are more "gas evolving" questions. I have no idea how to figure those out, and I keep taking random guesses.

So here's a friendly bump, since I would like to know the answer as well.

 

[folktale]

lol 4/28!!!

 

[fas376]

lol 4/28!!!

I think the reason we're all getting these questions wrong is because we don't know what's going on from a reaction standpoint...

Is it a redox reaction? I really don't know the appropriate time to use the different kinds of reactions..

 

[folktale]

I think the reason we're all getting these questions wrong is because we don't know what's going on from a reaction standpoint...

Is it a redox reaction? I really don't know the appropriate time to use the different kinds of reactions..

Yea, i'd say it's redox. Cu would be oxidized and HNO3 would be reduced.

 

[fas376]

Yea, i'd say it's redox. Cu would be oxidized and HNO3 would be reduced.

How do you know when it's going to be a redox reaction? The reactions themselves aren't bad, I just never know when it actually is one.

 

[folktale]

How do you know when it's going to be a redox reaction? The reactions themselves aren't bad, I just never know when it actually is one.

The presence of a solid metal in the beginning is a good indicator that it'd get oxidized in a reaction, although like most things, there's always some exception.

 

[fas376]

The presence of a solid metal in the beginning is a good indicator that it'd get oxidized in a reaction, although like most things, there's always some exception.

That's still a good rule of thumb though. Thank you!

 

[dsoz]

Well, since H2SO4 is a strong acid, it will readily dissociate, and I would think the SO4^2- anion would react with Cu. So the H2 gas evolved is more likely in this case.

But then HNO3 is also a strong acid, so I don't know.

NO!!!! Sorry for yelling, but copper is an UNREACTIVE metal. It does not react normally with acids. If you put copper in sulfuric acid, there is virtually no reaction, same with hydrochloric acid. Look up "standard reduction potentials" then REVERSE the reactions and the sign of the electrical potential (because you want to talk about oxidation, and the table is all about reduction). Metals like Mg, Zn, Fe or Al are called "active metals" because their oxidation would have a (+) value. This means they will react with most strong acid solutions to make H2 gas.

Metals like Cu, Ag, Au, Hg, Pb or Pt are "non-active metals" because their oxidation has a (-) value. this means that they will not get oxidized when H+ is getting reduced. (H+ has a standard reduction potential of 0v).

The only reason that copper acts with nitric acid is that nitric acid is a strong oxidizing agent as well as an acid. In dilute solution the reaction is:

(NO3)- + 4H+ + 3e- = NO + 2 H2O; E0 = 0.96. (from http://inorganicventures.com/tech/advice/spectroscopy/nitric-acid-oxidations)

Since it has a reduction potential that is +0.96 it can energetically drive the oxidation reaction of copper (-0.34v)

An interesting note... Ag has an oxidation potential of -0.8 so it also reacts with nitric acid. However, Au has an oxidation potential of -1.52v, thus it does NOT react with straight nitric acid. It will react with "aqua regia" a mixture of nitric and hydrochloric acids (read the wikipedia article on aqua regia if you want to know how it does it)

HTH

dsoz

 

[folktale]

NO!!!! Sorry for yelling, but copper is an UNREACTIVE metal. It does not react normally with acids. If you put copper in sulfuric acid, there is virtually no reaction, same with hydrochloric acid. Look up "standard reduction potentials" then REVERSE the reactions and the sign of the electrical potential (because you want to talk about oxidation, and the table is all about reduction). Metals like Mg, Zn, Fe or Al are called "active metals" because their oxidation would have a (+) value. This means they will react with most strong acid solutions to make H2 gas.

Metals like Cu, Ag, Au, Hg, Pb or Pt are "non-active metals" because their oxidation has a (-) value. this means that they will not get oxidized when H+ is getting reduced. (H+ has a standard reduction potential of 0v).

The only reason that copper acts with nitric acid is that nitric acid is a strong oxidizing agent as well as an acid. In dilute solution the reaction is:

(NO3)- + 4H+ + 3e- = NO + 2 H2O; E0 = 0.96. (from http://inorganicventures.com/tech/advice/spectroscopy/nitric-acid-oxidations)

Since it has a reduction potential that is +0.96 it can energetically drive the oxidation reaction of copper (-0.34v)

An interesting note... Ag has an oxidation potential of -0.8 so it also reacts with nitric acid. However, Au has an oxidation potential of -1.52v, thus it does NOT react with straight nitric acid. It will react with "aqua regia" a mixture of nitric and hydrochloric acids (read the wikipedia article on aqua regia if you want to know how it does it)

HTH

dsoz

Thank you for the correction. If it was zinc on the other hand, it would love to be oxidized. Cu definitely wants to be reduced, but NOT oxidized.

 

[MedPR]

Thank you for the correction. If it was zinc on the other hand, it would love to be oxidized. Cu definitely wants to be reduced, but NOT oxidized.

Cu gets oxidized, not reduced. Cu2+ might get reduced, but Cu certainly will not.

Edit: None of the neutral transition metals get reduced as far as I know.

 

[typicalindian]

NO!!!! Sorry for yelling, but copper is an UNREACTIVE metal. It does not react normally with acids. If you put copper in sulfuric acid, there is virtually no reaction, same with hydrochloric acid. Look up "standard reduction potentials" then REVERSE the reactions and the sign of the electrical potential (because you want to talk about oxidation, and the table is all about reduction). Metals like Mg, Zn, Fe or Al are called "active metals" because their oxidation would have a (+) value. This means they will react with most strong acid solutions to make H2 gas.

