OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD

Cathode
Positive
Reduction

The equation being reduced is the positive electrode and therefore in an electro-chemical cell the cathode. CPR :wink:

I THOUGHT in ELectrochemical cells, it is Cathode OXidation Negative and Reduction Anode Positive!!! :P

Reply 2801

thatblackguy

1

In electrochemical cells is the negative terminal equivalent to the positive electrode?

Reply 2802

username2640523

8

Original post by Ss0

any help appreciated.... entropy questions!!!!

What is the Enthalpy change for that question?

Reply 2803

LThomas694

2

Original post by Ss0

any help appreciated.... entropy questions!!!!

There are fewer moles of gas in the products so entropy decreases so delta S is negative, so -TdeltaS is positive. The reaction is exothermic so delta H is negative. As temperature increases the magnitude of TdeltaS increases, if the magnitude of TdeltaS is greater than delta H, delta G will be greater than 0 so the reaction will no longer be feasible

Reply 2804

KB_97

10

Original post by suibster

Rule: Oxidation= Negative terminal , Reduction=Positive terminal

Identify what's being oxidised and reduced.

O2 is being reduced so ------> Positive terminal

Negative terminal (Al) = -2.31 V

Ahh thanks. I read the question wrong and thought those where the two electrode potentials and didn't realise one was the cell potential.

I like your rule for figuring out which is the positive and negative terminal. Thank you!

Reply 2805

KB_97

10

is everyone expecting a hard paper because f324 was pretty easy? Apparantly their as papers this year were easy too so maybe they'll follow the trend or make this their hardest one yet.

Reply 2806

megafidget

1

Original post by KB_97

is everyone expecting a hard paper because f324 was pretty easy? Apparantly their as papers this year were easy too so maybe they'll follow the trend or make this their hardest one yet.

Last year this paper was 87 for an A* and 79 for an A, it was the highest ever...., so yeh I'm definitely expecting a tough one this year :frown:

Reply 2807

suibster

3

Original post by Ss0

any help appreciated.... entropy questions!!!!

Assuming the entropy is negative as the number of gas moles decreased.

So Delta G = Delta H - (T x -Delta S)

This makes T delta S positive so

Delta G = Delta H + TDeltaS

As temperature increases, TDeltaS increases, and when TDeltaS>DeltaH.
The Delta G is > 0. Reaction is not feasible if the free energy(Delta G) is bigger than 0.

Reply 2808

KB_97

10

Original post by megafidget

Last year this paper was 87 for an A* and 79 for an A, it was the highest ever...., so yeh I'm definitely expecting a tough one this year :frown:

Well f***. after the aqa core 3 disaster my only hope for a* is chemistry. It's never gonna happen.

Reply 2809

thad33

12

Does anyone have a nice way of explaining cathodes and anodes? I keep mixing them up. I know the direction that the electrons flow in but everywhere I look to help explain the terminals, they contradict each other.

Reply 2810

AqsaMx

5

Original post by RetroSpectro

You have to look at this in terms of the Kc expression

Kc = [N2O4] / [NO2]^2

Say we increase pressure by times 2. This means that

Kc = 2 / 2^2
= 2 / 4

Clearly we can see that the bottom of the Kc expression has increased by more than the top. However, the value of Kc cannot change in regards to pressure, therefore the equilibrium shifts right to increase N2O4 and restore the value of Kc.

Hope this helped

Thanks for all your help!

Reply 2811

username2703735

18

Original post by KB_97

is everyone expecting a hard paper because f324 was pretty easy? Apparantly their as papers this year were easy too so maybe they'll follow the trend or make this their hardest one yet.

I would say from reading many of the conflicting responses about the F324 paper to say it was easy is not entirely true. If anything I believe many here would have scored higher if they sat the 2015 F324 as it had less of the... well... nonsense like the fats and stitches questions. My chem teacher looked at the 2016 F324 and said he would be quite surprised if the grade boundaries changed too much from last year. As far as a pattern I'm not really sure one can be drawn for the F325 grade boundaries.
June 2015: A -79
June 2014: A-74
June 2013: A-75
January 2013: A-70
June 2012: A-76
January 2012: A-77
June 2011: A-72

(edited 7 years ago)

Reply 2812

username2703735

18

Original post by thad33

Does anyone have a nice way of explaining cathodes and anodes? I keep mixing them up. I know the direction that the electrons flow in but everywhere I look to help explain the terminals, they contradict each other.

Cathode, Positive, Reduction

Reply 2813

tcameron

19

Original post by VMD100

I would say from reading many of the conflicting responses about the F324 paper to say it was easy is not entirely true. If anything I believe many here would have scored higher if they sat the 2015 F324 as it had less of the... well... nonsense like the fats and stitches questions. My chem teacher looked at the 2016 F324 and said he would be quite surprised if the grade boundaries changed too much from last year. As far as a pattern I'm not really sure one can be drawn for the F325 grade boundaries.
June 2015: A -79
June 2014: A-74
June 2013: A-75
January 2013: A-70
June 2012: A-76
January 2012: A-77
June 2011: A-72

I agree, there were many sly questions in the f324 paper that I feel a lot, including myself, would have missed out marks on but I think the grade boundaries will be higher than last years but I doubt it'll be in the 50's for an A

Surprised last years f325 was the highest marks for an A in this unit as I feel there were lots of easier papers than last years, strange

(edited 7 years ago)

Reply 2814

HediIsLord

7

Original post by tcameron

I agree, there were many sly questions in the f324 paper that I feel a lot, including myself, would have missed out marks on but I think the grade boundaries will be higher than last years but I doubt it'll be in the 50's for an A

It won't be in the 50s, albeit a far nicer paper than last year's

I think people left the exam hall feeling content since it was relatively straight forward in most aspects, i.e. the NMR question

but lots of opportunities for small mistakes that filter the A/A* students out from the rest mean it wasn't as easy as most people say it was

Reply 2815

tcameron

19

Original post by HediIsLord

It won't be in the 50s, albeit a far nicer paper than last year's

I think people left the exam hall feeling content since it was relatively straight forward in most aspects, i.e. the NMR question

but lots of opportunities for small mistakes that filter the A/A* students out from the rest mean it wasn't as easy as most people say it was

hahah I guess I was one of the few who messed up my nmr question by not checking the number of proton environments - good thing it was only one mark

Reply 2816

tcameron

19

what is a conventional electrochemical cell? I always see it in questions but don't know what it actually is

Reply 2817

username2703735

18

Original post by tcameron

what is a conventional electrochemical cell? I always see it in questions but don't know what it actually is

Well the half-cells put together are technically an electrochemical cell but I am assuming it is referring to a battery?

Reply 2818

username2221225

7

Original post by tcameron

what is a conventional electrochemical cell? I always see it in questions but don't know what it actually is

It might be referring to a standard cell like battery which has all it's fuel/chemicals.
In comparison to a fuel cell, like a hydrogen fuel cell that needs a constant supply of fuel.