ocr a f325 revision thread

Can someone explain how to balance H+ + I- + NO2- ---> OH- + I2 + 2NO


Its the last question in January 2011

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?

not necessarily no, it doesnt always give equal concentrations

Resitting this paper, everything just went wrong in January

How would you define: Feasible and spontaneous

1 more day, I think I am ready... We'll see soon I guess

Surely this won't be asked tomorrow?


Posted from TSR Mobile

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.

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

Expect the unexpected, thats what I learnt from OCR

from the markscheme we have to divide the moles of V by five how do you know that you have to divide by 5

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?