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# ocr a f325 revision thread watch

1. (Original post by samzurai)
I found comparing the cations with the cations only and the Anions only as a good rule of thumb, with the charge most overruling and the f- ions is smaller than cl
I did that, those were the first two marking points. But the last point was to explain the pattern of lattice enthalpies.
2. (Original post by LegendX)
Indeed.
how comes I thought you got an A in Jan
3. Hi
Can someone explain to me what cathode and anode is with examples, and what it means when writing half equations from the positive/negative cathode/anode (i don't know)

Thank you
4. Stretch and challenge be nice to tomorrow to me babe

5. For the following questions, I have done equations for step 1 and step 2, however am confused as to how to formulate an equation for step 4.

Please could somebody explain how to do so. Thank you
6. Please can someone explain Q8 last part from jan 2013
7. (Original post by otrivine)
how comes I thought you got an A in Jan
Nope, i did not
8. I know it's a little late, but I've got some notes here if anyone wants something concise for last-minute revision

http://www.thestudentroom.co.uk/show....php?t=2378761
9. For a definition of rate, can you say rate of change of concentration?

Anyone doing physics will prefer it like that but the mark schemes seem to like change in concentration per unit time and it kind of annoys me.
10. (Original post by master y)
please can someone explain q8 last part from jan 2013
no one can its too advance for students!!!
11. (Original post by rival_)
In the mark scheme where does the "40" come from when working out conc^ in g dm^-3
When you work out the concentration, the volume you divide by is 25cm3. Although you have scaled up to 250cm3, that 250cm3 was originally made up with water from 25cm3.
12. (Original post by LegendX)
Gosh, i hate explaining over a forum but here goes:

You're reacting NaOH ( a base ) with an acid to form a salt, so you must do the moles of the acid - the moles of the naoh to see how many moles of acid are left over.
The mol of NaOH reacted = moles of the salt formed and you should know from your chemistry that the salt is going to decompose to form conjugate base and sodium ( this is where the large conjugate base reservoir comes from ) however the final step is realised that 50cm3 of volume came from each mixture so the total volume of the solution is 100cm3, convert that into dm3 and then work out the concentrations and then basically plug it into the formula
Thank you

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13. (Original post by slacky123)
http://www.docbrown.info/page07/SSqu...vol_calcs1.htm

I find these are really good practice!
thankyou!!!
14. (Original post by LegendX)
Nope, i did not

Good Luck !!
15. (Original post by KD35)
For a definition of rate, can you say rate of change of concentration?

Anyone doing physics will prefer it like that but the mark schemes seem to like change in concentration per unit time and it kind of annoys me.
I do physics and I find saying "per unit time" is better.
16. (Original post by Namod)
no one can its too advance for students!!!
Lool damn right, it took my teacher 20 mins to figure it out but on the examiners report some geniuses still managed full marks in that question

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17. (Original post by überambitious_ox)
Quick question:

When you are told to identify acid 1, base 1, acid 2 and base 2, how do you know from the LHS of the equilibrium, which to choose as the acid and which as the base?
Each side has an acid and a base as you know, but in the previous information it should tell you if say you had 2 acids on the LHS which one is strong and which one is weak. The strong acid will completely dissociate making it the acid and the weak will partially dissociate making it the base as it will accept a proton. Another method is to see which is the acid or base on the RHS, i.e. NO3- you label it as a base, then look for HNO3 on the LHS and that will be the acid for that pair. Then you can just fill in the other base/ acid pair 2
18. Just to be 100% sure, we don't need to know the transition element colours?
19. Just to be 100% sure, we don't need to know the indicator range and colour change?
20. (Original post by eggfriedrice)
Slightly confused about the mark scheme for question 1 d in the June 2012 paper.

I wrote F- ions are more attracted to Mg2+ ions than Na+ ions hence has a greater lattice enthalpy. And Na+ ions are more attracted to F- than Cl-, and hence NaF has a greater lattice enthalpy than NaCl.

Which would explain the pattern in lattice enthalpies.

However, the mark scheme just says F- has greater attraction for Na+ (but surely it'll have greater attraction for Mg2+).
Also Mg2+ has greater attraction for F- ions, however this doesn't explain why MgF2 is the most exothermic :s

Note; I got the two earlier marking points correct so I've already talked about charge density etc.
It's comparing the cl- ions to f- so f- has greater attraction to na compared to cl-

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