You are Here: Home

AQA - Unit 5 - Energetics, Redox and Inorganic Chemistry watch

1. (Original post by SkyHunter)
i'm confused!
Forget about the emf values since you're given the equation of reaction. Work out from the equation the species that is reduced and the species that is oxidised.
In cell rep, species that is reduced is on the right in this case (I2/I-) and the species that is oxidised is on the left (in this case Br2/Br-) .

emf = Eright - Eleft. and this gives a negative value.

WHENEVER YOU ARE GIVEN THE EQUATION, YOU NEED TO WORK OUT THE OXIDISED AND REDUCED SPECIES.
But if you are not given the equation, do it the normal way (which I think you should understand).
If you do not understand still, I will break it down more yh.
2. (Original post by Crowned Temmy)
Forget about the emf values since you're given the equation of reaction. Work out from the equation the species that is reduced and the species that is oxidised.
In cell rep, species that is reduced is on the right in this case (I2/I-) and the species that is oxidised is on the left (in this case Br2/Br-) .

emf = Eright - Eleft. and this gives a negative value.

WHENEVER YOU ARE GIVEN THE EQUATION, YOU NEED TO WORK OUT THE OXIDISED AND REDUCED SPECIES.
But if you are not given the equation, do it the normal way (which I think you should understand).
If you do not understand still, I will break it down more yh.
I understandddd! thanks a lot )) x
3. Hey can anyone please explain to me how and when Pt is used when writing conventional cell representations? left, right both?

And when writing cell equations we don't need to include H+ and H20 in the overall eq. do we?
4. Can anyone please how to do Born Haber cycles for enthalpy of solution? I don't get it!! Thanks.
5. (Original post by NRican)
Hey can anyone please explain to me how and when Pt is used when writing conventional cell representations? left, right both?
Both sides, I think it isn't used when it is done with hydrogen.
6. (Original post by Sparkly-Star)
Can anyone please how to do Born Haber cycles for enthalpy of solution? I don't get it!! Thanks.
I didn't bother I think in the exam calculation is enough? , just remember the equation E soln = E hydration + E lattice dissociation.
7. (Original post by NRican)
I didn't bother I think in the exam calculation is enough? , just remember the equation E soln = E hydration + E lattice dissociation.
Oh ok thanks, I didn't know that was the equation!! I have seen past questions on it that's why so I don't really get those type.
8. Who's doing an all-nighter? (Or in my case... half-nighters lol)
9. guys whats the O.S. of Cr in this compound Cr2O72- is it +6 or +12 and 2Cr3+ does 2 have any effect before Cr.thanx
10. The spec states that we should know the half cell equations for fuel cells. Do we have to know the half equations for other types of electrochemical cell as well?
11. (Original post by Sparkly-Star)
Both sides, I think it isn't used when it is done with hydrogen.
But I've seen it done on one side only
12. (Original post by Sparkly-Star)
Who's doing an all-nighter? (Or in my case... half-nighters lol)
meeee bt till 4 coz need to sleep for 3 hr after that then start cracking
13. (Original post by NRican)
But I've seen it done on one side only
Really? That's weird, I learnt it so that it is used on both sides.
14. (Original post by NRican)
But I've seen it done on one side only
(Original post by Sparkly-Star)
x
It's used when there isn't a solid involved.

E.g. Zn2+(aq) + 2e- ---> Zn(s)

doesn't need platinum as it has a solid (Zn) which acts as the electrode so it becomes:

Zn(s)|Zn2+(aq) ||

However,

Fe3+(aq) + e- ---> Fe2+(aq)

there is no solid involved here so a platinum electrode is needed to transfer electrons, and the cell becomes:

Pt | Fe2+(aq), Fe3+(aq) ||

So if you see a half cell equation with no solid involved, chuck in platinum

It can be left or right depending on if the half cell without the solid is more negative or more positive, and can appear on both if neither half cell involves a solid as it does with the hydrogen fuel cell:

Pt|H2(g)|OH-(aq), H2O(l) || O2(g) | H2O(l), OH-(aq)|Pt

Hope this helped
15. (Original post by jimmy303)
It's used when there isn't a solid involved.

