AQA CHEM5 A2 Chemistry - 19th June 2013

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Original post by frogs r everywhere

Its not a hard chapter. What do you need to know?

If an entity has a higher E standard value, it gets reduced.
If another entity has a lower e standard value, it gets oxidised.

This now apples to all questions to do with emf.

ok so whats a salt bridge? Thanks btw

Reply 921

louise_234

can someone please explain to me what the function of the electrolyte is in electrochemics cells? why is it in some cells and not others? and why is it needed in the hydrogen oxygen fuel cells ??

Reply 922

louise_234

Original post by ChemBoss

ok so whats a salt bridge? Thanks btw

a salt bridge allows ions to flow through and balances out the charges

Reply 923

coolstorybrother

do we need to know the flames colours for the p3 oxides?

Reply 924

SamEastie

Original post by coolstorybrother

do we need to know the flames colours for the p3 oxides?

yes

Reply 925

Gazfink

Original post by coolstorybrother

do we need to know the flames colours for the p3 oxides?

go back a couple pages. They're written there :-D

Reply 926

Gazfink

Original post by louise_234

can someone please explain to me what the function of the electrolyte is in electrochemics cells? why is it in some cells and not others? and why is it needed in the hydrogen oxygen fuel cells ??

Anelectrolyte is basically a fluid with ions dissolved into it. Sometimes a polymer fluid is used because this can conduct a current whilst remaining viscous to prevent leaking out. (This is why an electrolyte is used in mobile phone batteries) the hydrogen oxygen fuel cell needs an electrolyte because (I think) water may react if the reaction uses aqueous reactant. So the electrolyte is there to be inert and good at electrical conduction.

Reply 927

A sheesh

I've been so inconsistent in past papers.
ive done 3 and got 72,76,81

its so annoying, some papers seem to be harder than others,

though on all these I lost round 10 marks on colours since I haven't bothered to revise them yet :P

Reply 928

Dirtybit

Oh dear haven't started Past papers for this yet...

Reply 929

lmon

I am terrible at redox equilibria ! When writing the cell diagram how do you figure out which way around each half cell is ? :s-smilie:

Reply 930

RoaringLion

Have you guys learnt most of the content?

Posted from TSR Mobile

Reply 931

Dalts

Original post by lmon

I am terrible at redox equilibria ! When writing the cell diagram how do you figure out which way around each half cell is ? :s-smilie:

The more positive value is always on the right hand side, except for the standard hydrogen electrode which must always be on the left

Reply 932

BobLoblawLawBlog

Has anyone got a list of all the definitions you need to know for this exam?

Reply 933

louise_234

when doing the conventional representation of electrochemical cells, are there phase boundaries between aqueous and liquid? because on january 2010 paper question 3c it doesn't show phase boundaries between them, unless it is because the commas are seperating reactants and products?

Reply 934

DELETED ACCOUNT

Original post by RoaringLion

Have you guys learnt most of the content? Posted from TSR Mobile

There's not much content really

Reply 935

Chewy29

Original post by louise_234

No phase boundary as an aqueous solution is in liquid phase. At our level only 3 phases exist.

Posted from TSR Mobile

Reply 936

louise_234

Original post by Chewy29

No phase boundary as an aqueous solution is in liquid phase. At our level only 3 phases exist.

Posted from TSR Mobile

thank you

Reply 937

Jesss9

Original post by Dirtybit

Oh dear haven't started Past papers for this yet...

Same

I'm doing chem 2, 4 and 5 but I'm worried I'm leaving chem 5 too late :s-smilie:

Reply 938

BobLoblawLawBlog

Please can someone explain the answer to question 3(c) on the CHM5 Jan 2005 Paper. It's on electrochemical cells and I have no idea how they get the answer in the mark scheme.

Thanks.

AQA-CHM5-W-QP-JAN05.pdf122.4KB

AQA-CHM5-W-MS-JAN05.pdf162.0KB

Reply 939

Simonon

Original post by brittanna

Please can someone explain the answer to question 3(c) on the CHM5 Jan 2005 Paper. It's on electrochemical cells and I have no idea how they get the answer in the mark scheme.

Thanks.

The emf of the reaction "VO2+(aq) + 2H+(aq) + e– → VO2+(aq) + H2O(l)" is less than the one for MnO4- so V2+ makes V3+ which makes VO(2+) which makes VO2(+). The MnO4- must be reduced.

VO2+ +5 + -4 = +1

And the equation should be pretty straight forward once you know the final species because you just balance the oxygen and water etc.

Hope that helped because that first part is always hard to explain!