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[Collin Li]

1. Which of the following instrumental techniques is not associated with the properties of isotopes of an element?
a) H NMR, b) 13C NMR, c) mass spec, d)IR
apart from mass spec, i don't really understand how any of these are associated with the properties of isotopes...

Think about the fundamental building block that causes isotopes... the wonderful neutron! It's fairly undetectable, as is evident by virtue of it being last of the three subatomic particles to be discovered. It doesn't really have much bearing on chemical effects (which is why they didn't discover it until so late), but it is important in nuclear chemistry. Hence, NMR (nuclear magnetic resonance) is sensitive to isotopes, and obviously mass spectrometry is too (and we know about that rule where you need to have an odd number of protons or neutrons for NMR to be eligible - so we can see neutrons are important somewhere in NMR).

IR is the one that cannot distinguish among them.

[bec]

thanks!

[Mao]

IR is the absorbtion of energy in bonds, and neutrons have no bearings in bonds =]

[chem-nerd]

>99% of all H atoms are 1H, therefore we see splitting patterns from neighbouring H's (because each odd numbered 1H nucleus acts as a miny magnet). and its also why 'heavy' water (D2O) or other solvent containing deuterium can be used as it will contain even numbered nucleons.

~1% of all C atoms are 13C, therefore we rarely see splitting patterns from neighbouring C's because it's more likely the neighbour will be an even numbered nucleus of 12C (ie no miny magnet).

It's also the reason why you need a larger sampe for 13C NMR and why the spectrum takes longer to produce than in 1H NMR.

[beezy4eva]

I'm a bit confused with zwitterions. We were taught that in acidic solution, they took on extra protons so there was an NH3+ group and a COOH group, but in solutions for a question i just did it said there was an NH2+[/b] group and COOH group... does it depend on the degree of acidity in the solution?

No, it doesn't depend. NH2+ is just blatantly wrong. Ignore it. It should be NH3+ or NH2.

I disagree. Have a look at proline, which accepts a proton to become NH2+.

But if the question just ask about zwitterions in general, you would assume it would be NH3+. You could only really say NH2+ if ithe question specifically referred to proline.

[bec]

When hydrogen sulfide gas is passed into a solution of iron (III) nitrate, a pale yellow solid forms and the solution becomes pale green.

   The equation for this reaction is

   A   H2S + Fe3+ + 3H2O --> H2 + S + Fe(OH)3 + 3H+
   B   3H2S + Fe(NO3)3 --> Fe(OH)3 + 2HNO2 + 3S
   C   3H2S + 2Fe3+ ---> Fe2S3 + 6H+
   D   H2S + 2Fe3+ ---> 2H+ + S + 2Fe2+

how would we know? i can eliminate b because it's not balanced correctly, but i can't differentiate between the rest...

[ben4386]

When hydrogen sulfide gas is passed into a solution of iron (III) nitrate, a pale yellow solid forms and the solution becomes pale green.

The equation for this reaction is

   A   H2S + Fe3+ + 3H2O --> H2 + S + Fe(OH)3 + 3H+
   B   3H2S + Fe(NO3)3 --> Fe(OH)3 + 2HNO2 + 3S
   C   3H2S + 2Fe3+ ---> Fe2S3 + 6H+
   D   H2S + 2Fe3+ ---> 2H+ + S + 2Fe2+

methinks its C,  its not D as when you see ions like that it means its in solution

because a solid forms there has to be an ionic compound in the products, B doesnt balance, A has water as a reactant, even though the question says solution it doesn't really say water is a reactant. I would therefore say C.

[chem-nerd]

S is a pale yellow solid and the Fe2+ is likely to give the pale green colour so i would say D

[bec]

yep, it is D. but what if you didn't have the general knowledge to back that up?
(eg i knew sulphur was yellow but not that Fe2+ was green)

[chem-nerd]

well transition metals often have different colours for different oxidation states and most OH- salts are insoluble (covered in unit 1) 

without all the background knowledge, you'd probably be best to do it by elimination of incorrect answers like what has just been done. ie eliminate B and C due to no S (s) and eliminate A due to no change in transition metal oxidation state to explain the new solution colour.

with the background knowledge you had you would have at least been able to narrow it down to a 50-50 chance

[Mao]

you'd know that rust [iron(III)] has a orange/red colour [think of rust]
hence eliminates A,B and C using oxidation numbers.

[bec]

ok, thanks for that

another thing i'm a bit confused with is mass spec...when a question asks you to state the fragments corresponding to particular m/e values, are their particular rules you need to follow about WHERE a compound can be split up?
for example, if i've got propanoic acid and i need to find the fragment corresponding to the peak at m/e=29....how do i know if it's CH3CH2+ or, say, CHO+ ?

thanks

[Mao]

it is more likely to be CH3CH2+

the CHO+ ion [in this case referring to propanoic acid] would involve fragmenting -COOH from the molecule, severing the OH from -COOH, then attaching a H+. this process would require a lot of energy, hence not very likely [but not impossible]

on the other hand, CH3CH2+ would just be fragmenting at CH3CH2||COOH, not a big feat in comparison to the other one, hence this will be the answer.

[bec]

ok, that's pretty logical then. thanks!

[Collin Li]

I'm a bit confused with zwitterions. We were taught that in acidic solution, they took on extra protons so there was an NH3+ group and a COOH group, but in solutions for a question i just did it said there was an NH2+[/b] group and COOH group... does it depend on the degree of acidity in the solution?

No, it doesn't depend. NH2+ is just blatantly wrong. Ignore it. It should be NH3+ or NH2.

I disagree. Have a look at proline, which accepts a proton to become NH2+.

Good point. I'm not familiar with many of the alpha amino acids. I always assumed the amine group on alpha amino acids were primary (-NH2).

The amine group will be up by a H+ in acidic solutions, and the carboxylic acid group will be down by a H+ in basic solutions (and bear a negative charge).