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Edexcel - Chemistry Unit 2 - 4 June 2013

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Original post by geor
x


BTW, do we need to know the SN2 transition state and E1/E2 mechanisms?
Original post by geor
Haha wow.. very similar! Oxford, Durham, Imperial, Nottingham and probably UCL! :smile:


Cool. We could end up at the same uni :tongue:

Original post by geor
SN2 definitely, E2 kind of. You wouldn't be expected to draw it but my teacher said it's better to know it because they can ask you questions about it!


Thanks. :smile:

Are you doing the L6 Challenge?
Reply 922
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posthumus
OP
20
Original post by Big-Daddy
BTW, do we need to know the SN2 transition state and E1/E2 mechanisms?


What's E1 & E2 mechanism ? :confused:

I assume involving electrophiles (substitution or addition?), could you give me some examples please...

Thanks :smile:
How do we explain the meaning of london forces?
Original post by posthumus
What's E1 & E2 mechanism ? :confused:

I assume involving electrophiles (substitution or addition?), could you give me some examples please...

Thanks :smile:


They are the mechanisms of elimination. We only deal with leaving groups (e.g. X-) and that too only with E2 according to geor in the post above. Difference is that E2 is what usually happens when there is little alkyl substitution on the nearby C atoms, E1 when there is lots.

Want more details? Look it up! :tongue:
Why are group 2 with hydroxides more soluble as we go down the group and sulfate more soluble as we do up the group? :smile:
Reply 926
Avatar for Kurraiyo
Kurraiyo
7
June 2011 MCQ 16:

The enthalpy change of neutralization of an acid by an alkali is measured by adding 10.0 cm3 of hydrochloric acid to 10.0 cm3 of sodium hydroxide. 10.0 cm3 pipettes with an accuracy of ±0.04 cm3 are used to measure out both solutions.
The overall percentage error in measuring the total volume of the reaction mixture is:

A ±0.04%
B ±0.08%
C ±0.4%
D ±4.0%

Answer is C, can you explain why? :confused:
Reply 927
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Original post by bubblegummer
How do we explain the meaning of london forces?


It's an instantaneous dipole induced dipole. Electrons going around atoms, it's likely that there is more on one side than the other, causing an uneven distribution in charge, forming a dipole on the molecule, this causes another in the adjacent molecule and so on...:smile:
Original post by Kurraiyo
June 2011 MCQ 16:

The enthalpy change of neutralization of an acid by an alkali is measured by adding 10.0 cm3 of hydrochloric acid to 10.0 cm3 of sodium hydroxide. 10.0 cm3 pipettes with an accuracy of ±0.04 cm3 are used to measure out both solutions.
The overall percentage error in measuring the total volume of the reaction mixture is:

A ±0.04%
B ±0.08%
C ±0.4%
D ±4.0%

Answer is C, can you explain why? :confused:


Read the previous pages a long discussion has been going on for this question
Reply 929
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Tuya
4
Original post by geor
Now THAT would be cool! I am, are you? Have to ask - how are you preparing for it/are you preparing for it? Compared to the Olympiad this one seems to be a lot more problem solving and maths than actual chemistry, haha.


This has gone completely off topic, please use chat


Posted from the TSR iPad app
Reply 930
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own
Original post by Kurraiyo
June 2011 MCQ 16:

The enthalpy change of neutralization of an acid by an alkali is measured by adding 10.0 cm3 of hydrochloric acid to 10.0 cm3 of sodium hydroxide. 10.0 cm3 pipettes with an accuracy of ±0.04 cm3 are used to measure out both solutions.
The overall percentage error in measuring the total volume of the reaction mixture is:

A ±0.04%
B ±0.08%
C ±0.4%
D ±4.0%

Answer is C, can you explain why? :confused:


Multiply accuracy (uncertainty) by the volume (10 x 0.04) :smile:
Reply 931
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own
Original post by marseille_h
Why are group 2 with hydroxides more soluble as we go down the group and sulfate more soluble as we do up the group? :smile:


Of what I've read in text books there is little explanation of this, I believe all you need to know is singly charged anions with a group two element increases in solubility down the group, and doubly charged anions with a group two element increases in solubility up the group.
Original post by geor
Now THAT would be cool! I am, are you? Have to ask - how are you preparing for it/are you preparing for it? Compared to the Olympiad this one seems to be a lot more problem solving and maths than actual chemistry, haha.


Most likely, but I don't know the date of ours yet, and if it's too early my exams may still be on.

As for preparation, I agree, it's more problem solving and less knowledge than the Olympiad.

I suspect off the cuff that something on NMR is decently likely to come up, as it hasn't before I think (and I have met Wothers :tongue: ). Spectrometry is also one of my weakest topics in Chemistry. So I will take the opportunity to bring myself up to a decent level on NMR, mass spec and IR.

It's difficult just from A-level or even undergrad knowledge alone to feel prepared fully for this Challenge, because they sneek in things like Wothers' own personal classification method for hydrolysis/oxidation/reduction. It is unique to Wothers and Keeler and if you haven't studied under them directly you'll be meeting it for the first time (not that it's particularly hard), regardless of whether you are a GCSE student or a graduate. Other things like the solid state, it's good to have the knowledge, but they still slip in some pitfalls for you. Other than that, it is mostly just using the calculator. Thank God there aren't any long, painful mole calculations where I feel like I'm doomed to make a mistake somewhere just by accident, like in the Olympiad (if you remember how much I worried about the result!). :tongue:

Aside from that, not too sure. Do you have any ideas how else to prepare?
Original post by Tuya
This has gone completely off topic, please use chat


Posted from the TSR iPad app


Pardon us, I only just saw this post. We'll PM from here on.
Guys, lets keep this to Unit 2 discussion, let's not drift away.
Original post by own
Of what I've read in text books there is little explanation of this, I believe all you need to know is singly charged anions with a group two element increases in solubility down the group, and doubly charged anions with a group two element increases in solubility up the group.


I don't think this is the best general principle. Just go with carbonate and sulphate decreasing down the group and hydroxide increasing down the group (solubility). Good way to remember is that BaSO4 is used in hospital x-rays because it's insoluble.
Original post by Big-Daddy
I don't think this is the best general principle. Just go with carbonate and sulphate decreasing down the group and hydroxide increasing down the group (solubility). Good way to remember is that BaSO4 is used in hospital x-rays because it's insoluble.


Ok thanks :smile: so we don't need to know why i thought it might be related to the charge or something :rolleyes:
Reply 937
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Original post by Big-Daddy
I don't think this is the best general principle. Just go with carbonate and sulphate decreasing down the group and hydroxide increasing down the group (solubility). Good way to remember is that BaSO4 is used in hospital x-rays because it's insoluble.


Okay, just regurgitating what I have learned from cgp, what has become like a bible to me now. Shall remember that though, thanks :smile:
6 For parts (a) and (b), use your knowledge of intermolecular forces to predict the

compound with the highest boiling temperature.

(a) A HF

B H2O

C NH3

D CH4

(1)

(b) A 1-iodobutane

B 1-chlorobutane

C 2-methyl-2-iodopropane

D 2-methyl-2-chloropropane
Original post by own
It's an instantaneous dipole induced dipole. Electrons going around atoms, it's likely that there is more on one side than the other, causing an uneven distribution in charge, forming a dipole on the molecule, this causes another in the adjacent molecule and so on...:smile:


got it, thank you! :smile:

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