ocr a f325 revision thread

Scroll to see replies

Reply 1960

Theafricanlegend

Original post by X44

What is the hardest f325 past paper you have done guys? I wanna have a go!!

Posted from TSR Mobile

jan13

Reply 1961

chignesh10

Original post by eggfriedrice

Why when tin (II) chloride is added to Fe3+ and reduces it to Fe2+, I said the equation would be

Sn 2+ + Fe3+ -> Sn3+ - Fe 2+
But it should actually be Sn2+ + 2 Fe3+ -> Sn4+ + 2Fe2+

Why is my one wrong?

Question 8 b Jan 2013.

because the info in the table says it goes to sn4+

Reply 1962

eggfriedrice

Original post by chignesh10

because the info in the table says it goes to sn4+

Yup just noticed xD

Reply 1963

JimmyA*

Original post by shootingstars123

That makes sense, but the mark scheme says:

CH3OH +H2O ---> 6H+ +6e– +CO2

I'm not sure how they got this :s-smilie:

You times the half-equation that it gives you by 1.5 and then do it.

Reply 1964

bmpink

Original post by Tendael

The mark scheme says "add acid" or add H+. It is legacy so it could be something to do with Cr2072- that isn't on our spec anymore. Thanks for clearing up the Cl thing too.

Another question for anyone:
Suggest why the Cobalt(III) oxidation state is more stable in ammonia than in water. Just 1 mark. Quite confused

Ohhh ok, so basically the H+ will react with the OH-, in the H+ + OH- > H2O equilibrium, so that will shift to the right, and the first equilibrium will shift to the left (or right, can't remember which side the OH- ions were on, but that side) to replace some of the used OH-. It's actually really simple! It's just one of those things you can't see unless you get told the answer I fear!

Did someone else answer about the Cobalt(III) question? Is it to do with electronegativity?

Also do you know where I can get the legacy papers? A person put them all on in a comment once and I lost the page.

Reply 1965

bmpink

I find fuel cells questions really hard!!! Can anyone explain them to me!?

Reply 1966

Sarangtaec

Original post by eggfriedrice

And because you should multiply the ones involving Fe by 2 to balance out the electrons =)

Reply 1967

Sarangtaec

Does anyone know if an acid contains 2 H+s for example H2SO4, should I do pH = -log([H+]x2)?? Or do I just leave it as one?

Reply 1968

needtosucceed=)

Original post by Sarangtaec

Does anyone know if an acid contains 2 H+s for example H2SO4, should I do pH = -log([H+]x2)?? Or do I just leave it as one?

you only have to know how to calculate the ph of monoprotic acids :smile:

Reply 1969

_HabibaH_

Original post by X44

What is the hardest f325 past paper you have done guys? I wanna have a go!!

Posted from TSR Mobile

F325 - enjoy :devil3:

Reply 1970

Tendael

Original post by shootingstars123

That makes sense, but the mark scheme says:

CH3OH +H2O ---> 6H+ +6e– +CO2

I'm not sure how they got this :s-smilie:

Having exact same problem, wrote the same thing down with 4H and 4E instead. Don't really see where they got the extra 2H/e and H20 from.

Reply 1971

needtosucceed=)

Original post by Tendael

Having exact same problem, wrote the same thing down with 4H and 4E instead. Don't really see where they got the extra 2H/e and H20 from.

something about multiplying by 1.5 because theres 1.5 moles of o2. someone explained it a couple of pages back :smile:

Reply 1972

_HabibaH_

Original post by Tendael

Having exact same problem, wrote the same thing down with 4H and 4E instead. Don't really see where they got the extra 2H/e and H20 from.

Because there are 6H+ on the other side (LFS) so to balance that. And the e- then balance the 6H+ that you put down on the RHS.

Reply 1973

Sarangtaec

Original post by needtosucceed=)

you only have to know how to calculate the ph of monoprotic acids :smile:

Really?? Oh haha I'm just scared they'll do something sneaky like they always do -.- Don't want another run of January 2013... but would it be correct to multiply [H+] by 2?

Reply 1974

überambitious_ox

Could someone please help me with the question attached which is from the June '10 paper:

June 10.JPG

I have some ideas as to what to do (electron balancing/redox balancing), but not entirely sure...if someone could give me some help that would be great, thank you :smile:

Reply 1975

needtosucceed=)

Original post by Sarangtaec

Really?? Oh haha I'm just scared they'll do something sneaky like they always do -.- Don't want another run of January 2013... but would it be correct to multiply [H+] by 2?

no idea im afraid :/ in the specification it states only for monoprotic acids :/
it seems like a reasonable thing to do if it does come up :smile:

YESS 1st post on page100! thanks namod :wink:

(edited 10 years ago)

Reply 1976

Tendael

Original post by _HabibaH_

Because there are 6H+ on the other side (LFS) so to balance that. And the e- then balance the 6H+ that you put down on the RHS.

But if you don't put down the H20 there's only 4 hydrogens on the LHS. There are two waters on the RHS of the overall equation, and also two waters on the right hand side of the mentioned electrode, so why do you need to add in one to the LHS in (ii)?

Reply 1977

JimmyA*

Original post by Tendael

Times the half equation they give you by 1.5
Then do it and you'll see it works

Reply 1978

Tendael

Original post by JimmyA*

Times the half equation they give you by 1.5
Then do it and you'll see it works

Why though? I sort of understand that the equation would work if you multiply the original equation by 1.5 but if i'm doing this exam first time I would have no implication to do that. So what would be your reasoning for doing that without having seen the MS?