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AQA CHEM5 A2 Chemistry - 19th June 2013

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Original post by Lucy-1995
This may sound stupid, but whats valence??


valence is typically, the number of electrons needed to fill the outermost shell of an atom.
so carbon has a valence of 4 because it needs 4 electrons to fill its outer shell which is why it forms 4 bonds with other elements.
Reply 1221
Avatar for emma.bcx
emma.bcx
I came across these questions in the Jan 12 paper, and I got really confused..

8e) Diaminoethane (H2NCH2CH2NH2), like ammonia, can react as a base and as a ligand.

8ei) Write an equation for the reaction that occurs between an aqueous solution of aluminium chloride and an excess of aqueous diaminoethane.

8eii) Write an equation for the reaction that occurs between an aqueous solution of cobalt (II) sulphate and an excess of aqueous diaminoethane.

Ok so in the first question, the answer was 2[Al(H2O)6]3+ + 3H2NCH2CH2NH2 --> 2[Al(H2O)3(OH)3] + 3[H3NCH2CH2NH3]2+ ... so the diaminoethane is acting as a BRONSTED LOWRY BASE (proton acceptor).

In the second question, the answer was [Co(H2O)6]2+ + H2NCH2CH2NH2 --> [Co(H2NCH2CH2NH2)3]2+ + 6H2O .... so acting as a LEWIS BASE or a LIGAND (lone pair donor).

I understand how both these reactions work, but what I don't understand is how do I know which reaction to write for each question? Because they are both worded exactly the same! Or am I being completely stupid (which is actually rather likely) :wink:

X
All we need to know about DeltaE = hv is what it stands for dont we?
Original post by gooner1886
All we need to know about DeltaE = hv is what it stands for dont we?


Yes, no calculations
Original post by cuckoo99
valence is typically, the number of electrons needed to fill the outermost shell of an atom.
so carbon has a valence of 4 because it needs 4 electrons to fill its outer shell which is why it forms 4 bonds with other elements.


Oh okay thanks :smile:
Original post by RoaringLion
What synoptic stuff do we need to know?


Posted from TSR Mobile


Unit 5 is not synoptic unlike AQA biology.
Original post by frogs r everywhere
Unit 5 is not synoptic unlike AQA biology.

It is synoptic to Units 1 and 2, mostly Unit 1
Can someone explain to me why on the Jan12 paper, Q4(e)(i) the equation is 2Fe3+ + H2 --> 2Fe2+ + 2H+ instead of 2Fe2+ + 2H+ --> 2Fe3+ + H2 ????
Original post by tr12
Can someone explain to me why on the Jan12 paper, Q4(e)(i) the equation is 2Fe3+ + H2 --> 2Fe2+ + 2H+ instead of 2Fe2+ + 2H+ --> 2Fe3+ + H2 ????

You have to look at which is being oxidised and reduced in order to make the overall equation for the cell. In this instance, the Fe3+ electrode has a higher pd and is therefore being oxidised and so stays the way it is, whereas the H+ electrode is being reduced, so you have to flip the sign over before combining the two equations

I realise that sounds a little disjointed, ask for further clarification if needed :smile:
Original post by cuckoo99
Yes, no calculations



Delte E = energy gained by electrons
H = plancks constant
v = frequency of light absorbed

And thats it right?
Original post by gooner1886
Delte E = energy gained by electrons
H = plancks constant
v = frequency of light absorbed

And thats it right?


That is correct, thank God I do physics, didn't have to learn much new stuff for that part :biggrin:
Reply 1231
Avatar for emma.bcx
emma.bcx
I came across these questions in the Jan 12 paper, and I got really confused..

8e) Diaminoethane (H2NCH2CH2NH2), like ammonia, can react as a base and as a ligand.

8ei) Write an equation for the reaction that occurs between an aqueous solution of aluminium chloride and an excess of aqueous diaminoethane.

8eii) Write an equation for the reaction that occurs between an aqueous solution of cobalt (II) sulphate and an excess of aqueous diaminoethane.

Ok so in the first question, the answer was 2[Al(H2O)6]3+ + 3H2NCH2CH2NH2 --> 2[Al(H2O)3(OH)3] + 3[H3NCH2CH2NH3]2+ ... so the diaminoethane is acting as a BRONSTED LOWRY BASE (proton acceptor).

In the second question, the answer was [Co(H2O)6]2+ + H2NCH2CH2NH2 --> [Co(H2NCH2CH2NH2)3]2+ + 6H2O .... so acting as a LEWIS BASE or a LIGAND (lone pair donor).

I understand how both these reactions work, but what I don't understand is how do I know which reaction to write for each question? Because they are both worded exactly the same! Or am I being completely stupid (which is actually rather likely) :wink:

X
Original post by Sherlockedd
You have to look at which is being oxidised and reduced in order to make the overall equation for the cell. In this instance, the Fe3+ electrode has a higher pd and is therefore being oxidised and so stays the way it is, whereas the H+ electrode is being reduced, so you have to flip the sign over before combining the two equations

I realise that sounds a little disjointed, ask for further clarification if needed :smile:


How did you know Fe3+ had a higher pd?
What effect does the concentration of reactants have in electrochemical cells.
Example: In a zinc and Iron cell, what effect does changing concentrations of zinc or iron have on emf?
Can anyone tell me how you would write an equilibirum expression for this?
Original post by Sherlockedd
It is synoptic to Units 1 and 2, mostly Unit 1

Compared to biology, not really.
Well, A2 candidates are expected to know how to do simple molar calculations etc.
I did Chem5 in January with no knowledge of Chem4.
Original post by tr12
How did you know Fe3+ had a higher pd?

Well the SHE has a pd of 0 by definition and it shows on the table they give you in the paper that Fe3+ has a positive pd :smile:
Original post by frogs r everywhere
Compared to biology, not really.
Well, A2 candidates are expected to know how to do simple molar calculations etc.
I did Chem5 in January with no knowledge of Chem4.

Yeah, but chem 5 is not synoptic to chem 4, only to chem 1 and 2 :L
like in unit 2, do you get full marks just for the right answer in calculations or it is step by step marking? lol poor chemistry, been neglected by me over the weekend till bio5 has been castrated.
Original post by frogs r everywhere
Compared to biology, not really.
Well, A2 candidates are expected to know how to do simple molar calculations etc.
I did Chem5 in January with no knowledge of Chem4.


its sinoptic for AS topics. lots of stuff from unit 1 chemistry comes up in unit 5, like electronegativity and intermolecular forces. also stuff from unit 2 like enthalpy of formation and bond enthalpys.

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