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OCR B Salter's Chemistry by Design F335 - 15th June 2015

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Reply 80
Avatar for Odyhrot
Odyhrot
I know there's not much point speculating lol but do you think this paper will be crazy hard like F334 was or just a bit harder than normal (like what happened last year)? Feels a bit pointless doing past papers and doing well in them if the actual exam is going to be a million times more difficult.
Can someone explain to me why the biggest inaccuracy when working out PH comes from [HA] at start=[HA] at equilibrium
The answers seem to vary and depends on the context?
Original post by tealover96
Can someone explain to me why the biggest inaccuracy when working out PH comes from [HA] at start=[HA] at equilibrium
The answers seem to vary and depends on the context?

Normally its because to make this assumption you are assuming that no molecules of HA dissociate since being added to when equilibrium is achieved. If the pH is low then this is obviously not the case. Also you could subtract the conc of H+ you calculated from initial conc of HA added and say this isnt a good approximation of HA. (Think they asked that in june 13)
(edited 8 years ago)
Any help on nmr please


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Original post by LordGaben
Normally its because to make this assumption you are assuming that no molecules of HA dissociate since being added to when equilibrium is achieved. If the pH is low then this is obviously not the case. Also you could subtract the conc of H+ you calculated from initial conc of HA added and say this isnt a good approximation of HA. (Think they asked that in june 13)


Thanks...I understand the first bit now
So the questions come up twice...
Give approximation
Conc. Of acid at start = conc. of acid at equilibrium
Reasoning
June 13 – 0.00082 (H+) not much smaller then 0.004 (Conc. of acid); 0.004 - 0.00082/0.00318 is not a good appromximation (AW)
June 10 – H+ compared with HA is negligible

I guess both mark schemes are saying the same thing
But I'm still confused as to why this small/negligible difference makes it a bad approximation
Original post by tealover96
Thanks...I understand the first bit now
So the questions come up twice...
Give approximation
Conc. Of acid at start = conc. of acid at equilibrium
Reasoning
June 13 – 0.00082 (H+) not much smaller then 0.004 (Conc. of acid); 0.004 - 0.00082/0.00318 is not a good appromximation (AW)
June 10 – H+ compared with HA is negligible

I guess both mark schemes are saying the same thing
But I'm still confused as to why this small/negligible difference makes it a bad approximation

Between H+ and HA as it said in june 10?
If H+ is similar to HA it implies that the acid has almost fully dissociated so the approximation will lead to error as it assumes little dissociation.
(edited 8 years ago)
Original post by LordGaben
Between H+ and HA as it said in june 10?
If H+ is similar to HA it implies that the acid has almost fully dissociated so the approximation will lead to error as it assumes little dissociation.


Ahhh thanks a lot
Seems so obvious now!
Original post by EggsterminateMe
I know, but you see, I'm just memorising the reactions. I should've guess since benzene reaction undergo substitutions. Hydrogen is subsitituted for the chlorine?



Ah thank you.
I read the CGP explanation that was helpful. Watch a video on it on youtube.


Yes it is, a hydrogen for a chloride and hydrogen chloride is made from the remaining chloride.
Can anyone tell me what is up with the test bank?
I am really confused.
It is based around the specification - but is labelled in numbers, whilst the specification is in letters.
I have tried linking them up but they dont match entirely.
Anyone know the solution to this?
Original post by Shostakovish
Can anyone tell me what is up with the test bank?
I am really confused.
It is based around the specification - but is labelled in numbers, whilst the specification is in letters.
I have tried linking them up but they dont match entirely.
Anyone know the solution to this?


Whats the test bank...never heard of it :s-smilie:
Reply 90
Avatar for Shantellea
Shantellea
OP
2
Original post by Shostakovish
Can anyone tell me what is up with the test bank?
I am really confused.
It is based around the specification - but is labelled in numbers, whilst the specification is in letters.
I have tried linking them up but they dont match entirely.
Anyone know the solution to this?


Is this to do with the past paper questions?


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Erm, does anyone else feel NOWHERE near ready?
Original post by Shostakovish
Erm, does anyone else feel NOWHERE near ready?


Tell me about it... :bricks:
Can someone help me with enthalpy profiles for lattice enthalpy and all that. I don't understand any of the arrows
Ok I'm being an idiot but i'm confused about the nitrition reaction of benzene, there must be another product?! The atoms don't add up

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1434230024105.jpg40.3KB
Reply 95
Avatar for Shantellea
Shantellea
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2
Original post by hannnahcox
Ok I'm being an idiot but i'm confused about the nitrition reaction of benzene, there must be another product?! The atoms don't add up

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They do add up if you look at the bottom equation, it's only because NO2+ reacts with benzene but it has to be made in the nitrating mixture as HNO3 can't react with benzene


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Ohh ok so it's just present in the mixture too? Thanks :smile:

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Reply 97
Avatar for Shantellea
Shantellea
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Original post by hannnahcox
Ohh ok so it's just present in the mixture too? Thanks :smile:

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Yep that's how the electrophile is made, no problem😀


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Hey has anyone got the copy of june 14 f335? our teacher split the paper into 2 bits and we had to do the last question another time, and I've lost that section. has anyone got it?
Reply 99
Avatar for Shantellea
Shantellea
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Original post by ssaahhaanndd
Hey has anyone got the copy of june 14 f335? our teacher split the paper into 2 bits and we had to do the last question another time, and I've lost that section. has anyone got it?


Someone uploaded it onto google drive https://drive.google.com/file/d/0B-EbxUSm2w_1X00yT2hRN0VvdFU/view


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