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March 29, 2024, 10:01:24 am

Author Topic: VCE Chemistry Question Thread  (Read 2313454 times)  Share 

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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #1080 on: July 01, 2014, 08:32:26 pm »
+1
Just to clarify something:

In NMR, is chemical shift proportional or inversely proportional to frequency of the radio waves?

I thought that shielded atoms experience the magnetic field of the NMR machine to a lesser extent, and hence require less energy for resonance to be induced (ie. chemical shift is proportional to frequency), but TSFX says the opposite, that is shielded atoms require more energy for resonance to be induced. Does the discrepancy arises due to there apparently being 2 kinds of NMR machines (ones where the magnetic field varies and ones where the radio wave frequency varies)?
Please correct me if any of these information is wrong. And I believe VCAA does not require too much details about these:

Radio waves with sufficient energy can break the magnetic field, causing the nuclei to “flip” and create a peak.
Normally in NMR, they might vary the magnetic field so that if you have a high magnetic field, radio waves strength will not be strong enough to cause flipping. And if you have a low magnetic field, radio waves will be strong enough to cause flipping.
However, since different nuclei experiences different chemical environment, different magnetic field is needed in order for that nuclei to flip. Electrons play a vital role in the chemical environment because they shield the nucleus from the magnetic field, forcing them to experience less energy, hence, larger magnetic field strength is needed.
In short, a highly “exposed” nucleus requires lower magnetic field to cause flipping (especially those that close to a high electronegativity atom because these atoms will draw the electrons from the nucleus away). You can called these being “deshielded”.
Now, in the NMR spectrum, on the LHS, we call that the “downfield” region and on the RHS, we call that the “upfield” region. Downfield is the region of nuclei that experiences the MOST magnetic field and upfield is the region of nuclei that experiences the LEAST magnetic field. Since chemical shift is measure in ppm, we can say that when a peak is 7ppm, it means the magnetic field required to cause flipping of the nucleus is 7 millionths less than that of the TMS. Hence, higher chemical shift (the more downfield it is), the more exposed a nucleus.
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1081 on: July 01, 2014, 08:42:51 pm »
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Please correct me if any of these information is wrong. And I believe VCAA does not require too much details about these:

Radio waves with sufficient energy can break the magnetic field, causing the nuclei to “flip” and create a peak.
Normally in NMR, they might vary the magnetic field so that if you have a high magnetic field, radio waves strength will not be strong enough to cause flipping. And if you have a low magnetic field, radio waves will be strong enough to cause flipping.
However, since different nuclei experiences different chemical environment, different magnetic field is needed in order for that nuclei to flip. Electrons play a vital role in the chemical environment because they shield the nucleus from the magnetic field, forcing them to experience less energy, hence, larger magnetic field strength is needed.
In short, a highly “exposed” nucleus requires lower magnetic field to cause flipping (especially those that close to a high electronegativity atom because these atoms will draw the electrons from the nucleus away). You can called these being “deshielded”.
Now, in the NMR spectrum, on the LHS, we call that the “downfield” region and on the RHS, we call that the “upfield” region. Downfield is the region of nuclei that experiences the MOST magnetic field and upfield is the region of nuclei that experiences the LEAST magnetic field. Since chemical shift is measure in ppm, we can say that when a peak is 7ppm, it means the magnetic field required to cause flipping of the nucleus is 7 millionths less than that of the TMS. Hence, higher chemical shift (the more downfield it is), the more exposed a nucleus.
Hmm is that what ppm means? Alright, thanks for the reply. So it's the magnetic field that's used to flip the nuclei and not the radio waves?

Another question:

Why does 1,2-dimethylbenzene have only 2 hydrogen environments? No matter how I look at it I get 3.
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #1082 on: July 01, 2014, 09:18:47 pm »
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Hmm is that what ppm means? Alright, thanks for the reply. So it's the magnetic field that's used to flip the nuclei and not the radio waves?

Another question:

Why does 1,2-dimethylbenzene have only 2 hydrogen environments? No matter how I look at it I get 3.
Yeah, according to my knowledge, in NMR, we can either vary the magnetic field and keep the radio wave constant OR vary the radio wave and keep the magnetic field constant. If we keep a magnetic field constant and vary the radio wave frequency, different nuclei will resonate at different frequencies, then, we can measure this resonance and plot the graph. However, as long as I'm aware,in practice, it's it pretty easier to keep the radio wave constant and vary the magnetic field.


« Last Edit: July 01, 2014, 09:25:30 pm by nhmn0301 »
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1083 on: July 01, 2014, 09:27:35 pm »
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Yeah, according to my knowledge, in NMR, we can either vary the magnetic field and keep the radio wave constant OR vary the radio wave and keep the magnetic field constant. If we keep a magnetic field constant and vary the radio wave frequency, different nuclei will resonate at different frequencies, then, we can measure this resonance and plot the graph. However, as long as I'm aware,in practice, it's it pretty easier to keep the radio wave constant and vary the magnetic field.

