x Turn on thread page Beta
 You are Here: Home

# OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD watch

1. (Original post by VMD100)
CPR
Cathode = Positive = Reduction
uhh cathode = negative?
2. (Original post by RayMasterio)
guys can anyone help me with June 2015 question 2d part i, q 4c part ii, and part d iii
Im not sure if anyone has helped you yet?

2di) in step 2 you have formed something that is not in your overall equation. Hence it is an intermediate. Then if you look at step 1/step 2 reactants and compare it to the overall equation you will see that it has only used 1 mol of H2. But there is 2 mols of H2 in overall equation. So this must mean you need to react something e.g your intermediate with H2

step 3 : H2 + N2O -> N2 + H2O

(You can then check if your products in all your steps equal the overall equation)

4c) If you dilute an acid the ph increases (becomes less acidic). So your concentration of your acid has to change. The first part is calculating the change in concentration and then subbing in values.

So you need to initially workout in the no moles of acid= c x v
no moles = (25/1000)*0.480 = 0.012.

Then you take this value and work out the new concentration of acid in the 100cm3 c=n/v

c= 0.012/(100/1000) = 0.12

You can then use this a concentration of acid and sub into Ka equation

I cannot explain 4e(ii) because i dropped a mark in that question. the basis of it is that when you add metal to an acid it react

acid + metal -> salt + hydrogen

Therefore there will be a decrease in [acid] and an increase in the [conjugate base]

This is pretty poorly explained, I'm not amazing at writing my thought process down. I hope I have helped a little. If you need me to explain more I will try too.

PLEASE CORRECT ME IF I AM WRONG
3. (Original post by postexamtalk)
uhh cathode = negative?
Nope
http://hyperphysics.phy-astr.gsu.edu...electrode.html

This diagram is clearer

The reactions we are doing generate electrical energy, not require it.
4. (Original post by Rust Cohle)
Use your Q from previous question in this rearranged Q = mCdT

For iii:
dT = Q/(mc). Then x1000 if you used kJ (as opposed to J) for Q.
Same number of moles as in (ii) except occupying a larger volume.

For ii:
m = sum of reactants = 35+35 = 70.
For the n, yes, you use only the moles of the limiting reagent (whichever has lower moles). Here they're both equal moles (which is common for acid+base) so irrelevant.

-Q is always around -54.6 kJ for future reference

-Q/n = -4.827 kJ (exothermic)
thanks so much. your worked solution for the charge of Vn+ is amazing too good luck tomorrow
5. F325 will be impossible. I'm calling it. F324 was freakishly easy, and they can't make F325 easy again or too many people will get an A*. Prepare your anus lads and lasses.
6. (Original post by itsConnor_)
for ci https://gyazo.com/2d3b610ec0ec6ae199ede503f4da0794

how do you get first equation here:?

https://gyazo.com/053605e63dbfd229d81453c968d2033e

I thought I- would react with Fe3+ with accordance to electrode potentials?
In accordance to electrode potentials, the one with the highest electrode potential is forward. The one with the more negative electrode potential has the reverse reaction, balance and add together and remove electrons.
7. Is there a set of rules for determining half equations when given the full equation? I get confused as to which species are involved in each half equation.
8. (Original post by Lucy985)
Is there a set of rules for determining half equations when given the full equation? I get confused as to which species are involved in each half equation.
The best rule to go by is that both half equations have to be balanced, and they have to combine to give the full equation. Normally they give you one half and a full - you have to work out the other half.
9. (Original post by VMD100)
Nope
http://hyperphysics.phy-astr.gsu.edu...electrode.html

This diagram is clearer

The reactions we are doing generate electrical energy, not require it.
Do we need to know anything about electrolytic cells or do we need to know both?

