AQA CHEM4 - 13th June 2012
Chemistry exam discussion - share revision tips in preparation for GCSE, A Level and other chemistry exams and discuss how they went afterwards.
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Re: AQA CHEM4 - 13th June 2012thanks but why don't the two ch2 groups count as one?(Original post by Casshern1456)
because you get 5 groups starting from the most electronegative substance the oxygen, so when you count the protons at every different occurring position along the chain the Hydrogen it is in will affect how many peaks there are. The 2 CH3 group on the second Carbon count as 1 for example.
Attachment 153061 -
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Re: AQA CHEM4 - 13th June 2012What I meant by the two CH3 group counting as one is because they're both attached to the same Carbon atom only in a different position they are not further away from each other like the other Hydrogen's. So the Oxygen affects them equally thus they are only 1 peak.(Original post by rss.914)
thanks but why don't the two ch2 groups count as one? -
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Re: AQA CHEM4 - 13th June 2012I was also thinking the same thing, as they would both give a triplet (area/integration value 2)(Original post by rss.914)
thanks but why don't the two ch2 groups count as one?
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Re: AQA CHEM4 - 13th June 2012See above(Original post by Casshern1456)
What I meant by the two CH3 group counting as one is because they're both attached to the same Carbon atom only in a different position they are not further away from each other like the other Hydrogen's. So the Oxygen affects them equally thus they are only 1 peak.
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Re: AQA CHEM4 - 13th June 2012I will ask some of my Oxbridge chem friends - they're bound to know!(Original post by rss.914)
im still slightly confused lol
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Re: AQA CHEM4 - 13th June 2012Is this the Nelson Thornes book? if so, what's the page number? I'll help(Original post by swert)
can anyone help with the exam style question on the aqa book for unit 4 first chapter....1st quetion work oout the order of reaction with resperct to A how would i do such a quetion i worked out what the other two are but not A
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Re: AQA CHEM4 - 13th June 2012This is the reply of my friend...(Original post by rss.914)
im still slightly confused lol
The molecule isn't fully symmetrical so it still has a far reaching effect
The groups on the outside of the C=O are different and they have an effect on the nmr of the groups inside
They do have the same splitting pattern, but in different peaks - it's asking how many overall peaks there are, not for the splitting pattern
This website explains it well...
Hydrogen atoms attached to the same carbon atom are said to be equivalent. Equivalent hydrogen atoms have no effect on each other - so that one hydrogen atom in a CH2 group doesn't cause any splitting in the spectrum of the other one.
But hydrogen atoms on neighbouring carbon atoms can also be equivalent if they are in exactly the same environment. For example:
IMAGE
These four hydrogens are all exactly equivalent. You would get a single peak with no splitting at all.
You only have to change the molecule very slightly for this no longer to be true.
IMAGE
Because the molecule now contains different atoms at each end, the hydrogens are no longer all in the same environment. This compound would give two separate peaks on a low resolution NMR spectrum. The high resolution spectrum would show that both peaks subdivided into triplets - because each is next door to a differently placed CH2 group.
http://www.chemguide.co.uk/analysis/...ghres.html#top (scroll down to the bottom to see the images)
Hope this helps!
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Re: AQA CHEM4 - 13th June 2012ohhh i see, thanks alot(Original post by skylight17)
This is the reply of my friend...
The molecule isn't fully symmetrical so it still has a far reaching effect
The groups on the outside of the C=O are different and they have an effect on the nmr of the groups inside
They do have the same splitting pattern, but in different peaks - it's asking how many overall peaks there are, not for the splitting pattern
This website explains it well...
Hydrogen atoms attached to the same carbon atom are said to be equivalent. Equivalent hydrogen atoms have no effect on each other - so that one hydrogen atom in a CH2 group doesn't cause any splitting in the spectrum of the other one.
