Chemistry 3/4 2013 Thread

[memarani]

Thanks, both of you! 

[9_7]

Hey guys just wondering if you could help me on two questions on volumetric!!

Why isn't an indicator required when the titration is performed using potassium permanganate?? and why do we record the volume of solution A in a burette by reading from the top rather than from the bottom of the meniscus?

Thanks a lot guys! 

[REBORN]

The two redox species (MnO4-/Mn2+) are different colours, so an indicator will not be required.

[Stick]

Teach your cat NMR. You'll gain a better understanding when you explain to others in simple terms.

I teach my mum Biology as a revision tool and she actually understands it. XD So I can verify that this does work.

I think Einstein once said that if you cannot explain a topic simply, you don't know it well enough.

[clıppy]

Could someone please explain how to explain NMR peaks.
When I'm doing NMR and I'm asked to explain why e.g. Peak A is a quartet, how do i explain this fluently?
I know the hydrogen in that environment is being affected by hydrogens in the neighbouring environment (which in this case would be a CH3, correct?) but how do i say this in terms of it being magnetically affected (with the dipoles, and the spinning, and whatnot)

[Alwin]

Could someone please explain how to explain NMR peaks.
When I'm doing NMR and I'm asked to explain why e.g. Peak A is a quartet, how do i explain this fluently?
I know the hydrogen in that environment is being affected by hydrogens in the neighbouring environment (which in this case would be a CH3, correct?) but how do i say this in terms of it being magnetically affected (with the dipoles, and the spinning, and whatnot)

1. Peak A is a quartet, so by the n+1 rule the there must be 3 neighbouring hydrogen atoms (hopefully you can continue on from here)

2. It does not necessarily mean that the neighbour is -CH3. Consider the hydrocarbon: CH3CH2CH2CH(CH3)OH. The hydrogen in bold has 3 neighbouring hydrogens

3. As for the actual reason for splitting, I don't recall this specifically being on the study design (you may want to check this). Most simply, you could say that the neighbouring hydrogen atoms causes a variation of the chemical shifts of hydrogen atoms in that particular bonding environment, but the real real reason is uni chem level, electron spin, cloud density etc. If you are really interested, then I can attempt a simple explanation for you, but I doubt a question asking, "why does peak splitting occur on high resolution NMR occur?" will appear on the exam.

[clıppy]

1. Peak A is a quartet, so by the n+1 rule the there must be 3 neighbouring hydrogen atoms (hopefully you can continue on from here)

2. It does not necessarily mean that the neighbour is -CH3. Consider the hydrocarbon: CH3CH2CH2CH(CH3)OH. The hydrogen in bold has 3 neighbouring hydrogens

3. As for the actual reason for splitting, I don't recall this specifically being on the study design (you may want to check this). Most simply, you could say that the neighbouring hydrogen atoms causes a variation of the chemical shifts of hydrogen atoms in that particular bonding environment, but the real real reason is uni chem level, electron spin, cloud density etc. If you are really interested, then I can attempt a simple explanation for you, but I doubt a question asking, "why does peak splitting occur on high resolution NMR occur?" will appear on the exam.

The first explanation probably satisfies most questions like this for me to answer. It was just confusing because looking at some book answers it was mentioning spinning, and creating tiny magnets which affects other environments.

[lzxnl]

I must stress that the idea of an electron spinning isn't quite accurate. Electrons have wave properties too and we don't really associate waves as spinning, do we? It is true, however, that the "electron spin" is related to its angular momentum.

I teach my mum Biology as a revision tool and she actually understands it. XD So I can verify that this does work.

I think Einstein once said that if you cannot explain a topic simply, you don't know it well enough.

Stick, I'm fairly sure that Einstein knew his theory of general relativity well enough, but I doubt he could explain it simply.

[Alwin]

I must stress that the idea of an electron spinning isn't quite accurate. Electrons have wave properties too and we don't really associate waves as spinning, do we? It is true, however, that the "electron spin" is related to its angular momentum.

Don't mean to nit pick, but think about the context too. this is a chem board and even though NMR uses physical properties not chemical properties for its analysis, wave particle duality is not essential to analysis.

