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# ocr a f325 revision thread watch

1. Can someone help me with hydration enthalpies?

When you explain the difference between hydration enthalpies, are charge and charge density separate marking points, because for instance when comparing Mg2+ and Na+, you talk about the ionic radii, but doesn't the ionic radius becoming smaller also increase the charge density of the ion?
So Mg2+ has greater ionic charge and greater charge density, as separate things?

Confused myself!
2. (Original post by Tendael)
Quick question.

Cr207(2-) + 2OH <---> 2CrO4(2-) + 2H2O

In that equation, what could you add to shift the equation from right to left? I thought Water, but it isn't. Any help?

Also...

Is it only a hydrogen half cell which has a gas feed of H2? Say you have a system like:

1/2Cl2 + e- <--> Cl-

Could that have a chlorine gas feed?

Thanks in advance if anyone takes a look at these!
Hey, yeah you only use H2 gas when you have a hydrogen half cell. If it's Cl in different oxidation states, it's a solution of the 1 moldm-3 of each of the ions.

Don't know what you mean by the other question? Do you just mean shift the equilibrium? Because if you increase the pressure it will shift to the LHS. But if you add CO2 there will be a new equilibrium set up, the Co2 reacts with H20 > H2CO3. So the H2CO3 + OH– > H2O + HCO3–, and the first equilibrium will shift left to replace some of the used OH-.
3. (Original post by bmpink)
Can someone help me with hydration enthalpies?

When you explain the difference between hydration enthalpies, are charge and charge density separate marking points, because for instance when comparing Mg2+ and Na+, you talk about the ionic radii, but doesn't the ionic radius becoming smaller also increase the charge density of the ion?
So Mg2+ has greater ionic charge and greater charge density, as separate things?

Confused myself!
Charge density takes into account both the radius and the charge of an ion. However, I would talk about the charge, then the ionic radius and then sum it up with charge density afterwards. OCR mark schemes can be very finicky and it's good to stay on the safe side.
4. (Original post by herothing11)
Charge density takes into account both the radius and the charge of an ion. However, I would talk about the charge, then the ionic radius and then sum it up with charge density afterwards. OCR mark schemes can be very finicky and it's good to stay on the safe side.
Thanks, so charge density is the overall combination of the effects of ionic radius and charge?
5. (Original post by bmpink)
Thanks, so charge density is the overall combination of the effects of ionic radius and charge?
Yeah. So Mg2+ will have a higher charge densitry than Ca2+ as they both have the same charge but Mg2+ has a smaller ionic radius
6. Well after just doing June 2011 and get 81/100 in the time conditions allowed at the time that I really need to learn all the different types of reactions and equations there could be, if I knew them to a good standard I could've and should've got 10 more marks very easily :-/
7. guys any predictions? this one will be harder than jan13? or easier? and what types of question will come up? any ideas?
8. jan 2011 question 7a, i dont get why they divided by 4?

if anyone could help i'd appreciate it!
9. (Original post by amin666)
guys any predictions? this one will be harder than jan13? or easier? and what types of question will come up? any ideas?
I reckon it should be easier considering how hard januarys paper was. may be of a similar level of difficulty to june 2012. no idea what will come up though, except the standard kc/k calculations, acids, electrode potentials and drawing complex ions maybe.
10. (Original post by amin666)
guys any predictions? this one will be harder than jan13? or easier? and what types of question will come up? any ideas?
Genuinely have no idea after the January paper. That was such a horrible exam and the grade boundaries were still higher than an easier paper (thanks to Michael Gove). I can't imagine it would be harder than that.
11. can anyone help with Jan 2012, 3 eii?
Out of interest how do you know what UMS it is??? thanks
I said 141
13. (Original post by DudeBoy)
Here you go.
Thank you!!
14. on Some mark schemes eg June 12 question 5b : http://www.ocr.org.uk/Images/135166-...ments-june.pdf
It says 'IF more than one example shown, mark example giving lower mark' In blue what does that even mean???
on Some mark schemes eg June 12 question 5b : http://www.ocr.org.uk/Images/135166-...ments-june.pdf
It says 'IF more than one example shown, mark example giving lower mark' In blue what does that even mean???
if youve put down 2 examples/equations theyll mark them both. whichever one has received the lower mark is the final mark you get for that question
16. (Original post by chignesh10)
I said 141
he means how did you work out it was 141..
17. Where can i find sets of just stretch and challenge questions?
18. (Original post by needtosucceed=))
if youve put down 2 examples/equations theyll mark them both. whichever one has received the lower mark is the final mark you get for that question
So does that mean even if you have the correct answer which 2/2 but on the safe side do another example which is only 1/2, you get 1/2?
19. http://www.ocr.org.uk/Images/64622-m...ts-january.pdf

how is the ratio found in 3 e 11? thanks

i get that 0.0200 mol from mass / mr of agcl
but where does 0.0100 come from...

got,it was iin the question,need to start reading more precisely
20. 3.25g of zinc were added to 50.0 cm3 1.50 mol dm-3 copper sulphate solution (an excess) in an insulated calorimeter. The initial temperature of the copper sulphate solution was 22 0C and after stirring, a final temperature of 46 0C was recorded.
how would you do this?
Calculate the heat transferred to the solution by the reaction.
Assume that all the solutions have the density of water (1 g cm-3) and the specific heat capacity of water 4.18 J g-1K-1.

for this one , I did 4.18 x temperature change x 50 ? but they used 100 and not 50

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