ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Greatness on March 24, 2011, 03:41:15 pm
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How well do you think we should know the Spectroscopy/Chromatography like equipment wise, how they work and the steps in using them? And knowing how to interpret the results is the more critical thing yeah?
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I haven't done much spectroscopy yet, but in chromatography, knowing the specific methods is not necessary (just a general knowledge should suffice). You should know (for chromatography) which type of substance works best with each type of chromatography and what the advantages/disadvantages are of each.
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Alright yeah its just that we're going through this stuff so fast in class... We've covered all of chromatography and most spectroscopy techniques in like a 2 weeks (10 lessons).. we're going to finish start/finsh NMR tomorrow. And i dont think that we were goin into enough detail.
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Its pretty hectic, we only spent 1.5 periods on chromatography :o
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Wow you guys are destroying it.
Our class has a SAC in 10 days on chromatography and we've spent one double writing 3 dot points.
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Just want to remind you guys that all the spectroscopy, chromatography in particular, will make a lot more sense and have a far greater application once you've done all the organic chem that's in Unit 3. That's why I'm moving on to organic with my students next week.
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Yeah I agree, Chromatography and Spectroscopy only makes sense combines with organic Chemistry. However, you do need to know about what each part of the equipment are used for. e.g, what is the injection port?
Here's the key knowledge required as copied from the study design
• principles and applications of chromatographic techniques and interpretation of qualitative and
quantitative data from thin layer chromatography (TLC), high performance liquid chromatography
(HPLC) and gas chromatography (GC);
• principles and applications of spectroscopic techniques and interpretation of qualitative and
quantitative data from atomic absorption spectroscopy (AAS), infrared spectroscopy (IR), mass
spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and visible and ultraviolet
spectroscopy (visible-UV);
• matching analytical technique/s to a particular task.
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Yeah I agree, Chromatography and Spectroscopy only makes sense combines with organic Chemistry. However, you do need to know about what each part of the equipment are used for.
+1 Thats why we are doing AOS2 now (right after chromatography) instead of jumping to spectroscopy
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Just want to remind you guys that all the spectroscopy, chromatography in particular, will make a lot more sense and have a far greater application once you've done all the organic chem that's in Unit 3. That's why I'm moving on to organic with my students next week.
I disagree. Most of spectroscopy and chromatography can be understood with unit 1 contents. In fact, most absorption spectroscopy and chromatography have no connection to organic chemistry. Application of absorption spectroscopy and chromatography is simply using a calibration curve. Understanding the principles almost always boil down to the structure of an atom (unit 1), trends in periodic table (unit 1) and intermolecular interaction (unit 1).
Knowledge of organic chemistry is only required for tackling IR, NMR and MS.
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Would methylbutane be the same as pentane?
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Would methylbutane be the same as pentane?
Same molecular formula,
it is a structural isomer?
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hmm yeah a question asked to draw methylbutane, but the answer actually has 2 methyl butane....
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You mean 2-methylbutane right? If so, then that's the systematic name for it.
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Yeah, it'd need to be 2-Methyl Butane. If it were Methyl Butane, then it implies that it is 1-Methyl Butane, in which case it'd have a 5 Carbon chain, and would be called Pentane...
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Yeah. Sorry for tthe confusion. The question wants you to draw the structural formula for methylbutane. But the answer has the structural formula for 2-methylbutane.
So what im asking is methlybutane and 2-methylbutane are different things?
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I've always known it as 2-methylbutane. But now I'm thinking about it, I'm just wondering if it's allowable to exclude the number in front of 2-methylbutane because methylbutane implies that it's attached to the 2nd carbon atom. It can't be 3-methylbutane since you always number from the end with the nearest side chain (so it would be 2-methylbutane) and, 1-methylbutane or 4-methylbutane would just be pentane. Cis and tran doesn't apply here either. Hmm. Can anyone clarify?
In any case, I think your sheet meant 2-methylbutane. ^^
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Yeah. Sorry for tthe confusion. The question wants you to draw the structural formula for methylbutane. But the answer has the structural formula for 2-methylbutane.
So what im asking is methlybutane and 2-methylbutane are different things?
I'd say it's a typo :P
They aren't the same things, just like Propanol = Propan-1-ol, but =/= Propan-2-ol
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There are quite a few errors between quetsoins and answers from the heinemann textbook....
-Zien, i dont understand how methylbutane implies that it is 2-methylbutane :s
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But that example isn't applicable here though. Whereas Propan-1-ol can be oxidised into its corresponding aldehyde and then its corresponding carboxylic acid , propan-2-ol can only be oxidised into its corresponding ketone. They're in totally different 'environments.' 2-methylbutane and 3-methylbutane have the same 'environment', only a naming rule that we start counting from the end with the nearest chain disallows 3-methylbutane as a name. They're the same.
