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

1. (Original post by InItToWinItGetIt?)
But then why don't you divide by 50 to get moles in 5cm3 and then do c = n/v, instead of going straight to c = n/v?
Because that mole calculation is for the 25cm3 diluted sample which is taken from the whole 250cm3 sample. In the 250cm3 sample there will be 10x as many moles of the H2O2 than in the 25cm3 sample. These moles will represented the amount of moles that were originally dilluted. As the question asks you to calculate the original conc of the H2O2, you need to get the moles in the 250cm3, not just the25cm3 sample
2. Im starting to make some very silly errors!!
Here goes another question:

What is the Ar of Fe2+?

anyone??

nevermind it was my periodic tables fault, it has Mr's to 1 dp so ive been getting my answers wrong!!
3. Could someone explain the colorimetry section in 15.3? This and redox titrations are the only things I have left to do wooooo.
4. (Original post by Sparkly-Star)
I see you've posted this loads of times and I wish I knew the answer but I don't. Can anyone else PLEASE answer this thanks?
Haha!!

Yeah, I thought I was being ignored
Thanks
5. In the NT book, transition metal table. The very last chromium one is Cr(OH)3 which is written different to the rest but is this the same as Cr(H2O)3(OH)3.
6. HEEEELP! Right, the melting point of al2o3 is lower than MgO because Al3+ is more polarizing so there is a degree of covalent character... but why is it not higher then?! In lattice enthalpies a degree of cov character increases the lattice dissociation because the bonding becomes stronger?

Am i missing something here
7. (Original post by Sparkly-Star)
In the NT book, transition metal table. The very last chromium one is Cr(OH)3 which is written different to the rest but is this the same as Cr(H2O)3(OH)3.
Yes, sometimes the H2O isn't shown for some reason ( I think I read that somewhere)
8. (Original post by rrelish)
When writing the conventional cell rep. e.g in JAN 10 question 3C
http://store.aqa.org.uk/qual/gce/pdf...W-QP-JAN10.PDF

How do you know if to write the OH- before the H2O or H2O before the OH- ? or does it not matter

Thank you
I thought the OH- was part of the reactants, so you have to put it before H2O which is the product
9. (Original post by Sparkly-Star)
I see you've posted this loads of times and I wish I knew the answer but I don't. Can anyone else PLEASE answer this thanks?
think of it this way:

| reduced form| oxidised form ||oxidised form|reduced form|

Putting H2O is never essential, so i never put it to make the answer easier to comprehend
10. can someone please explain what this means from the spec?

"be able to explain why values from mean bond enthalpy
calculations differ from those determined from enthalpy
cycles"

is it simply because it is only a MEAN value averaged over various compounds?

thanks
11. we don't have to memorise the fuel cells half equations right? if we do thats ridiculous lol
12. (Original post by Sparkly-Star)
I see you've posted this loads of times and I wish I knew the answer but I don't. Can anyone else PLEASE answer this thanks?
It doesn't matter. They're just 2 different ligands datively bonded to the metal ion. So long as you say which ligands are present, in what proportion and remember your [...] with charge (if any), you'll be fine!
13. (Original post by dents)
can someone please explain what this means from the spec?

"be able to explain why values from mean bond enthalpy
calculations differ from those determined from enthalpy
cycles"

is it simply because it is only a MEAN value averaged over various compounds

thanks
hole in one.
14. (Original post by dents)
can someone please explain what this means from the spec?

"be able to explain why values from mean bond enthalpy
calculations differ from those determined from enthalpy
cycles"

is it simply because it is only a MEAN value averaged over various compounds?

thanks
Exactly that! Say you have a tertiary carbocation and a primary carbocation, the C-H bond will need less energy.

Also, remember molecules which have a covalent character. Theoretical enthalpy is very different to actual enthalpy.

Hope this helps.
15. (Original post by jimmy303)
Meh. I typed it out from one tab to another. I got H2O and O2 mixed up. At least we got it sorted...
Good thing I spotted that out! Would of cost you a mark
16. (Original post by englishman129)
we don't have to memorise the fuel cells half equations right? if we do thats ridiculous lol
No we don't, just need to UNDERSTAND them.
17. (Original post by ceeebs)
HEEEELP! Right, the melting point of al2o3 is lower than MgO because Al3+ is more polarizing so there is a degree of covalent character... but why is it not higher then?! In lattice enthalpies a degree of cov character increases the lattice dissociation because the bonding becomes stronger?

Am i missing something here

The covalent character decreases the affect of the strong attraction between negatively charged ions in the ionic lattice - so the electrostatic forces are weaker.

Hope that helps
18. Whats the equation for Cr being oxidised by h2O2? cant seem to find it in the book :/
19. I really don't know what we're supposed to know from the 'cells' part of the spec. How much detail do we need?

As in fuel cells.
20. Okay last chapter, bring it on.

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