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

1. (Original post by thesmallman)
Hey, I tried to solve a really difficult redox titration question on this thread- but couldn't seem to be able to - Can you help please??

http://www.thestudentroom.co.uk/show...php?p=65895832
It is very unsatisfying to not have the original source question to check the wording BUT, and I'm not saying this is the original version, if you assume that the titres for each oxidizing agent when added together comes to 25.50 cm3 then you can work through the problem and you will get to 0.51M as a final answer.
2. (Original post by tcameron)
How does the hydrogen fuel cell actually work?
I don't understand all the stuff about in alkaline or acidic conditions
If you had a jar of oxygen and a jar of hydrogen and reacted them together (with a spark) you would get water and a lot of heat.

The hydrogen fuel cell works by separating the conversion of these gases into water into separate transformations and capturing the released energy not as heat but as an electrical current.
3. (Original post by Dentistry101)
Attachment 552460

Basically you find the moles of butanoic acid originally and the moles of Naoh. The moles of butanoic acid in equilibrium are the moles of butanoic acid you calculated minus the moles of Naoh you calculated. Then find the concentration of butanoic acid doing c x v= n using the new moles you calculated and you get 0.1
(Original post by thesmallman)
the butanoic acid is in excess (0.0125 mol), whereas NaOH is 0.0025 mol.

So, moles of butanoic acid remaining= 0.0125-0.0025=0.01

conc of butanoic acid= 0.01/0.1=0.1 mol dm^-3
Thanks guys!!

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4. (Original post by Messier31)
Same!! We basically had to teach ourselves it and i'm still pretty inconsistent on the exam qs

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Probably because your teacher didn't understand how to do it. Not good.
5. (Original post by TeachChemistry)
It is very unsatisfying to have the original source question to check the wording BUT, and I'm not saying this is the original version, if you assume that the titres for each oxidizing agent when added together comes to 25.50 cm3 then you can work through the problem and you will get to 0.51M as a final answer.
Ahhh I got the answer now- thanks a lot for your help!!! Got me thinking for so long ahaha
6. (Original post by TeachChemistry)
If you had a jar of oxygen and a jar of hydrogen and reacted them together (with a spark) you would get water and a lot of heat.

The hydrogen fuel cell works by separating the conversion of these gases into water into separate transformations and capturing the released energy not as heat but as an electrical current.
Thank you
but what has it got to do with electrode potentials?
7. (Original post by Messier31)
Same!! We basically had to teach ourselves it and i'm still pretty inconsistent on the exam qs

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Me too, I always lose marks on that section
I'm just hoping my practical mark and F324 will keep up an A*
8. (Original post by Serine Soul)
Me too, I always lose marks on that section
I'm just hoping my practical mark and F324 will keep up an A*
Same, those and the low F325 boundaries
9. (Original post by tcameron)
Thank you
but what has it got to do with electrode potentials?
10. Studying on your own is always the best. You get so much more done imo.
11. (Original post by TeachChemistry)
how does the topic on electrode potentials lead into the stuff about fuel cells?
12. Does anyone have a link to last year's F325 paper and mark scheme?!
13. (Original post by tcameron)
how does the topic on electrode potentials lead into the stuff about fuel cells?
Because the operation of fuel cells depends on the half reactions and electrode potentials.

e.g. under alkaline conditions

2H2O + 2e <--> H2 + 2OH- E = -0.83V
0.5O2 + H2O + 2e <--> 2OH- E = +0.40V

Since the top reaction is more negative than the bottom one, the top reaction will run from right to left. Overall, hydrogen and oxygen is converted to water and the electrons in those half equations that are being transferred are the current for the cell.

Use values in this table to construct a similar whole cell from two half cells under acidic conditions. You will see that E cell is the same.

https://en.wikipedia.org/wiki/Standa...al_(data_page)
14. Anyone have any method to remember colour changes?

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15. (Original post by ImNervous)
No not really
But I guess it would be because the chloride ions that are part of the complex have formed co - ordinate bonds which are essentially dative covalent bonds and are stronger than ionic bonds. I'm not too sure though. And have you checked the answer on the mrkscheme to make sure its complex B? If not could you please check. Thank you very much
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Ah, that would make sense
Yeah, ive just looked and your answer and reasoning were correct 👍

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16. What are the colour changes we need to know for this exam??
Would we get asked about the colour of any transition element?- so confused lol
17. (Original post by thesmallman)
What are the colour changes we need to know for this exam??
Would we get asked about the colour of any transition element?- so confused lol
I don't think we'd just be asked about any but we definitely need to know the colour changes associated with the precipitation reactioms of Cu2+, Fe2+ and Fe3+, and Co2+
We also need to the coloirs of three ligand substitution reactilns, I need to remind myself which ones.
We need to know that dichromate changes from orange to green too. Ive seen a question that asked about the difficulty of using permanganate in a titration, which required knowledge of its two oxidation states and their respective colours I believe
18. (Original post by thesmallman)
What are the colour changes we need to know for this exam??
Would we get asked about the colour of any transition element?- so confused lol
the book's image for transition metal colours
19. I don't know what I'm doing with titration curves. Can someone explain?

e.g. https://gyazo.com/b9b5bf5f7a6fbaf84bbcb3bd78916460
20. Can someone please elaborate on this statement from the Spec:

"explain the changes that take place at eachelectrode in a hydrogen–oxygen fuel cell"

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