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# ocr a f325 revision thread watch

1. (Original post by Jamesnorniron)
Can someone explain how to balance H+ + I- + NO2- ---> OH- + I2 + 2NO

Its the last question in January 2011
First balance the number of nitrogens:
H+ + I- + 2NO2- ---> OH- + I2 + 2NO

Then balance the number of hydrogens:
2H+ + I- + 2NO2- ----> 2OH + I2 + 2NO

Then balance the Iodines:
2H+ + 2I- + 2NO2- -----> 2OH + I2 + 2NO

Then the oxygens:
2H+ + 2I- + 2NO2- -----> 2OH- + I2 + 2NO

Check that the charges are balanced and then you're done.
2. (Original post by chignesh10)
was that your first attempt at JAN 2013?
Hi

Am I correct in thinking that in buffers, Pka=Ph because when the acid dissociates it gives equal concentrations of the acid and its base?
3. (Original post by otrivine)
Hi

Am I correct in thinking that in buffers, Pka=Ph because when the acid dissociates it gives equal concentrations of the acid and its base?
not necessarily no, it doesnt always give equal concentrations
4. (Original post by chignesh10)
not necessarily no, it doesnt always give equal concentrations
But in the book on page 153 it says that?
5. Any predictions for things that might come up tommorow that haven't already in recent past papers?

How is everyone preparing for it today? Past papers?
Resitting this paper, everything just went wrong in January
Hey bro
7. 1 more day, I think I am ready... We'll see soon I guess
8. How would you define: Feasible and spontaneous
9. (Original post by otrivine)
How would you define: Feasible and spontaneous
Surely this won't be asked tomorrow?

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10. (Original post by DudeBoy)
1 more day, I think I am ready... We'll see soon I guess
Want to revise

Define Lattice Enthalpy

Suggest why the enthalpy change of hydration is always exothermic
11. (Original post by MathsNerd1)
Surely this won't be asked tomorrow?

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Expect the unexpected, thats what I learnt from OCR
First balance the number of nitrogens:
H+ + I- + 2NO2- ---> OH- + I2 + 2NO

Then balance the number of hydrogens:
2H+ + I- + 2NO2- ----> 2OH + I2 + 2NO

Then balance the Iodines:
2H+ + 2I- + 2NO2- -----> 2OH + I2 + 2NO

Then the oxygens:
2H+ + 2I- + 2NO2- -----> 2OH- + I2 + 2NO

Check that the charges are balanced and then you're done.
How do you know you have to balance the hydrogens, is there not only one on each side?
13. (Original post by sidmanny)
I worked out how to do it, using kw again but I got the right answer this time. I dunno how I didn't get this answer first time, must have forgotten to log it or something.
So you're finding pH, [H+]= kw/[OH-]
[OH-]=100x[H+], kw=1.00x10^-14
so [H+]=1.00x10^-14/100[H+]
[H+]^2= 1.00x10^-14/100
[H+]= square root of 1.00x10^-14/100
Then you minus log the [H+], -log10(1x10-8
which gives 8
Very nice
14. (Original post by otrivine)
Expect the unexpected, thats what I learnt from OCR
But it has to be on the spec which surely these aren't in this way?

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15. :'( this exam will break me.
16. (Original post by BrightStar57)
from the markscheme we have to divide the moles of V by five how do you know that you have to divide by 5
the sample titrated against the MnO4 was a 5th (10cm^3) of the original solution (50cm^3)
17. (Original post by otrivine)
Want to revise

Define Lattice Enthalpy

Suggest why the enthalpy change of hydration is always exothermic
Lattice enthalpy:
The enthalpy change that occurs when one mol of an ionic molecule is formed from gaseous ions.

Enthalpy change of hydration is always exothermic because bonds are formed?

What is the effect on Kc when changing the pressure in a system?
18. (Original post by DudeBoy)
Lattice enthalpy:
The enthalpy change that occurs when one mol of an ionic molecule is formed from gaseous ions.

Enthalpy change of hydration is always exothermic because bonds are formed?

What is the effect on Kc when changing the pressure in a system?
correct

No effect on Kc as Kc only changes by/ is affected by temperature.

1) Balance the following equation using oxidation numbers

Pt + HNO3 + HCL -----> H2ptCl6 + NO2 + H2O
19. Using quadratic equation to find pH

Ka=[H+][A-]/[AH]
Ka=[H+]^2/[AH-H+]
Ka[AH-H+]=[H+]^2
[H+]^2-Ka[-H+]-Ka[AH]
I doubt it will come up but you never know
20. Guys please can someone clear this up for me... I know the half equations for a hydrogen oxygen fuel cell are:
2H+ + 2e- ---> H2
2H+ +2e- +0.5O2 ---> H2O
but I'm confused as to which ones occur at the anode and cathode. My teacher started off by saying the first one occurs at the anode and the second at the cathode but then confused me by saying that it doesn't happen at the normal electrode or something??? :/
Thanks

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