.. at least that's what I'm thinking?
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There are quite a few errors between quetsoins and answers from the heinemann textbook....
-Zien, i dont understand how methylbutane implies that it is 2-methylbutane :s
Zien is saying that the only way a methyl butane can exist is if it's attached to the 2-C, as attached to 1 or 4 would be Pentane, and 3 would be the same as 2-C
I'd still say it's a typo though, I'm fairly sure they don't "assume" these things hahaha
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Ohhh ok :)
Next question: Which is notation is more correct when youre writing chloropropane or 1-chloropropane. I know that it doesnt really matter but ive heard that having the 1 there is safer.
Also if you are given a structural formula how do you know if there is branching? Is it if there is just a CH? Cos i get confused with all the brackets and stuff in those crazy molecules >>
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Lol a) Yeh, I always write 1-chloropropane. You don't want a what bitch examiner or teacher marking you down, for not having it, so better prevention than cure.
b) I think its a matter of practise
CH3 (CH2)3 CHOH (CH2)2 CH3
From there, we'd know that there would be a branching in the CH sector because of the OH, then we'd just count from the shortest carbon chain.
In other cases you'd have
CH3CHCH3(CH2)5CH3
which would be a methyl branching out, indicated by the CH3 adjacent to CH and CH2.
I hope your doing heinemen, page 142, on question 11, they have exercises that help with the branching.
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Alright cool! Yeah i was doing qn 11 took me some time tho :P
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Ohhh ok :)
Next question: Which is notation is more correct when youre writing chloropropane or 1-chloropropane. I know that it doesnt really matter but ive heard that having the 1 there is safer.
Also if you are given a structural formula how do you know if there is branching? Is it if there is just a CH? Cos i get confused with all the brackets and stuff in those crazy molecules >>
There are some molecules where the two types of notation are less vital [ but I would still be safe and write the number] (e.g. 2-chloro-2-methylbutane vs 2-chloromethylbutane because the methyl group has to be on carbon 2.) BUT with the example you gave (chloropropane) there are different types of chloropropane so you HAVE to write 1-chloropropane or 2-chloropropane, because chloropropane is ambiguous.
And yes, the most helpful thing to remember is that each C atom usually has a total of 4 bonds - so if it only has one H atom bonded to it, then its 3 other bonds are probably to another C or a heteroatom/s.
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Zien and scocliffe09 are correct. the '2' in 2-methylbutane is redundant. Naming only needs to go so far until there are no ambiguities in the structure. methylpropane is also fine. methylpentane is not okay (since there is 2-methylpentane and 3-methylpentane), however ethylpentane is okay.
Substituted functional groups generally always require numbering. This is since they can substitute at any carbon, hence for anything larger than 2 carbons they will always require a number.
Not having a number does not imply substitution at position 1.
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Thanks Mao. Looks like ill have to make writing the carbon no.s a habit!
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That reminds me, I was wondering, unless the question specifically asks to 'show all bonds' will they accept line drawings for organic molecules?
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That reminds me, I was wondering, unless the question specifically asks to 'show all bonds' will they accept line drawings for organic molecules?
I would assume so. It is after all what every chemist uses. :P
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That reminds me, I was wondering, unless the question specifically asks to 'show all bonds' will they accept line drawings for organic molecules?
My teacher said to avoid it
(by line drawing, I assume you mean just drawing the skeleton of the molecule, ie. no carbons or hydrogens shown)
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i think they'd allow it o.o, it looks much cleaner (of course unless they specify to show all bonds)
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Chrom/Spec tech questions:
1) Do we need to know the difference of normal phase and reverse phase chromatography? Or are tsfx just being ghey?
2) Has anyone encountered a question that requires you to calculate the area of a peak so A=1/2*b *h? Hence we can find the % of a certain substance
3) Shuold we know thee relationships between wavelength, frequency and energy
4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol
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Chrom/Spec tech questions:
1) Do we need to know the difference of normal phase and reverse phase chromatography? Or are tsfx just being ghey?
It can't hurt to be aware of RP. Reverse phase was on the 09 exam, and is legit course material. The main thing is to understand the implications of reverse phase i.e. molecules are eluted in the opposite order.
2) Has anyone encountered a question that requires you to calculate the area of a peak so A=1/2*b *h? Hence we can find the % of a certain substance
I don't remember doing any questions like this but I spose it's legitimate again.
