Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed) Watch

Poll: How pumped up are you for this exam?-(warning)-(bad jokes arene this poll!)
"Titanium-I'm not going to corrode (even at high temperatures)" (A*) (22)
16.67%
"Benzene's my middle name, give me the paper in a week and I'll ace it!" (A) (27)
20.45%
"Yeah, I'm fairly electrophillic (positively charged) about the exam" (B) (27)
20.45%
"I'm in the middle of the salt bridge, but I will pass-eventually" (C) (21)
15.91%
"I'm feeling rather electroNegative about this exam" (D) (18)
13.64%
"Benzene, what's that?" (E) (6)
4.55%
"Chemistry, what's that?" (F) (11)
8.33%
This discussion is closed.
posthumus
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(Original post by GeorgeL3)
Yeah you're right, the LORA rule will always give a positive Ecell value, ie. it shows the reaction with that will
occur with the reactants you've got.
The answer should only be negative if for example they ask you to work out whether a specific reaction will occur or give you a full equation to work out the value for.
In that case you have to look at which species is being oxidised/reduced and decide from there which needs the sign swapping.
(Original post by Mollymod)
I gave you an example at the bottom of page 26
Thanks guys I think I get it now ....

So basically when I find the difference in electrode potentials of each half equation - I am finding the Electrode cell potential of the feasible direction of the reaction !

So say if I do that and I get my value for E ... from the equation that's also been formed I can determine if that is for oxidation or reduction ? (or will it always be reduction)

If the question states that they want you to evaluate E for a reaction in a particular direction... if the overall equation you obtained from working out the feasible direction is what they ask in the question then its fine if its asking you in the question the opposite direction, you quite simply switch the sign of the E you worked out previously to negative ? (and state that this reaction is not feasible).


I hope you guys get what I'm saying & Hopefully I've got it

Thanks again
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Mollymod
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(Original post by posthumus)
Thanks guys I think I get it now ....

So basically when I find the difference in electrode potentials of each half equation - I am finding the Electrode cell potential of the feasible direction of the reaction !

So say if I do that and I get my value for E ... from the equation that's also been formed I can determine if that is for oxidation or reduction ? (or will it always be reduction)

If the question states that they want you to evaluate E for a reaction in a particular direction... if the overall equation you obtained from working out the feasible direction is what they ask in the question then its fine if its asking you in the question the opposite direction, you quite simply switch the sign of the E you worked out previously to negative ? (and state that this reaction is not feasible).


I hope you guys get what I'm saying & Hopefully I've got it

Thanks again
Well it's a redox equation. Something get's oxidised and something gets reduced, depending on the reduction potential will depend which equation is Red or which is Ox.
I think your E will always be based on the reduction reaction as the forward/reversed cells are based on reduction potentials, but I'm not 100% on that as I don't fully understand your question. The words are all jumpy after a while for me

I think you've got it nailed though, that last description looks right to me!
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posthumus
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(Original post by Mollymod)
Well it's a redox equation. Something get's oxidised and something gets reduced, depending on the reduction potential will depend which equation is Red or which is Ox.
I think your E will always be based on the reduction reaction as the forward/reversed cells are based on reduction potentials, but I'm not 100% on that as I don't fully understand your question. The words are all jumpy after a while for me

I think you've got it nailed though, that last description looks right to me!
ahaha I know, I realized at the end I might just be getting too excited and not making much sense

Well is electrode potential of a reduction half-equation ALWAYS greater than the oxidation half-equation ?

If that's the case then it would always be reduction, I think

But yh in general I think I'm good to move on & when I do practice questions I should get the hang of it anyway (which half-equations to switch).
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Mollymod
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(Original post by posthumus)
ahaha I know, I realized at the end I might just be getting too excited and not making much sense
Well is electrode potential of a reduction half-equation ALWAYS greater than the oxidation half-equation ? [QUOTE] Yes. Definitely, it'll always be the more positive one. It means the equation really does want to go that way (more than the other equation)

(Original post by posthumus)
But yh in general I think I'm good to move on & when I do practice questions I should get the hang of it anyway (which half-equations to switch).
Practice questions really is the way forward.

