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Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

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Original post by Don Pedro K.
Guys, I just don't get the mark scheme's logic here for this question...Anyone care to explain? (June 15)

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The salt of a weak acid is a strong base so the propanoate ions in solution act as a base. They accept protons from the water leaving OH- ions in solution so the resulting pH is slightly alkaline hence above 7
Original post by Supermanxxxxxx
Does the sn2 or sn1 mechanism form a racemic mixture


Sn1 forms a racemic mixture since a planar carbocation intermediate is formed, in which the nucleophile is able to attack the C+ atom from both sides of the plane.

In Sn2, the nucleophile can only attack the Cdelta + atom from the opposite side to that of the leaving group and you get a five-bonded transition state, so you only get one enantiomer :smile:
(edited 7 years ago)
Original post by Whizbox
The salt of a weak acid is a strong base so the propanoate ions in solution act as a base. They accept protons from the water leaving OH- ions in solution so the resulting pH is slightly alkaline hence above 7


Okay but I thought pH is only dependent on [H+]?
Original post by Don Pedro K.
Okay but I thought pH is only dependent on [H+]?


Well yes, but the propanoate ions react with the H+ ions I think so their concentration goes down. To be perfectly honest I'm not really sure
Original post by Don Pedro K.
Okay but I thought pH is only dependent on [H+]?


The reason the production of OH- ions causes an increase in PH is because of the following equilibrium of water:

H2O --> OH- + H+

Increase in OH- concentration pushes the position of equilibrium to the left to produce more water so H+ concentration decreases which causes PH to increase :smile:
Original post by Whizbox
Well yes, but the propanoate ions react with the H+ ions I think so their concentration goes down. To be perfectly honest I'm not really sure


Ah, it's okay haha :smile: Another question, do you know why methanal doesn't form a racemic mixture with HCN?
Original post by setarcos
The reason the production of OH- ions causes an increase in PH is because of the following equilibrium of water:

H2O --> OH- + H+

Increase in OH- concentration pushes the position of equilibrium to the left to produce more water so H+ concentration decreases which causes PH to increase :smile:


Ahhhh okay, I see!! haha thanks :smile:
Original post by Don Pedro K.
Ah, it's okay haha :smile: Another question, do you know why methanal doesn't form a racemic mixture with HCN?


Methanal is HCHO, so has two of the same groups bonded to it, therefore it isnt chiral hence no racemic mixture.
Original post by Don Pedro K.
Ah, it's okay haha :smile: Another question, do you know why methanal doesn't form a racemic mixture with HCN?


Because it doesn't contain a chiral carbon atom, it has 2 hydrogens bonded to the central carbon atom so doesn't display optical isomerism
Original post by KungMaule
Methanal is HCHO, so has two of the same groups bonded to it, therefore it isnt chiral hence no racemic mixture.


Original post by Whizbox
Because it doesn't contain a chiral carbon atom, it has 2 hydrogens bonded to the central carbon atom so doesn't display optical isomerism


Ah yes of course ahah :smile: Alright thanks guyssss!
What marks are you guys aiming for out of 90?
Original post by Funky_Giraffe
What marks are you guys aiming for out of 90?


Ideally ~75+ :smile: you?
Original post by Don Pedro K.
Ideally ~75+ :smile: you?


Same really. I need to try and maximise my marks on this paper to give me some leeway for unit 5 so the higher I can get, the better. Somehow need an A* in Bio/Chem for medschool...! :tongue:
Original post by Funky_Giraffe
What marks are you guys aiming for out of 90?


Ideally I'd say I want to get over 80, but more likely over 70.
i literally cannot revise no further, ive reached my optimum, im 100% ready, i dont know what to do with my time, any advice ? lol or suggest some topics that might slip me up ?
Original post by KungMaule
Ideally I'd say I want to get over 80, but more likely over 70.


Yeah. I suppose a lot of it depends on the difficulty of the paper. From some of the past papers I've done I've managed low 80s but I suppose the fact it was easier to do well was reflected in the grade boundaries? It all depends what comes up really
Guyss will someone be posting the mark scheme???????
Original post by tayloryeah
i literally cannot revise no further, ive reached my optimum, im 100% ready, i dont know what to do with my time, any advice ? lol or suggest some topics that might slip me up ?


same I've done every single past paper and re read my notes so all we can do is hope for the best:smile:
Original post by Funky_Giraffe
What marks are you guys aiming for out of 90?


Over 80 is the goal
Okay, say there is a big 20 mark question on the paper, what topic do you people hope its on?
Personally I'd want it on entropy, because the questions on entropy are so easy to pick up marks on.

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