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Edexcel A2 Chemistry 6ch04/05 JUNE 2015

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Can someone please explain these 2 questions to me?

Thanks

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Original post by Undisclosed 15
Can someone please explain these 2 questions to me?

Thanks

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The pH calculation:

20x1/1000 - 10x1/1000 = 0.01 (mol of acid-mol of base to give mol of unreacted acid)
0.01x1000/(20+10)= 1/3 (mol of unreacted acid/total volume)
-log(1/3)= 0.47712... =0.48 so C

and the buffer:
the question asks for a buffer with pH 5 (acidic buffer) so it cant be C or D as those involve bases and it cant be B as it include HCl which is a strong acid- weak acids and their conjucate base make up buffers which Ethanoic acid and Sodium Ethanoate are therefore A!

Hope this makes sense!
(edited 8 years ago)
So wat questions do u expect to come for chem unit 4 on wed
Original post by Raghadabuali
So wat questions do u expect to come for chem unit 4 on wed


do you really think its possible to predict questions?
My teacher expects a transesterification question
Original post by Ripper Phoenix
do you really think its possible to predict questions?
Original post by Ripper Phoenix
do you really think its possible to predict questions?


Plus, there's enough space on the paper to ask questions from every topic.
There's always an entropy question, there's always a rate question, there's always an equilbria question, always an acid/base equlibria question and there's always organic questions.

The only thing that may not show up is a high-mark, detail intensive question on chromatography, and even that we can't be sure about.
Original post by Raghadabuali
My teacher expects a transesterification question


i'm not going to lie but i had the same topic in my head too! :smile:
Original post by thymolphthalein
Plus, there's enough space on the paper to ask questions from every topic.
There's always an entropy question, there's always a rate question, there's always an equilbria question, always an acid/base equlibria question and there's always organic questions.

The only thing that may not show up is a high-mark, detail intensive question on chromatography, and even that we can't be sure about.


basically it will be a unit 4 paper :tongue:
Original post by Ripper Phoenix
i'm not going to lie but i had the same topic in my head too! :smile:

then can u pls tell me abt transesterification important points
Original post by Ripper Phoenix
basically it will be a unit 4 paper :tongue:


Ahah, yeah but I guess weird things could happen.

I think I mentioned it here before but Unit 2 IAL went something like this:

Section B: Only Green Chemistry
Section C: Only Organic Chemistry
Section A: Group 2, Group 7, Bonding, Shapes, Kinetics, Mechanisms, etc.

Felt like a waste to have studied all the things that came in Section A because the MCQs were so bloody easy anyway.

But if the reverse had happened, people who didn't study would have been totally screwed. So it's not wise to rely on 'predictions'. :biggrin:
Do you know whats going to be predicted on the whole C2 paper on Wednesday?
Original post by Raghadabuali
then can u pls tell me abt transesterification important points


Basically there's two types of transesterification.

Reaction with another organic acid:
Acid part of an ester being replaced by an acid reactant in the presence of an acid catalyst.
Ethyl ethanoate and methanoic acid react to form ethyl methanoate and ethanoic acid.
This is the type used in manafacture of low-fat margarine.

Then there's reaction with another alcohol.
The alcohol part of the ester is replaced by the alcohol reactant in the presence of an acid catalyst.
Ethyl ethanoate and methanol react to form methyl ethanoate and ethanol.
This is used in the manafacture of biodiesel where the reactant alcohol is usually methanol/ethanol which is generally reacted with a triglyceride(an ester) to form a 1,2,3-triol.

That's pretty much it.
Thank you alottt
June 1AL paper is amazing could only hope for that again (doubt it)


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Original post by Raghadabuali
Thank you alottt


Going to go just a bit further and add this part because they occasionally ask about the efficiency of it and whatnot:

Most commonly used method for producing biodiesel involves base** catalyzed transesterification of vegetable oils (which mainly consist of the triglycerides mentioned before) with methanol/ethanol producing the transester and the 1,2,3-triol.

According to my notes, this process is very efficient with a high conversion yield and economical because it uses low temperature and readily available catalysts.

**Just to be clear: my notes mention acid catalysts for simple transesterification such as methyl ethanoate and ethanol reacting, but lists a strong alkali such as sodium hydroxide for biodiesel formation.
Original post by AnishaJayne
June 1AL paper is amazing could only hope for that again (doubt it)


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I sat for that exam paper. Missed the A by a few marks because I hadn't learnt recrystallization then and messed up the entire kc question. :ashamed2:

Along with a few other moronic mistakes obviously, but those were the main culprits. :unimpressed:
Hey guys does anyone know why this is C?
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Original post by Ripper Phoenix
where is the q?
Reply 498
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Original post by AnishaJayne
Hey guys does anyone know why this is C?
ImageUploadedByStudent Room1433611979.127307.jpg


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H2O = 1x10-7
1. [H+] = 10/1000 x 1 = 0.01 mol
2. [OH-] = 990/1000 x 1x10-7 = 9.9x10-8
3. Excess = 0.01 - 9.99x10-8 = 9.99x10-3
4. Conc= 9.99x10-3 x 1000/(990+10) = 9.99x10-3
5. -log(9.99x10-3) = 2
I know its a long method but it got me there!
(edited 8 years ago)
Original post by MeeraP07
The pH calculation:

20x1/1000 - 10x1/1000 = 0.01 (mol of acid-mol of base to give mol of unreacted acid)
0.01x1000/(20+10)= 1/3 (mol of unreacted acid/total volume)
-log(1/3)= 0.47712... =0.48 so C

and the buffer:
the question asks for a buffer with pH 5 (acidic buffer) so it cant be C or D as those involve bases and it cant be B as it include HCl which is a strong acid- weak acids and their conjucate base make up buffers which Ethanoic acid and Sodium Ethanoate are therefore A!

Hope this makes sense!


Thanks

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