Guys, how likely is it that they'll ask some Acid/Base titrationy things.
I've been getting a little bit worried about the whole synoptic thing. I feel like my brain will explode with all this information it has to carry for the exam. And this is just Chemistry...
I realise I've forgotten how to calculate Kb and things, or my calculator is heavily glitching on me.

I tried calculating the pH of a strong base and let's say..
0.1mol dm-3 of NaOH.
I did 0.1/1x10-14 = 1x10-13 and then -log(1x10-13)
is my working right or have I heavily messed up? Because my calculator
then gave me pH -13 o.O

and long equilibrium calculations; I'm paranoid of not knowing absolutely everything, and I have to ace this; I've grown to love Chem too much so I kind of want to do well
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GeorgeL3
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[QUOTE=Mollymod;42076362]Well is electrode potential of a reduction half-equation ALWAYS greater than the oxidation half-equation ?
Yes. Definitely, it'll always be the more positive one. It means the equation really does want to go that way (more than the other equation)



Practice questions really is the way forward.

Guys, how likely is it that they'll ask some Acid/Base titrationy things.
I've been getting a little bit worried about the whole synoptic thing. I feel like my brain will explode with all this information it has to carry for the exam. And this is just Chemistry...
I realise I've forgotten how to calculate Kb and things, or my calculator is heavily glitching on me.

I tried calculating the pH of a strong base and let's say..
0.1mol dm-3 of NaOH.
I did 0.1/1x10-14 = 1x10-13 and then -log(1x10-13)
is my working right or have I heavily messed up? Because my calculator
then gave me pH -13 o.O

and long equilibrium calculations; I'm paranoid of not knowing absolutely everything, and I have to ace this; I've grown to love Chem too much so I kind of want to do well
To find the pH of a strong base I think there were two different ways:
1. Work out pOH and do 14 - pOH to get pH.
This just involves using pH = -log[H+] and replacing pH with pOH and H+ with OH-
so pOH = -log0.1 = 1
pH = 14 - 1 = 13
2. Say [OH-][H+]=1x10^-14 and then rearrange it.
[H+] = 1x10^-14 / 0.1 = 1x10^-13
pH = -log1x10^-13 = 13 <- Yeah I guess your calculator must have messed up, have you tried turning it off and on again? haha


Also I would just make sure you know how the sodium thiosulphate titration works with Iodine and also the potassium manganate one with iron since I think they mention them in the specification.
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Mollymod
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(Original post by GeorgeL3)
Well is electrode potential of a reduction half-equation ALWAYS greater than the oxidation half-equation ?
To find the pH of a strong base I think there were two different ways:
1. Work out pOH and do 14 - pOH to get pH.
This just involves using pH = -log[H+] and replacing pH with pOH and H+ with OH-
so pOH = -log0.1 = 1
pH = 14 - 1 = 13
2. Say [OH-][H+]=1x10^-14 and then rearrange it.
[H+] = 1x10^-14 / 0.1 = 1x10^-13
pH = -log1x10^-13 = 13 <- Yeah I guess your calculator must have messed up, have you tried turning it off and on again? haha


Also I would just make sure you know how the sodium thiosulphate titration works with Iodine and also the potassium manganate one with iron since I think they mention them in the specification.
Yeah, I wont be using my dodgy calculator for the exam. It's so old and rusty and all the numbers have disappeared. I'll post a picture later :P Thanks for that, yeah I've got it right. I'll practice those briefly before the exam but I'll try not to stress too much

Aha yeah, alright. So back-titration things/Redox Titrations and the 1:5 Fe/MnO4 ratio
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AS01
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(Original post by Mollymod)
I need BCC Almost sure that the B will be Chemistry.
And yeah, it's a lot of memorisation but I like memorising things, it's almost fun when I test myself, although it's not really proving if I'm good at Chemistry or not, it's just a memory game really :P

Yeah, TM's are a bit iffy but thankfully it should be easy to sort out once I fully understand the theory. We've got quite a while left which is good

I've just started looking at a couple of Unit 1 things for my retake which is 23rd of May. What grades do you need?
I need 3As. I think I am sorted with redox now phew!
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Mollymod
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(Original post by AS01)
I need 3As. I think I am sorted with redox now phew!
Ooh that's high :P
What university is this for?

Awesomeness. I'm spending today drawing out all unit 1 mechanisms, and looking at Parts Per Million

I'm going to power through Mass Spec and Calorimetry this evening. Tomorrow will be Unit 5 day where I'll transition-metal and probably Redox things up
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posthumus
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If anyone is retaking Unit 2, I've made a thread here : http://www.thestudentroom.co.uk/show....php?t=2310012
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AS01
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(Original post by Mollymod)
Ooh that's high :P
What university is this for?

Awesomeness. I'm spending today drawing out all unit 1 mechanisms, and looking at Parts Per Million

I'm going to power through Mass Spec and Calorimetry this evening. Tomorrow will be Unit 5 day where I'll transition-metal and probably Redox things up
its for medicine so most unis want 3As
Hmm that's good! I think I should do the same for mechanisms
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Mollymod
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(Original post by AS01)
its for medicine so most unis want 3As
Hmm that's good! I think I should do the same for mechanisms
Oh in that case, well done for getting into Medicine!
So many mechanisms, it's mad!
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AS01
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(Original post by Mollymod)
Oh in that case, well done for getting into Medicine!
So many mechanisms, it's mad!
I am not that lucky. I didnt get any offer. I am taking gap year but that's the grade I have to aim for
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David Tennant
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(Original post by AS01)
I am not that lucky. I didnt get any offer. I am taking gap year but that's the grade I have to aim for
I'm sorry you didn't get in, do you mind me asking where you applied at?
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David Tennant
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(Original post by posthumus)
If anyone is retaking Unit 2, I've made a thread here : http://www.thestudentroom.co.uk/show....php?t=2310012
Thanks posthumous, I can link your forum to my Op if you'd like
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posthumus
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(Original post by David Tennant)
Thanks posthumous, I can link your forum to my Op if you'd like
Yh that would be great, thanks

However by the looks of it... not many people are retaking unit 2 !
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Mollymod
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(Original post by posthumus)
Yh that would be great, thanks

However by the looks of it... not many people are retaking unit 2 !
or the year 12s haven't woken up yet :P

(Original post by AS01)
I am not that lucky. I didnt get any offer. I am taking gap year but that's the grade I have to aim for
Well at least you can get your As at the end of this year and get yourself sorted early. Better luck in a year, you deserve it!
It's good that you're still going for the dream though. Props!
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posthumus
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(Original post by Mollymod)
or the year 12s haven't woken up yet :P
Ahaha yh I was thinking ! Last year I would start studying for my exams like 1 or 2 weeks before them.... boy did I learn by lesson
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Mollymod
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(Original post by posthumus)
Ahaha yh I was thinking ! Last year I would start studying for my exams like 1 or 2 weeks before them.... boy did I learn by lesson
at least you learned before it was too late :P
And you had the chance for January exams; for some, January just isn't an option. It's either June or nothing.

I revised last year, but I revised and didn't make sure that I had properly understood things. It's all well and good knowing what to say, but it's about knowing when, how and why to say it. My teacher famously says 'skip out all the bs of a question' when it's full of words that you don't really need to do the calculation, but I took it too far and stopped reading questions altogether; and really, that's how I lose my marks which accounts for MCQ being my worst section :P
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LeaX
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Does anyone know if we need to know the names and structures of different amino acids? Such as glycine, alanine, cysteine, phenylalanine, and aspartic acid. Thank you in advance.
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AS01
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(Original post by David Tennant)
I'm sorry you didn't get in, do you mind me asking where you applied at?
I applied to St.George's, Queen Mary, Sheffield and Birmingham. Did you get offers?
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