But hydrogen atoms on neighbouring carbon atoms can also be equivalent if they are in exactly the same environment. For example:
IMAGE
These four hydrogens are all exactly equivalent. You would get a single peak with no splitting at all.
You only have to change the molecule very slightly for this no longer to be true.
IMAGE
Because the molecule now contains different atoms at each end, the hydrogens are no longer all in the same environment. This compound would give two separate peaks on a low resolution NMR spectrum. The high resolution spectrum would show that both peaks subdivided into triplets - because each is next door to a differently placed CH2 group.
http://www.chemguide.co.uk/analysis/...ghres.html#top (scroll down to the bottom to see the images)
Hope this helps!
so you know if they asked the number of peaks for
ch3-co-ch2-ch2-co-ch3 then there would only be two peaks right because its symmetrical? -
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Re: AQA CHEM4 - 13th June 2012page 17 question 1 and thanks(Original post by skylight17)
Is this the Nelson Thornes book? if so, what's the page number? I'll help
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Re: AQA CHEM4 - 13th June 2012That's correct - 2 peaks (The two CH3 groups are in the same environment, as are the CH2).(Original post by rss.914)
ohhh i see, thanks alot so you know if they asked the number of peaks for
ch3-co-ch2-ch2-co-ch3 then there would only be two peaks right because its symmetrical? -
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Re: AQA CHEM4 - 13th June 2012for the formation of a racemic mixture does it only occure from attack of a planar c=o or from a planar carbocation?(Original post by skylight17)
That's correct - 2 peaks (The two CH3 groups are in the same environment, as are the CH2). -
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Re: AQA CHEM4 - 13th June 2012A - 2(Original post by swert)
page 17 question 1 and thanks
B - 1
C - 0
Some of these questions it's easier to work out the later parts of the question and then come back to the first part (as in this example)...
Take experiment 2 ---> 3
We know that C is order 0 - that can be ignored
B is order 1 - when conc is doubled, rate doubles. Do this calculation to the initial rate (3.2 x 10-3) x 2 = 6.4 x 10-3
To get from 6.4 to 1.6 it has gone down by 4 - from looking at conc of A (this has halved) it must be rate order 2. Think of it in reverse; going down 2x will make rate go down 4x
Does that make sense? -
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Re: AQA CHEM4 - 13th June 2012A racemate will be formed in a reaction when a trigonal planar reactant or intermediate is approached from both sides by an attacking species.(Original post by rss.914)
for the formation of a racemic mixture does it only occure from attack of a planar c=o or from a planar carbocation?
There is an equal chance of attack from above or below the plane of the carbon atom (C=O) - both enantiomers are formed (racemic mixture)
It can also be formed by electrophilic addition of HBr to an unsymmetrical alkene - the bromide can attack the planar carbocation from both sides.
So the answer to your question is yes!
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Re: AQA CHEM4 - 13th June 2012yes sort of i will follow your steps see if i cna figure the rest out...thanks dude(Original post by skylight17)
A - 2
B - 1
C - 0
Some of these questions it's easier to work out the later parts of the question and then come back to the first part (as in this example)...
Take experiment 2 ---> 3
We know that C is order 0 - that can be ignored
B is order 1 - when conc is doubled, rate doubles. Do this calculation to the initial rate (3.2 x 10-3) x 2 = 6.4 x 10-3
To get from 6.4 to 1.6 it has gone down by 4 - from looking at conc of A (this has halved) it must be rate order 2. Think of it in reverse; going down 2x will make rate go down 4x
Does that make sense? -
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Re: AQA CHEM4 - 13th June 2012when drawing the repeating unit of the polyester Terylene that is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.why is it just c=o on one size of the benzene ring and O=CH2CH2C=O on the other side(Original post by Doctor.)
Really need to get back to CHEM4
Think I will dedicate 9-5 tomorrow to CHEM4 If anyone has questions on Chapters 4-11, hit me! 
Think I will dedicate 9-5 tomorrow to CHEM4 