Posted from TSR Mobile
10. (Original post by Dinasaurus)
In accordance to electrode potentials, the one with the highest electrode potential is forward. The one with the more negative electrode potential has the reverse reaction, balance and add together and remove electrons.
but I- E potential is lower than Fe, so surely that means Fe3+ would react with I- and not Fe2+? cofused
11. (Original post by itsConnor_)
https://gyazo.com/49069f767bf5ea95d7e870c6a979a5e2 How do we know that 2 mol X reacts with 2 mol Cr? 1.456/1.021 = 1.43, do we just round 1.43 up to 1.5? seems a bit off :/
No, 2 moles of X reacts with 3 moles of Cr I believe. If you write out the half equations you get this ratio:

Cr3+ + 3e- --> Cr (we know it gains 3 electrons because it says chromium(III))

X --> X2+ + 2e- (X is losing mass, so it must be forming X2+ aqueous ions)

The ratio of electrons in each equation is 3 (Cr) : 2 (X)

The no. of moles of Cr is 0.028 mol, so divide that by 1.5 (3/2) to get no. of moles of X.

Then work out the Mr which is 54.6 g mol-1 and this essentially has same atomic mass as manganese. (54.9)
This is how I worked it out and it got me the right answer
12. (Original post by M.Branson98)
No, 2 moles of X reacts with 3 moles of Cr I believe. If you write out the half equations you get this ratio:

Cr3+ + 3e- --> Cr (we know it gains 3 electrons because it says chromium(III))

X --> X2+ + 2e- (X is losing mass, so it must be forming X2+ aqueous ions)

The ratio of electrons in each equation is 3 (Cr) : 2 (X)

The no. of moles of Cr is 0.028 mol, so divide that by 1.5 (3/2) to get no. of moles of X.

Then work out the Mr which is 54.6 g mol-1 and this essentially has same atomic mass as manganese. (54.9)
This is how I worked it out and it got me the right answer
Ohh thanks so much

(typo in my initial question, meant 2mol X with 3mol Cr)
13. (Original post by VMD100)
Nope
http://hyperphysics.phy-astr.gsu.edu...electrode.html

This diagram is clearer

The reactions we are doing generate electrical energy, not require it.
I'm confused so which one do we do?? So is cathode positive or not
14. (Original post by BioStudentx)
F325 will be impossible. I'm calling it. F324 was freakishly easy, and they can't make F325 easy again or too many people will get an A*. Prepare your anus lads and lasses.
too many people can't get A*'s there's always a set amount who do get A*s, it doesn't increase just because a paper is easy. It doesn't work that way
15. (Original post by itsConnor_)
but I- E potential is lower than Fe, so surely that means Fe3+ would react with I- and not Fe2+? cofused
Oh you meant equation 2 and the reaction with Fe3+ going forward is higher so it goes forward, as 0.77 > 0.54.

The 2I- forward equation is lower as 0.54<0.77, so you have to reverse it so 2I- reacts with Fe3+
16. (Original post by tcameron)
I'm confused so which one do we do?? So is cathode positive or not
We are studying the one on the left, As I said CPR.
Galvanic cells
The book states about using transition metals to coat another creating an alloy- basically to galvanize something.
17. (Original post by Dinasaurus)
Oh you meant equation 2 and the reaction with Fe3+ going forward is higher so it goes forward, as 0.77 > 0.54.

The 2I- forward equation is lower as 0.54<0.77, so you have to reverse it so 2I- reacts with Fe3+
omg nvm i read the mark scheme wrong! and it catalyses the reaction because regenerated in reaction 2, got it thx
18. (Original post by tcameron)
too many people can't get A*'s there's always a set amount who do get A*s, it doesn't increase just because a paper is easy. It doesn't work that way
Uhmm, I have nothing to say other than you're severely misinformed? Not sure who told you this.

http://www.bstubbs.co.uk/a-lev.htm

doesn't look constant to me? That definitely isn't a set amount...
19. (Original post by BioStudentx)
F325 will be impossible. I'm calling it. F324 was freakishly easy, and they can't make F325 easy again or too many people will get an A*. Prepare your anus lads and lasses.
i just hope there are no long word questions, only calculations and equations
20. (Original post by itsConnor_)
Ohh thanks so much

(typo in my initial question, meant 2mol X with 3mol Cr)
I didn't realise you are hoping to study chemistry at uni and you made Bath your insurance! I'm hoping to study chemistry also, but I've made Bath my firm!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: September 26, 2016
Today on TSR

### Four things top students are doing

Over the Easter break

### Her parents made her dump me

Poll
Useful resources

Can you help? Study Help unanswered threadsStudy help rules and posting guidelines

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE