Rod's Chemistry 3/4 Questions Thread

[Rod]

Hey guys,

Just a question on the unit 4 course here, in chapters 18-22 we would learn about the production of ammonia, ethane and etc. Apparently those four chapters are like a detailed study? So we only pick one out of the four and only need to know the one of those chapters for the exam?

Thanks

[clıppy]

Detailed studies have been removed from the exam (from the 2013-2016 study design) but your school will still have a SAC on it. Your teacher will probably tell you which one your class will be studying as the SAC will be on that.

[Rod]

Detailed studies have been removed from the exam (from the 2013-2016 study design) but your school will still have a SAC on it. Your teacher will probably tell you which one your class will be studying as the SAC will be on that.

So we don't necessarily need to know a single chapter from 18-22 for the exam? But will need to study one for a SAC during school? That's pretty good!

[Edward21]

So we don't necessarily need to know a single chapter from 18-22 for the exam? But will need to study one for a SAC during school? That's pretty good!

Yeah you can look past those chapters Forget ethene completely, it's not existent on the study design now, you'll either study sulfuric or nitric acid or ammonia production. If you study sulfuric, contact me and I can send you the questions I had on my SAC to practice ALSO try and get your hands on checkpoints 2010,11,12 [any of those unit 4 ones] (not 2013-14) it has a whole section in the Unit 4 area for these detailed studies (question banks) that the later combined 3/4 versions from 2013 don't have..

[Rod]

Yeah you can look past those chapters Forget ethene completely, it's not existent on the study design now, you'll either study sulfuric or nitric acid or ammonia production. If you study sulfuric, contact me and I can send you the questions I had on my SAC to practice ALSO try and get your hands on checkpoints 2010,11,12 [any of those unit 4 ones] (not 2013-14) it has a whole section in the Unit 4 area for these detailed studies (question banks) that the later combined 3/4 versions from 2013 don't have..

Awesome! Thank you !

[Rod]

Hey guys, can I please get some help on UV-visible spectrometry;

So I'm just going to jot down some stuff I know, please let me know if any of my explanations are incorrect, and please add any other theory that I have not included.

I am finding this part of chemistry very hard to get my head around, and extremely boring.

Okay, so UV-visible spectrometry is one of six types of spectrometry techniques we learn in the chemistry course. It can be used for qualitative analysis, but mainly quantitative analysis. The first thing I was thinking of before reading 'mainly used for quantitative analysis', I was just like 'for f*** sake, why can't we just use AAS then, why do we have to learn this useless stuff?' - But the reason why we use UV instead of AAS because apparently an AAS machine costs a million while a UV costs 600 dollars, so that's one difference, and the other because AAS cannot analyse samples that are organic and non-metal, but a UV-visible can.

In simple steps, when a scientist is to use a UV-visible spectrometer, he would first analyse the sample to find out what it is, and also what colour and wavelength of light the sample best absorbs. He would then, by using many samples create a absorbance versus concentration curve, which the scientist can then use to find the unknown. Beer's law (A=ecl) can also be used in this situation.

In complex steps, when a scientist is to use a UV-visible spectrometer, he shines the light source into a monochromator, the monochromator chooses the best wavelength and colour of light to go into the sample solution. The absorbance of the light/wavelength is then measured my a detector which records the results.

Ok so that's all I know. I wasn't able to incorporate what a 'reference cell' is into it. And it felt terrible when I was typing it up, I didn't use my notes/text book but I had to really take my time in writing it up.

Thanks everyone.

[PB]

A reference cell is basically the cell that is being analysed, except without the actual analyte. So this includes the glass test tube and whatever solvent was used to dissolve the organic analyte, but NONE of the analyte itself. The chemist usually runs the reference cell through the spectrometer before the analysis of the actual analyte so that the absorbance of the glassware and the solvent can be recorded. Then the computer does some weird stuff with this knowledge, so that when you actually analyse the analyte, the absorbance readings would be solely attributed to the absorbance of the substance you are analysing! 
So basically, the purpose of the reference cell is to factor in and remove any interferences from the testtube and the solvent in which the organic analyte being held in We call these undesirable interferences - background interferences.

[PB]

when a scientist is to use a UV-visible spectrometer, he would first analyse the sample to find out what it is, and also what colour and wavelength of light the sample best absorbs.

You need to know how he finds out the identity of the chemical! Basically, the spectrometer scans the substance with the entire UV- Visible spectrum of light, producing a characteristic graph of Absorbance vs Wavelength unique to the compound. The graph looks something like this http://commons.wikimedia.org/wiki/File:UV-vis_spectrum_of_host-guest_binding.jpg
The computer can then match the graph to its database of graphs and match the shape to its couterpart molecule.