For anyone reading this who doesn't do physics and is thinking what the hell is going on, in essence hydrogen protons align themselves to the powerful external magnetic field, somewhat akin to tiny magnets. However, they can align with or against the field. As the sample is rotated in the field, the hydrogen protons switch alignment, oscillating with then against the magnetic field. The frequency of oscillation (measured as chemical shift) can vary due to electron shielding or strong electronegative element nearby eg oxygen. Neighbouring Hydrogen atoms have a similar effect, but instead of shifting the entire peak like say an oxygen atom in a hydroxyl bond does, it causes variations in chemical shift.

btw carbon 13 NMR is a different theory altogether from 1-hydrogen NMR so don't get me started on that too haha

EDIT: sorry, was in a rush when I typed this so a few of the terms were incorrect. hopefully didn't confuse anyone

[jgoudie]

From my understanding it is not the electron spin you are looking at with NMR it is the 'spin' that the nuclei have, hence 'nuclear' magnetic resonance.  This also explains why NMR can only be performed on atoms with an odd number of nuclei  The basic way of explaining why you have splitting is due to the interference of the neighbouring hydrogens.  The signals get a little disrupted and thus you see the numerous amounts of peaks depending on the level of disruption.

Hope this doesn't confuse anyone, but thought i would just clarify.

Don't mean to nit pick, but think about the context too. this is a chem board and even though NMR uses physical properties not chemical properties for its analysis, wave particle duality is not essential to analysis.

For anyone reading this who doesn't do physics and is thinking what the hell is going on, in essence electrons align themselves to the powerful external magnetic field, somewhat akin to tiny magnets. However, they can align with or against the field. As the sample is rotated in the field, the electrons switch alignment, oscillating with then against the magnetic field. The frequency of oscillation (measured as chemical shift) can vary due to electron shielding or strong electronegative element nearby eg oxygen. Neighbouring Hydrogen atoms have a similar effect, but instead of shifting the entire peak like say an oxygen atom in a hydroxyl bond does, it causes variations in chemical shift.

btw carbon 13 NMR is a different theory altogether from hydrogen-1 so don't get me started on that too haha

[Alwin]

From my understanding it is not the electron spin you are looking at with NMR it is the 'spin' that the nuclei have, hence 'nuclear' magnetic resonance.  This also explains why NMR can only be performed on atoms with an odd number of nuclei  The basic way of explaining why you have splitting is due to the interference of the neighbouring hydrogens.  The signals get a little disrupted and thus you see the numerous amounts of peaks depending on the level of disruption.

Hope this doesn't confuse anyone, but thought i would just clarify.

Yes, you're completely right. Sorry, I was thinking about electron characteristics in Physics and got carried away. Meant to say proton spin which can be effected by electron shielding.

Expanding on your explanation, a spinning charge generates a magnetic field that results in a magnetic moment proportional to the spin. In the presence of an external magnetic field in NMR, two spin states exist with or against the field, +1/2 or -1/2. An even mass number (eg Carbon-12) is unaffected by the field because the nucleons can 'cancel' out each other's spin (simply put, half can spin one way and the other half can spin the other).

Hence, Carbon-13 must be used in 13-Carbon NMR because of this odd number of nucleons gives it an overall spin. As I said previously (this is just a clarification) they act as tiny magnets and flip in the field etc.

Sorry if my first post confused anyone, thanks jgoudie for picking up on my typos.

[jono88]

Just making sure but is 1,1-dimethylethene the same as 2-methylprop-1-ene?

[jgoudie]

That does make sense, however as far as IUPAC naming is concerned the systematic name is 2-methylprop-2-ene.  Due to the longest carbon chain getting the preference (i.e. propene).

Just making sure but is 1,1-dimethylethene the same as 2-methylprop-1-ene?

[FlorianK]

For Lithium-Ion-Batteries, what does the (1-n) mean in the chemical equations with Li_(1-n)

https://en.wikipedia.org/wiki/Lithium-ion_battery

[9_7]

Hey guys!

So my result compared to my teachers result was 0.01% off. (Her's was 0.3% compared 0.29% which was mine) What error could have possibly happened?