Please Help-Chem Questions.

[bec]

now a question about NMR...
Why can NMR only be used to detect substances with an odd mass number? isn't it just the protons that are affected by the magnetic field? so, provided the number of protons is odd, why is it relevant how many neutrons there are?

[Collin Li]

You don't need to know why. It's got to do with the fact that electrons exist in pairs in an orbital (remember that orbitals hold  0, 1 or 2 electrons?). When they are paired, their spins are opposite and "cancel" each other out, but when they are unpaired this does not happen. Therefore, odd numbers (i.e: there are unpaired electrons) are important to the operation of NMR.

[bec]

But wouldn't that mean that even mass numbers are acceptable sometimes?
eg. Could you use NMR to analyse nitrogen, since even though its mass number is 14, there are 7 electrons/protons...

[Collin Li]

No idea. Neutrons might have some influence on magnetic activity, after all they are, roughly speaking, a proton plus an electron.

[bec]

hmm ok i won't worry about it then, thanks coblin

[bec]

i'm trying to write summary notes for chem and i've realised i have a lot of questions...here's batch #1....

1. Why is AAS more accurate than AES: couldn’t you use the detecting instrument, and the same setup, in AES?

2. UV-Vis Can be used for atoms, ions and molecules…………why is that special? Don’t the others? If not, why not?

3. In IR-spec, the equipment includes a “sample and reference cell or disc made out of NaCl, KBr or similar" – why? What’s “similar” to NaCl?

4. Do we need to know what sp², sp³ stretches are?

[Collin Li]

1. AAS and AES are like apples and trucks. You seem to continue to confuse the two. AAS is more comparable with UV-visible as they are both types of absorption spectroscopy. On the other hand, AES is emission spectroscopy, and in VCE you really only need to be aware of the qualitative aspect of AES. It has quantitative abilities, and I think the Heinemann textbook has a good bedtime story read about it. AES is more comparable to the flame test (and it is strictly better than it, except in cost and time perhaps).

2. Where does it say it is "special"? AAS is restricted to metals only (because of the cathode lamp, which requires metal cathodes to operate it). GC and HPLC are better suited to molecules. Molecules are most susceptible to chromatography because of polarity and non-polarity. In that way, I guess UV-visible is special, since it can deal with anything - as long as it absorbs any wavelength of light in the UV-visible spectrum.

3. Similar compounds to NaCl are just relatively inert salts. If you remember from electrochemistry, these ions are weak reductants and oxidants, which means they will not react very readily. They are stable compounds, similar to the types of compounds that you would also use in a salt bridge.

4. Possibly, but not by those names. You should know what a "stretch" is, but you definitely do not need to know about the "sp" part. That is something you will learn in first year university.

[bec]

Thanks Coblin

You're right, I always get confused with AAS and AES...
Is this pretty much right?

AES:
- Like a flame test but with hotter flame, slit (NOT wavelength selector), prism that splits emitted energies into emission spectrum (coloured lines, black background)
- Only qualitative (for VCE...i checked in the textbook)

AAS:
- Hollow source cathode lamp (made of same metal as analyte) "beams" through a flame
- Sample sprayed into flame --> atomic vapour
- Monochromator and slit select WAVELENGTH (as opposed to just the slit in AES)
- Machine measures how much of that wavelength was absorbed
- Analyse sample solutions, draw calibration curve, plot point of sample to find concentration

[Collin Li]

Sounds pretty good to me. Try to remember it diagrammatically (AAS particularly) rather than in a series of rote-learned dot points. It will help you understand it better. Perhaps you already have, and you're just translating it into words for me to see, which is great!

[bec]

Sounds pretty good to me. Try to remember it diagrammatically (AAS particularly) rather than in a series of rote-learned dot points. It will help you understand it better. Perhaps you already have, and you're just translating it into words for me to see, which is great!

woohoo, finally. and yeah, i know what the setup looks like and haven't memorised any dotpoints, i just wrote them out here because i can't use pictures...

anyway, second round of questions:

1. How does quantitative analysis with IR work? (p95 of heinemann, there's about a paragraph on it). My understanding: Choose a significant peak on the spectrum of "unknown" analyte; figure out what it is (let's say it's CH₃); plot a series of samples with known CH₃ concentration; read the CH₃ concentration of unknown off the graph. Is that about right? We never did it at school...

