OCR A2 Chemistry Unit 5, F325 15th June 2015

So you have those DENCH questions on flash cards?

If I see a dodgy Q especially unstructured redox that I have no clue on how to answer , I'd firstly try to understand the method then try the question again the next day

I'll then write it down as simply as possible for future refrence incase a similar Q comes up and at least I'll have an idea on what going on

Reply 1321

HAnwar

17

Original post by TheLegalDealer

Hope that explains it and at the end its simple you just balance the no. Of oxygens by adding 3 water molecules on the left and 6 H+ on the right

Giving 5V2+(aq) + 3MnO4+(aq) + 3H2O(l) --------> 5VO3–(aq) + 3Mn2+(aq) + 6H+(aq)

If your still stuck this will help

http://youtu.be/NHP0V9mDD-Y

You know in the third step where you try and work out the number of moles for Vn+ and you divide by 5, in the mark scheme they give another possible method which you can use, and they times the number of moles of MnO4 by 5.
Do you know why they times by 5?

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Reply 1322

User1686961

12

Original post by TheLegalDealer

If I see a dodgy Q especially unstructured redox that I have no clue on how to answer , I'd firstly try to understand the method then try the question again the next day

I'll then write it down as simply as possible for future refrence incase a similar Q comes up and at least I'll have an idea on what going on

Thats a very good idea. No wonder your bossing F325. Ill try that thanks!

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Reply 1323

Yua

14

Original post by TheLegalDealer

Hope that explains it and at the end its simple you just balance the no. Of oxygens by adding 3 water molecules on the left and 6 H+ on the right

Giving 5V2+(aq) + 3MnO4+(aq) + 3H2O(l) --------> 5VO3–(aq) + 3Mn2+(aq) + 6H+(aq)

If your still stuck this will help

http://youtu.be/NHP0V9mDD-Y

oh **** im stupid

Reply 1324

TheLegalDealer

9

Original post by Yua

oh **** im stupid

I didn't see it at first too :biggrin:

Reply 1325

plasmaman

18

Original post by Mutleybm1996

Am I the only one that found this question easy........

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You and I bud. Seems more like an F321 question.

Reply 1326

Captain Anonymous

14

Do we have to know about reaction with butan-2-enal for f324?

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Reply 1327

MollyMcFly1

13

Could someone please explain to me where they get the 3:2 ratio from on this Jan 2013 question? There's always one question in every paper that stumps me even after staring at it for ages :mad:

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Reply 1328

Yua

14

Original post by MollyMcFly1

Could someone please explain to me where they get the 3:2 ratio from on this Jan 2013 question? There's always one question in every paper that stumps me even after staring at it for ages :mad:

Cr2(SO4)3 so Cr oxidation state is +3

XSO4 so X is in a state of +2

if cr gains electrons (as it gained mass) then 2Cr3+ ----> 2Cr

if x loses electrons then X -----> X2+

combining is 2Cr3+ + X --------> 2Cr + X2+

then balance it so charges are same

+6 on left and 2+ on right

times X2+ by 3 so you must times X by 3

2Cr3+ + 3X --------> 2Cr + 3X2+

Reply 1329

MollyMcFly1

13

Original post by Yua

Cr2(SO4)3 so Cr oxidation state is +3

XSO4 so X is in a state of +2

if cr gains electrons (as it gained mass) then 2Cr3+ ----> 2Cr

if x loses electrons then X -----> X2+

combining is 2Cr3+ + X --------> 2Cr + X2+

then balance it so charges are same

+6 on left and 2+ on right

times X2+ by 3 so you must times X by 3

2Cr3+ + 3X --------> 2Cr + 3X2+

Thank you very much!! :smile:

Reply 1330

Straighthate

2

does my answer make sense?

suggest two step mechanism for: 2NO(g) + O2(g) ---> 2NO2(g)
rate equation = k[NO]^2

my answer:
step 1: 2NO ---> NO2 + N
step 2: O2 + N ---> NO2

the answer in the book is:
step 1: 2NO ---> N2O2
step 2: N2O2 + O2 ---> NO2

i know there are often different possible mechanisms but the book only mentions one, but is mine right?

Reply 1331

LA_95

1

What are the advantages and disadvantages of using methanol as a fuel instead of hydrogen? I know methanol is a liquid and so easier to store and transport is an advantage but I don't know the rest.

Reply 1332

Baloney26

9

Original post by LA_95

What are the advantages and disadvantages of using methanol as a fuel instead of hydrogen? I know methanol is a liquid and so easier to store and transport is an advantage but I don't know the rest.

One I saw on a markscheme is that methanol is more widely accepted as a fuel politically and by the public, also maybe that methanol can be produced from biomass?

Reply 1333

LA_95

1

Original post by Baloney26

One I saw on a markscheme is that methanol is more widely accepted as a fuel politically and by the public, also maybe that methanol can be produced from biomass?

Ahh, thanks, what about disadvantages? :/

(edited 8 years ago)

Reply 1334

User1686961

12

Original post by LA_95

Ahh, thanks, what about disadvantages? :/

Doesn't methanol produce CO2 in combustion? Maybe that's a disadvantage.

Reply 1335

LA_95

1

Does anyone have an idea what topics are due to come up on this paper? I don't think enthalpy change of neutralisation or solution/hydration born haber cycles have been on for a while? What does everyone think?

Reply 1336

username1436301

1

Does anyone have the June 2014 F334 and F335 papers? I have become ever so bored of doing the past papers available on the OCR website and it would be nice to have a change :smile:

Reply 1337

bluejayy

1

Could you upload unit 5? Please.