E.g. Zn2+(aq) + 2e- ---> Zn(s)

doesn't need platinum as it has a solid (Zn) which acts as the electrode so it becomes:

Zn(s)|Zn2+(aq) ||

However,

Fe3+(aq) + e- ---> Fe2+(aq)

there is no solid involved here so a platinum electrode is needed to transfer electrons, and the cell becomes:

Pt | Fe2+(aq), Fe3+(aq) ||

So if you see a half cell equation with no solid involved, chuck in platinum

It can be left or right depending on if the half cell without the solid is more negative or more positive, and can appear on both if neither half cell involves a solid as it does with the hydrogen fuel cell:

Pt|H2(g)|OH-(aq), H2O(l) || H2O(l) | O2(g), OH-(aq)|Pt

Hope this helped
Thanks a lot
16. (Original post by jimmy303)
It's used when there isn't a solid involved.

E.g. Zn2+(aq) + 2e- ---> Zn(s)

doesn't need platinum as it has a solid (Zn) which acts as the electrode so it becomes:

Zn(s)|Zn2+(aq) ||

However,

Fe3+(aq) + e- ---> Fe2+(aq)

there is no solid involved here so a platinum electrode is needed to transfer electrons, and the cell becomes:

Pt | Fe2+(aq), Fe3+(aq) ||

So if you see a half cell equation with no solid involved, chuck in platinum

It can be left or right depending on if the half cell without the solid is more negative or more positive, and can appear on both if neither half cell involves a solid as it does with the hydrogen fuel cell:

Pt|H2(g)|OH-(aq), H2O(l) || H2O(l) | O2(g), OH-(aq)|Pt

Hope this helped
Thank you, that did really help! I had no idea about all this! Shame on me.
17. theres barely any synoptic in unit 5 chem ! its not like bio lol
i dont think there was any in the jan11 paper
i probs wont revise any synoptic
18. (Original post by al_habib)
guys whats the O.S. of Cr in this compound Cr2O72- is it +6 or +12 and 2Cr3+ does 2 have any effect before Cr.thanx
6+ for Cr2O72-

3+ for 2Cr3+ but counts as +6 due to the '2'

19. (Original post by choc1234)
theres barely any synoptic in unit 5 chem ! its not like bio lol
i dont think there was any in the jan11 paper
i probs wont revise any synoptic
There's the periodicity stuff that's a little synoptic :P
20. (Original post by Sparkly-Star)
Can anyone please how to do Born Haber cycles for enthalpy of solution? I don't get it!! Thanks.
errrrr ok, ill try

?Hsol = ?Hlatt dissociation + ?Hhyd of gaseous ions

because lattice enthalpy and lattice dissociation both have the same enthalpy change symbols, the way to distinguish between them is the value.
because making bonds is exothemic, -ve value. (lattice enthalpy)
and because breaking bonds is endo thermic, +ve value. (lattice dissociation)

so now the born haber cycle (in terms of the diagram with the levels),

start with the solution of a ionic compound, eg. NaCl(s) ---> Na+(aq) + Cl-(aq)

so, ?Hlatt. diss. is +ve, so the products on the diagram increase to a higher energy level ......(Na+(g) + Cl-(g))

?Hhyd is -ve, so the products of this reaction is of a lower energy, but still higher than NaCl(s) .......(Na+(aq) + Cl-(aq))

Hess' law states ?H is the same independant of the route. so if we rewind back from (Na+(aq) + Cl-(aq)) to (Na+(g) + Cl-(g)) and then finally to NaCl(s), this is ?Hsol

the same can be applied to the cycle

write the solution reaction.
then from the ionic compound, write the lattice diss. and its products.
then from the products, the hydration of each of the ions.
bam. complete cycle

i hope this isnt too much waffle. it'd be much easier if i had a diagram

EDIT: silly formatting, the question marks are meant to be triangles for delta

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: August 20, 2011
Today on TSR

Are you living on a tight budget at uni?

From budgets to cutbacks...

University open days

1. University of Cambridge
Wed, 26 Sep '18
2. Norwich University of the Arts
Fri, 28 Sep '18
3. Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 29 Sep '18
Poll
Useful resources

Can you help? Study Help unanswered threadsStudy help rules and posting guidelines

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE