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Author Topic: VCE Chemistry Question Thread  (Read 2313254 times)  Share 

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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #870 on: May 18, 2014, 12:53:46 pm »
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can someone explain to me how the answer is C, much thanks :)
start counting from COOH to find the longest carbon chain. methyl group at 6th carbon and propyl group at 5th carbon.
Hence 6 methyl 5 propyl octanoic acid
« Last Edit: May 18, 2014, 01:06:59 pm by swagsxcboi »
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #871 on: May 18, 2014, 12:58:32 pm »
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I have my aspirin sac on Monday and I have answered some revision questions
Could someone please check that I am correct?
- What's the purpose of adding Sulfuric acid?
The H2SO4 acts as a catalyst and speeds up the reaction
- What is the limiting reagent in the reaction?
The limiting reagent is salicylic acid as ethanoic anhydride is added in excess
- Why is water added at the end of the reaction?
Water is added at the end of the reaction to allow for crystallisation to occur and it cools down the reaction
-Why is cold water used?
Cold water is used to lower the solubility of aspirin. Water is also used to ensure that all other solutes and reactants are washed away.

Feel free to add anything
Much appreciated :)

For the sake of being meticulous, I'd probably specify that concentrated sulfuric acid is used to increase the rate of the condensation reaction (i.e. acts as a catalyst) by which acetylsalicylic acid - aspirin - is produced.


Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #872 on: May 18, 2014, 01:07:00 pm »
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can someone explain to me how the answer is C, much thanks :)

The first thing to do is count the longest (parent) chain. When a carboxyl group is present, that must be counted in the parent chain. The longest parent chain (including the carboxyl group) contains 8 carbon atoms. Hence, this is octanoic acid (a number is not required to specificy the location of the carboxyl group because it must be present at the terminal carbon atom). The next thing to do is notice that we have a methyl (CH3) group coming off one of the carbons, and a propyl (CH3CH2CH3) group coming off another carbon. The important thing to do now is note what number carbon they're coming off. Counting from the carbon of the carboxyl group (i.e. Carbon one), we can see that the propyl group is coming off carbon number 5, while the methyl group is coming off of carbon 6.

So, we know three things now:

Parent chain: octanoic acid
We also have 5-propyl and 6-methyl

So, our compound:

6-methyl-5-propyl octanoic acid, C.

Einstein

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Re: VCE Chemistry Question Thread
« Reply #873 on: May 18, 2014, 06:32:43 pm »
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i got a different answer, and you can probably see where i sourced it from, can you tell me what i did wrong?

our teacher said you cant name the ethyls, methlys etc if they are a part of the chain.

Rishi97

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Re: VCE Chemistry Question Thread
« Reply #874 on: May 18, 2014, 07:09:26 pm »
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Relating to the aspirin practical, can anybody think of any sources of errors that could've resulted in a higher / lower percentage yield?
Thank you :)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #875 on: May 18, 2014, 07:11:04 pm »
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i got a different answer, and you can probably see where i sourced it from, can you tell me what i did wrong?

our teacher said you cant name the ethyls, methlys etc if they are a part of the chain.

Why can't you name the ethyl and methyl groups here? If you count your parent hydrocarbon chain as going up and then to the left (that one), you'll find that you can name it quite easily.
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Einstein

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Re: VCE Chemistry Question Thread
« Reply #876 on: May 18, 2014, 07:21:17 pm »
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Why can't you name the ethyl and methyl groups here? If you count your parent hydrocarbon chain as going up and then to the left (that one), you'll find that you can name it quite easily.



im not sure what you mean here , sorry

swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #877 on: May 18, 2014, 07:23:12 pm »
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i got a different answer, and you can probably see where i sourced it from, can you tell me what i did wrong?

our teacher said you cant name the ethyls, methlys etc if they are a part of the chain.
start counting from the COOH...
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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #878 on: May 18, 2014, 07:30:25 pm »
+1
apologies for shitty drawing but hope this helps
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Einstein

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Re: VCE Chemistry Question Thread
« Reply #879 on: May 18, 2014, 08:20:21 pm »
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start counting from the COOH...

ahh i got it, is that because the double bond takes precedence?

also how did you know to count from cooh then move to left and not right if that makes sense (in terms of determining longest chain) is this right?

psyxwar

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Re: VCE Chemistry Question Thread
« Reply #880 on: May 18, 2014, 10:21:57 pm »
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do we need to worry about multiplicity for H1 NMR regarding peak splitting? ie. CH3-CH2-CH2-CH2-CH3, the hydrogen environment (first CH2) being split into 12 peaks.
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psyxwar

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Re: VCE Chemistry Question Thread
« Reply #881 on: May 18, 2014, 10:23:19 pm »
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ahh i got it, is that because the double bond takes precedence?

also how did you know to count from cooh then move to left and not right if that makes sense (in terms of determining longest chain) is this right?
he went left because counting left gives you a longer chain. With COOH the carbon there is always the carbon number 1 and you start counting from that.
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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #882 on: May 18, 2014, 10:40:59 pm »
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do we need to worry about multiplicity for H1 NMR regarding peak splitting? ie. CH3-CH2-CH2-CH2-CH3, the hydrogen environment (first CH2) being split into 12 peaks.

This may sound weird but did you do a STAV paper? I did one today and saw the exact same question lol.

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #883 on: May 18, 2014, 10:59:32 pm »
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do we need to worry about multiplicity for H1 NMR regarding peak splitting? ie. CH3-CH2-CH2-CH2-CH3, the hydrogen environment (first CH2) being split into 12 peaks.

Most likely not

This may sound weird but did you do a STAV paper? I did one today and saw the exact same question lol.

So keen

ahh i got it, is that because the double bond takes precedence?

also how did you know to count from cooh then move to left and not right if that makes sense (in terms of determining longest chain) is this right?

It's not the double bond that takes precedence; the COOH functional group takes precedence.

In rare cases, it may be simpler to not treat the COOH as the parent functional group, but you won't come across these in VCE.
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Einstein

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Re: VCE Chemistry Question Thread
« Reply #884 on: May 19, 2014, 07:54:08 am »
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It's not the double bond that takes precedence; the COOH functional group takes precedence.

In rare cases, it may be simpler to not treat the COOH as the parent functional group, but you won't come across these in VCE.

Yeap, thats what i meant. Thanks for the help :)