"production of gas" questions

[thebillsfan]

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i hate this questions with a passion. but as i'm taking my test on friday, i am looking for a systematic way to figure them out. i'm talking, of course, about any question that says "what gas will be produced when the chemist does this?"

For CO2, whenever there's bicarbonate involved, I think it's safe to assume we're talking about carbon dioxide production.

But what trips me up is NO and H2. I saw a question that had HNO3 getting reduced. What gas is produced? H2 or NO? how do you know? I think both are possible. Are there any general guidelines for these types of gas questions? and are there other gases that we should be on the lookout for?

 

[Hemichordate]

I'm wondering about this too.

Also, during electrochemistry experiments (hooking up anode/cathode in different solutions, finding out E value) is H2 gas usually released?

 

[RogueUnicorn]

hey duder.. start winding down on the review with this week to get yourself fresh and ready.

with re: HNO3 reduction, isn't the gas produced N2?

 

[thebillsfan]

hey duder.. start winding down on the review with this week to get yourself fresh and ready.

with re: HNO3 reduction, isn't the gas produced N2?

no i dont believe it is. that woulld mean two HNO3 molecules have to react w/ each other, and anyway, i'm pretty sure i saw it being NO gas in the problem i did.

and yes, i do need to stop this content review. its getting out of hand. hemi, youre also taking the exam on friday, no?

 

[RogueUnicorn]

what you say makes sense, except that you need two HNO3 to make H2 gas anyway.. one HNO3 that reduces to NO will leave HO2, which doesn't make sense to me.

edit: what's the reducing agent?

 

[Hemichordate]

no i dont believe it is. that woulld mean two HNO3 molecules have to react w/ each other, and anyway, i'm pretty sure i saw it being NO gas in the problem i did.

and yes, i do need to stop this content review. its getting out of hand. hemi, youre also taking the exam on friday, no?

Yeah, the answer for that one was NO and not H2.

And yeah, I'm also taking it next Friday. One more practice test for me this week.

 

[thebillsfan]

yup me too i saved 9 for last (didnt want the verbal on 10 to be a confidence killer, but it was actually not that bad for me, ended up with an 11 but *should have* gotten a 13)

bleargh, not sure what the reducing agent is...are you quizzing me or do you want to know to answer the question? ha..i think it was some solid metal.

 

[RogueUnicorn]

i just want to figure it out, because now it's bugging me. =)... depending on the reaction i guess it could be that HNO3--> H+ + NO + O2, but still, it'd be nice to have the full reactant side to figure out the product side

 

[thebillsfan]

i will try and find it for you in the next day or two. O2 wasnt an option, and I dont think it was produced because that would make it a very hard question (if not impossible to answer)

 

[RogueUnicorn]

what i'm thinking right now is...

metal(s) + NO3- (since HNO3 is fully dissociated in H2O) --> metal oxide + NO

did the answer choice have any other N containing compounds? because if NO is the only answer choice, it's the obvious choice as nothing else gets reduced

 

[thebillsfan]

no2? is that a gas

 

[RogueUnicorn]

no2? is that a gas

NO2 is not the right answer because that leaves N at a state of same oxdation as NO3. and here we have the solution to your original question =)

look for the only logical reduction product

EDIT: thanks to wanderer100 for pointing out my brainfart, it seems NO2 might be a reasonable choice as well. in this case, then, the metal reducing agent would be key in solving..

 

[wanderer]

I believe H2SO4 (or SO4 2-) can be reduced to SO2(g) under certain conditions. Br(g) can form from Br - as well.

 

[thebillsfan]

ahh...thats hard though. i just doesnt seem obviously logical if H2 is an answer choice as well. remind me why it couldnt be H2? just b/c HNO3 wouldnt give up its electrons to the hydrogen?

 

[RogueUnicorn]

H+ has a VERY low reduction potential if i remember correctly. think about it, it's in the same family as the alkali metals. it's not going to be able to strip away electrons from very many things

EDIT: in fact, the H+ reduction potential is the standard from which all other reduction potentials are calculated. i.e. 0V

 

[wanderer]

You can have
In NO3 -, N is +5 oxidation (not very comfortable)
in H+, H is +1 (pretty stable)
so N gets reduced first.

 

[RogueUnicorn]

oxidation state of n in no3- is +1

 

[wanderer]

Please explain....showing the oxidation number of the oxygens as well.

 

[RogueUnicorn]

........+
[-o]--n==o
........|
........o-

ignore the dots, they're for placement purposes only

 

[wanderer]

That's formal charge, not oxidation state.

 

[RogueUnicorn]

That's formal charge, not oxidation state.

lol you're right