2. NMR is "the only technique that can be used to determine exact 3D structure of biological molecules that cannot be crystallised"....what does this even mean? What's crystalisation, and why does it help you analyse things?

3. Do we need to know/understand the relationship C=λv?

4. For mass spec, how much do we need to know about the process of fragmentation and the formation of free radicals? Because basically all I know is that there's an electron beam, it shoots electrons off atoms (somehow) to make them into cations, electrons are now unpaired so they split off into two parts, one part is a cation, one is a free radical...the radical gets vaccuumed away and the cation is detected and plotted on the graph? And this can happen in a variety of different combinations, hence the different peaks on the spectrum?

[Mao]

1. How does quantitative analysis with IR work? (p95 of heinemann, there's about a paragraph on it). My understanding: Choose a significant peak on the spectrum of "unknown" analyte; figure out what it is (let's say it's CH₃); plot a series of samples with known CH₃ concentration; read the CH₃ concentration of unknown off the graph. Is that about right? We never did it at school...

that is roughly right

it is exactly the same principle as other spectroscopy, where known concentrations are used to make a calibration curve. It is very similar to UV-Vis, but where that graph is concentration vs absorbance at a particular wavelength, this is concentration vs transmittence at a particular wavenumber.

The initial IR reading serves to identify which wavenumber should be used (a particular peak), much like how the UV Vis machine is first optimized to a particular wavelenth to allow maximum absorbance.

3. Do we need to know/understand the relationship C=λv?

from my understanding, we only need to understand that higher wavelength means lower frequency and lower energy, vice versa. We wouldnt be needed to use the actual formula.

4. For mass spec, how much do we need to know about the process of fragmentation and the formation of free radicals? Because basically all I know is that there's an electron beam, it shoots electrons off atoms (somehow) to make them into cations, electrons are now unpaired so they split off into two parts, one part is a cation, one is a free radical...the radical gets vaccuumed away and the cation is detected and plotted on the graph? And this can happen in a variety of different combinations, hence the different peaks on the spectrum?

that is already far more than required. What we need to know is that the compound is ionised in a flame and becomes a cation (dont need to know how), and in the process it may be broken down into smaller parts. Then they are sent down a curved tube with magnetic plates on both sides. The cations curves according to the mass/charge ratio (z), which means they move at different angles, and reach the detector at different positions (or hit the wall, in which case the magnetic plates are adjusted and the detector recalibrated accordingly), and this gives a graph of relative percentage abundance vs mass/charge. For the purpose of this course, we also need to know how to identify the peaks (guessing/from quesion/data from other analytical techniques, etc)

[bec]

great, thanks mao!

so with mass spec, we probably wouldn't need to understand what this is on about? (page 113 from the book)

The relative intensities of the ions depend on:
- the energy of the bombarding electrons
- the stability of the ion fragments formed
- the ease with which ions can lose atoms.


...because i don't. And I've pretty much given up being curious about chem now and just want to learn what I absolutely have to...which is a bit sad but pretty much necessary...

[Mao]

bec that is VCE, uni is a lot better =D

I have never heard of any of those things, and I dont think you'll need them

the study design for Chem is very vague, but this is what it says:

principles and applications of spectroscopic techniques and interpretation of qualitative and quantitative data from...

my interpretation is the general principle of how it works, what it works with, what are the results, and how to read the results...
that applies to all the techniques

[bec]

awesome, it sounds pretty easy when you put it like they did in the study design!
i'm starting to think i complicate things by about 1000%....

[Collin Li]

2. NMR is "the only technique that can be used to determine exact 3D structure of biological molecules that cannot be crystallised"....what does this even mean? What's crystalisation, and why does it help you analyse things?

You don't need to worry about this: it's just a contextual statement that is demonstrating the power of NMR. To crystallise something basically means you have the ability to create a purified sample of the product. If you can do that, usually you can identify what it is - because of what type of reagents you need, etc. NMR doesn't require this - it feels the 3d shape through magnetism, rather than through chemical reaction.