Also, through this graph, you will be able to find out which wavelength of light (eg. colour - both the same thing) the compound absorbs best. This would be the x-value of the maximum turning points on the graph I linked you above.
Hope that helps

[Rod]

A reference cell is basically the cell that is being analysed, except without the actual analyte. So this includes the glass test tube and whatever solvent was used to dissolve the organic analyte, but NONE of the analyte itself. The chemist usually runs the reference cell through the spectrometer before the analysis of the actual analyte so that the absorbance of the glassware and the solvent can be recorded. Then the computer does some weird stuff with this knowledge, so that when you actually analyse the analyte, the absorbance readings would be solely attributed to the absorbance of the substance you are analysing! 
So basically, the purpose of the reference cell is to factor in and remove any interferences from the testtube and the solvent in which the organic analyte being held in We call these undesirable interferences - background interferences.

Understood!!! You are really good at explaining stuff  , thank you so much!

[PB]

Understood!!! You are really good at explaining stuff  , thank you so much!

I do try.. X-)  haha jks, thanks! I appreciate it

[Rod]

You need to know how he finds out the identity of the chemical! Basically, the spectrometer scans the substance with the entire UV- Visible spectrum of light, producing a characteristic graph of Absorbance vs Wavelength unique to the compound. The graph looks something like this http://commons.wikimedia.org/wiki/File:UV-vis_spectrum_of_host-guest_binding.jpg
The computer can then match the graph to its database of graphs and match the shape to its couterpart molecule.

Also, through this graph, you will be able to find out which wavelength of light (eg. colour - both the same thing) the compound absorbs best. This would be the x-value of the maximum turning points on the graph I linked you above.
Hope that helps

Damn I missed this post! Just got to read it!

Rightyo, so to sum up:

Reference cell:
A reference cell is used during UV-visible spectroscopy and infra-red spectroscopy. They are ultimately used in order to bring 100% accuracy of the analyte's absorption of the energy, either UV light or infra red radiation. A scientist can do this by first shining the light into the reference cell, which does not contain the analyte, the computer can then determine any disturbances that absorb the light as well as how much the cell absorbs. Then when calculating, the computer can take this away from the normal results so ONLY the analyte's absorbance is recorded.

Identifying a substance via UV spectroscopy:
In order to qualitatively determine the substance via UV spectroscopy, the scientist must expose all wavelengths and colours of UV light in the electromagnetic spectrum and then record it in a absorbance verses wavelength graph. He then can compare this to his database of graphs and determine what the substance is. He can do this because indentical substances will always have the same absorbances to the same wavelengths of lights.

Are you happy with that?
Thanks again

[PB]

Preeecisely Good work!

[Rod]

So I was reading about some uses of infra-red spectroscopy, and one of them being that is is used for forensic science. Apparently in crime scenes they  analyse carpet and find the molecular structure of the fibre, and I do understand how they can do that. But how the hell would that solve anything? It's not like the murderer/rapist had brought in carpet from his house to use during the crime? Have I missed something?

Thanks

[PB]

lols, ignore all the other crap about it is used in CSI lab or how it is used to convict a murderer XD, it will NOT be tested by VCAA obviously. Just focus on how IR works. Infra-red can be used to qualitatively determine the identity of a molecule and that has many uses in identifying organic compounds!

Btw, just a heads up - forensic investigations was removed from the course last year so you don't have to waste your time studying it in the textbook

[Rod]

Hey guys,

So I'm just having some trouble with NMR. I know how to analyze and apply it, I know how to get the shift, split, size and environment, I know the functions of each and finally I know how to interpret a HNMR graph to find out the complete structure of a biomolecule. I also know how to interpret a CNMR graph, know the instrumentation well and have got all questions in the textbook correct. Is this all I have to know about it?

I have been struggling with the theory. I have been spending loads of time trying to understand all the processes of the NMR and how it actually works. Need some help. To be specific, I don't understand the spin stuff. So yeah, an atom has a spin and different molecules have different intakes of radiowave energy levels to change that spin. But how does that help us achieve the NMR graph? I don't understand the changes in magnetic fields, and overall how all this spin and energy stuff brings out the graph. Finally I don't understand the TMS, PB thought me what a reference cell is, and apparently a TMS acts as a reference cell, but how? And can someone please help me visualise what's actually happening when an atoms's particles 'spin', or 'change spin' or achieve magnetic fields.

Thank you