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

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Original post by Serine Soul
I can't believe how badly I've been taught redox and fuel cells

I need to go over it 100 times :cry:


Students find this hard because teachers do not explain the difference between voltaic and electrolytic cells.


http://chemwiki.ucdavis.edu/Core/Analytical_Chemistry/Electrochemistry/Electrolytic_Cells/Electrolysis
Original post by itsConnor_
Why is the conc of butanoic acid 0.1?

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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
Original post by TeachChemistry
Yes batteries run down because current has been allowed to flow.

Think about what you are measuring (emf) as a push from the cell. You need to measure the magnitude of that push by, in effect, pushing back using electrical resistance. If you resist enough, current will be negligible.

If you are ok with physics think about the resistance that an ammeter must have, because it goes in series to the other components, and the resistance a voltmeter must have because it goes parallel to the component you are measuring.

I will draw a picture and upload it.


image.jpeg
image.jpg
Original post by itsConnor_
Why is the conc of butanoic acid 0.1?

Posted from TSR Mobile


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
(edited 7 years ago)
Original post by TeachChemistry
image.jpeg


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?? :frown:


http://www.thestudentroom.co.uk/showthread.php?p=65895832
Do I need to be doing any specific ABG/CER revision before EEE- do they ever ask random AS questions?
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


Original post by Dentistry101
image.jpg

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

Thanks guys :smile:

Gonna try an Edexcel one I found and see if I can do it this time!
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?? :frown:


http://www.thestudentroom.co.uk/showthread.php?p=65895832


Your chemical equations are correct. If you work forward from a supposedly correct answer of 0.51M you will find that the titre values for both oxidizing agents are nearly the same value so there is something wrong in the question.

And I think I might have just spotted the error in the question. BRB.
Original post by Serine Soul
I can't believe how badly I've been taught redox and fuel cells

I need to go over it 100 times :cry:


Same!! We basically had to teach ourselves it and i'm still pretty inconsistent on the exam qs


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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?? :frown:


http://www.thestudentroom.co.uk/showthread.php?p=65895832


If you get the question number and the paper and put it into youtube I think MaChemGuy did a really clear explanation it
Could anyone help with june 14, with the laat part of the last question?

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Original post by Dentistry101
Multiply what by 24? Could you write your working out please? Why did you use V/24=N?



So basically you find the emperical formula as normal as N2O
because gdm-3 x24 = gmol-1 which is the Mr to confirm that the molecular mass is N2O as the answer
x24 because number of moles = v/24 for a gas, you just have to make the association with density and how much a gas occupies a space
How does the hydrogen fuel cell actually work?
I don't understand all the stuff about in alkaline or acidic conditions
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?? :frown:


http://www.thestudentroom.co.uk/showthread.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.
(edited 7 years ago)
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.
Original post by Dentistry101
image.jpg

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!! :smile:



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1466272527020.jpg46.9KB
Original post by Messier31
Same!! We basically had to teach ourselves it and i'm still pretty inconsistent on the exam qs


Posted from TSR Mobile


Probably because your teacher didn't understand how to do it. Not good.
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
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?
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* :cry:

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