3) Shuold we know thee relationships between wavelength, frequency and energy
Yes, it's basic physics and important to differentiating between NMR, IR and UV/vis/AAS. long wavelength = low frequency = low energy. short wavelength = high frequency = high energy
4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
often a prism is used to split light into a 'rainbow' i.e, to spread light out, then a slit is used to select a specific wavelength. otherwise a machine can be used to filter the light so only a single wavelength can get through.
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol
Yep pretty much - I would just change the "measures the amount when the substance" to " measures the amount of light leftover after the substance has absorbed some of the light"
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1) Pretty sure we aren't. Came across a Q in a prac exam with it, but it explained how it works enough to answer the question...
2) Nope. Peaks are usually too steep, so peak height is used (from what I've seen)
3) Pretty sure we don't, although it's probably handy to know what wavenumber is
4) I'll wait for someone else to answer this question properly...
5) Yeah that's pretty much correct, they'll first run a test sample that is *pure* so as to erase any indifferences caused by the atmosphere and so on, and then this will be calibrated to 0% absorbance. Then, when the actual sample is run, it will be comparable to this 0% calibration...
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Sweeeet! See Luke ive been studying for chem :P
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Sweeeet! See Luke ive been studying for chem :P
Good good, I'll pester you tomorrow.
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4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol
4) There are a number of techniques. The most common one I've seen in questions uses the ol' prism for dispersion; you physically just split up all the light into its component wavelengths. Other things I've seen are fancy mirrors, which from what I can tell in essence do the same thing. I can't imagine we need to know details though.
5) If we're going to be technical, that's not quite correct. Absorbance is the negative log of the transmittance, the transmittance being the ratio of the intensity of the light after over the intensity of the light before. Thus, absorbance isn't merely the "difference" between the two values =)
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Thanks Aurelian! Yeah i know we dont need to know the finer details, but personally i find that by knowing how something works it help me understand/remember stuff more easily.
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Also, since the interference from the flame is constant and reproducible, we can eliminate it by 'chopping' the light into pulses. The pulses of light are transformed into alternating current, detected and recorded, whilst the flame interference is transformed into direct current and not recorded.
^Might want a second opinion on that though; I'm still quite ambiguous over all this extra detail. :p
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Hmm i read about 'chopping' the light, but yeah we probably dont need to go into this much detail :P
Another question: If we have to find mols, masses etc of something that has a really big molecular formula, can we just write something like 'let NiC8H14O4 or whatever = x'? So then yuo can just write: n(x), m(x), C(x) etc.
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Hmm i read about 'chopping' the light, but yeah we probably dont need to go into this much detail :P
Another question: If we have to find mols, masses etc of something that has a really big molecular formula, can we just write something like 'let NiC8H14O4 or whatever = x'? So then yuo can just write: n(x), m(x), C(x) etc.
Where'd you read about 'chopping' the light up into pulses? :O
To answer your question, I'd assume so, as long as its legible? It's a risk though.
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It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.
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It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.
Also, (for UV), they have a mirror-like prism that deflects the beam into a sample cell and the cell in question, which will instantaneously measure the interference present...
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It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.
I think that's correct; the light is pulsed so that the emitted light doesn't interfere with the detector (or so i was told :) )
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in the reaction: ethene + HCl -> 1-chloroethane, what state is the HCl in?
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Aqueous, as always.
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o.O that's what i thuoght but apparently you have to use heat for the reaction or something, so it's in gaseous state... Well thats what my teacher said (something along those lines)
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To calculate the concentration of somethiing in g L^-1 we multiply the concentration of x by its molar mass?? :/
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Yup!
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1) How is galactose found in the body - like in what form? Because in my notes it says it's not found as a free monosaccharide.
2) My teacher said you have to becareful when you call a bond an 'amide' or 'peptide' link. He said someting along the lines of, you use amide if youre talking abuot general CONH bonds, but peptide link when its in a protein. Are examiners fussy about this?
3) Do we have to know that glucose has 5 OH groups etc?
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1) How is galactose found in the body - like in what form? Because in my notes it says it's not found as a free monosaccharide.
2) My teacher said you have to becareful when you call a bond an 'amide' or 'peptide' link. He said someting along the lines of, you use amide if youre talking abuot general CONH bonds, but peptide link when its in a protein. Are examiners fussy about this?
3) Do we have to know that glucose has 5 OH groups etc?
1) I'm pretty sure we break poly/disaccharides down, which is how galactose is present. E.g. as lactose (present in milk, for example) is broken down into glucose and galactose, the body now has to deal with both of those monosaccharides.
2) Yep. Can't hurt to get it right. ANY CONH bond is an amide bond, no matter where it is. If it's in a protein, we can call it a peptide bond too. Same with ether bonds - when they're in carbohydrates, suddenly we call them glycosidic linkages.
3) It's in the data book, so yes :)