If you split them in half between the 4 and 5 C, you see that they are symmetrical from both sides and hence, it has 2 H environments.
Ok thanks!

Really? But aren't the 3 and 6 hydrogens, the 4 and 5 hydrogens and the methyl hydrogens exposed to different chemical environments?
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #1084 on: July 01, 2014, 09:56:10 pm »
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Ok thanks!

Really? But aren't the 3 and 6 hydrogens, the 4 and 5 hydrogens and the methyl hydrogens exposed to different chemical environments?
Oops sorry, yeah I think there should be 3H environments too :( whilst if we have 1,4-dimethyl benzene, that can be 2.
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1085 on: July 01, 2014, 10:05:31 pm »
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Oops sorry, yeah I think there should be 3H environments too :( whilst if we have 1,4-dimethyl benzene, that can be 2.
tsfx y u do dis :(

http://www.chemicalbook.com/SpectrumEN_95-47-6_1HNMR.htm

The actual spectrum seems to show 2 peaks as well though. Maybe it's something beyond the VCE course?
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nhmn0301

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Aurelian

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Re: VCE Chemistry Question Thread
« Reply #1087 on: July 01, 2014, 10:49:10 pm »
+2
Hey guys,

There are three different environments formally, but in practice two of these environments have pretty much the same chemical shift, because they are very similar. As a result, two of the signals overlap with each other to give a messy looking multiplet at around 7.1 ppm. In light of this fact, in that spectrum from the Chemical Book this signal has been attributed jointly to all four of the aromatic hydrogens.

In short: formally, there are three environments, but in practice two of these environments behave as one.
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Valyria

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Re: VCE Chemistry Question Thread
« Reply #1088 on: July 02, 2014, 11:29:00 am »
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Hey guys,

If a question involving calculations is split into multiple parts and the answer found in part (a) was adjusted to 3 significant figures, in part (b) do we use the adjusted figure or the figure on the calculator that has more than 3 significant figures? I thought the latter as it seems more accurate but a few questions from checkpoints uses the adjusted figure for future calculations.

Thanks :)
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saba.ay

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Re: VCE Chemistry Question Thread
« Reply #1089 on: July 02, 2014, 12:06:40 pm »
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Hey guys,

If a question involving calculations is split into multiple parts and the answer found in part (a) was adjusted to 3 significant figures, in part (b) do we use the adjusted figure or the figure on the calculator that has more than 3 significant figures? I thought the latter as it seems more accurate but a few questions from checkpoints uses the adjusted figure for future calculations.

Thanks :)

Always use the full figure for calculations, as it would be more accurate. But, when writing the final answer for part b) you have to consider the significant figures of that full figure as found in (a). Does that make sense ? Hope so. :P
 

Valyria

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Re: VCE Chemistry Question Thread
« Reply #1090 on: July 02, 2014, 01:16:49 pm »
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Always use the full figure for calculations, as it would be more accurate. But, when writing the final answer for part b) you have to consider the significant figures of that full figure as found in (a). Does that make sense ? Hope so. :P

So basically, the answer in part (b) should contain the same number of significant figures as the answer in part (a)?
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saba.ay

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Re: VCE Chemistry Question Thread
« Reply #1091 on: July 02, 2014, 01:34:25 pm »
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So basically, the answer in part (b) should contain the same number of significant figures as the answer in part (a)?

Assuming that the smallest number of significant figures belongs to the value found in part (a), then yes. If the smallest number of significant figures belongs to another value introduced in part (b), then you'd obviously use that to determine the significant figures for the answer of part (b).

Valyria

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Re: VCE Chemistry Question Thread
« Reply #1092 on: July 02, 2014, 02:24:07 pm »
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Assuming that the smallest number of significant figures belongs to the value found in part (a), then yes. If the smallest number of significant figures belongs to another value introduced in part (b), then you'd obviously use that to determine the significant figures for the answer of part (b).

I thought the least precise piece of information provided within the stem of the question dictated how many significant figures we used for our answers :/
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Re: VCE Chemistry Question Thread
« Reply #1093 on: July 02, 2014, 02:28:44 pm »
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It does, really. Think of it like this: if your least precise piece of information is to two significant figures, you're not going to round below that. That means in part a, the lowest amount of significant figures that you could have is two, because you're not going to go below that. So when you get to part b, you'll be using the lowest amount in the actual question, which is two.

However, consider this scenario: in part a, you round to two figures. But, in part b, you don't use the data from the question that's to two figures, but you do use your answer to part a. That means, the lowest amount of significant figures is coming from your answer to part a, which is two. So, you use that answer to guide your answer to part b.

Hope that all made sense. n.n;

psyxwar

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Re: VCE Chemistry Question Thread
« Reply #1094 on: July 02, 2014, 02:31:56 pm »
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So in gas chromatography the more volatile compounds are eluded first, right? Alkenes are more volatile than alkanes, does that mean they are eluded before alkanes? Asking cuz TSFX lists it the other way round
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