Metals like Cu, Ag, Au, Hg, Pb or Pt are "non-active metals" because their oxidation has a (-) value. this means that they will not get oxidized when H+ is getting reduced. (H+ has a standard reduction potential of 0v).

The only reason that copper acts with nitric acid is that nitric acid is a strong oxidizing agent as well as an acid. In dilute solution the reaction is:

(NO3)- + 4H+ + 3e- = NO + 2 H2O; E0 = 0.96. (from http://inorganicventures.com/tech/advice/spectroscopy/nitric-acid-oxidations)

Since it has a reduction potential that is +0.96 it can energetically drive the oxidation reaction of copper (-0.34v)

An interesting note... Ag has an oxidation potential of -0.8 so it also reacts with nitric acid. However, Au has an oxidation potential of -1.52v, thus it does NOT react with straight nitric acid. It will react with "aqua regia" a mixture of nitric and hydrochloric acids (read the wikipedia article on aqua regia if you want to know how it does it)

HTH

dsoz

Going off of what you said...if we were to assume that H2 gas formed, that would imply that Hydrogen got reduced, and that Copper must have been oxidized, right? Since the reduction potential of hydrogen is 0 and copper has an oxidation potential of -0.34V. That would make the standard electric potential for the reaction -0.34V then, which would cause the standard free energy change of the reaction to be a positive (unfavorable) value. I figure this by using the delta G = -n*F*E* which would be (-)*(+)*(-) giving you a positive G value. Is that why you should be swayed away from picking H2 as the answer? As opposed to when NO gas is formed, the nitrogen is the species being reduced, but since it has such a large positive reduction potential, the delta G of the reaction would still be negative, so the formation of NO gas is more likely to happen than the formation of H2 gas? I don't know why I just thought of this, and I highly doubt that this is a way to think of it during the testing situation. Hell, I'm not even sure if what I just wrote makes any sense...someone read it and let me know if it makes sense

edit: also, how are we supposed to know that nitric acid is a strong oxidizing agent? is there some general trend i'm missing or is it one of things things that we just need to know?

 

[folktale]

Cu gets oxidized, not reduced. Cu2+ might get reduced, but Cu certainly will not.

I meant Cu2+. Thanks for catching that.

 

[MrNeuro]

this question was bullllllll

for the CFC one about adding a catalytic amount. Do all radicals act as catalysts (i think they do)?

 

[typicalindian]

this question was bullllllll

for the CFC one about adding a catalytic amount. Do all radicals act as catalysts (i think they do)?

You really can't tell without being given a mechanism. The key to that question was just to notice that the Cl radical was used and then reproduced, which is characteristic of a catalyst.

 

[EnginrTheFuture]

I picked H2 as well... had a feeling they were eluding to H2 reduction potential being = 0 and I couldn't remember if Cu liked to be oxidized with such little time left

Essentially they wanted us to narrow it down to the two we did and then realize that Cu will not reduce H's to H2 (g). This would give a - deltaE.

As for radical reactions. There is almost always a catalytic amount of radicals on the MCAT. Because of this the radicals rarely find each other in the sea of reactants, they continue propagating the radical chain reaction and take a while to terminate (find each other).

 

[typicalindian]

I picked H2 as well... had a feeling they were eluding to H2 reduction potential being = 0 and I couldn't remember if Cu liked to be oxidized with such little time left

Essentially they wanted us to narrow it down to the two we did and then realize that Cu will not reduce H's to H2 (g). This would give a - deltaE.

As for radical reactions. There is almost always a catalytic amount of radicals on the MCAT. Because of this the radicals rarely find each other in the sea of reactants, they continue propagating the radical chain reaction and take a while to terminate (find each other).

Easy way to remember this is to remember the "money metals" like the metals used to make coins. Coins don't rust easily, and rusting = oxidation. What are "money" metals? platinum, gold, silver, mercury, copper.

 

[Danlee07]

I think you overthought this question...

The 1st sentence tells you Cu is unreactive. 2nd sentence tells you the nitric acid is a catalyst for the gas emission (It took a strong acid for some reaction). And the thing that is produced is a gas with N and O. Hydrogen can't come off...Most of the time it comes off a strong acid and a reactive metal.

 

[EnginrTheFuture]

Easy way to remember this is to remember the "money metals" like the metals used to make coins. Coins don't rust easily, and rusting = oxidation. What are "money" metals? platinum, gold, silver, mercury, copper.

Nice, I had read this before somewhere but didn't let it soak in... now I've got it in my head forever! Thanks

 

[pfaction]

Not done with this topic yet.

So I just read TBR Gen Chem Ch. 10, on Electrochemistry. It said that Cu, Ag, Au, Pt do not like to be oxidized - they prefer to be reduced.

So let's switch the question up.

If I place a Cu strip in H2SO4, and gas evolves, what gas has come out?

 

[MrNeuro]

Not done with this topic yet.

So I just read TBR Gen Chem Ch. 10, on Electrochemistry. It said that Cu, Ag, Au, Pt do not like to be oxidized - they prefer to be reduced.

So let's switch the question up.

If I place a Cu strip in H2SO4, and gas evolves, what gas has come out?

CuSO4 is my first guess but that involves Cu > Cu2+ which is oxidation

i still don't understand how to determine what evolves

but i'd guess SO2

after doing some googling

apparently it can be H2 and SO2 it just has to do w/ the conditions

i think the take home is the first experiment where Cu in H2O yielded no reaction

 

[bajoneswadup]

Why can't the MCAT gods be nice and give us an electrode potential WHYYYY