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# Aqa chem 4/ chem 5 june 2016 thread

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1. (Original post by Suits101)
Bingo!

Finally someone who agrees.
Can't believe people fell for that ahahah
2. (Original post by Aerosmith)
Just to clarify - for the enthalpy and entropy calculations you had to take into account standard conditions. So you should have divided the equation by 2 to get so2 + 1/2o2 ----> so3 as it is the formation of 1mol of a compound. Then work it out
You get the same answer regardless, if they were to be so cruel you'd only lose a mark.
3. (Original post by Suits101)
Anyone confirm?

SA has no effect on emf

Environmental advantage: doesn't need to be electrically recharged

Why not carbon-neutral: hydrogen is produced by electrolysis which requires electricity that's produced from combustion of fossil fuels releasing CO2
I thing SA does have an effect. It will increase the rate of the redox process at the electrode, and so the overall reaction would 'proceed further' .

Not carbon neutral because fossil fuels burnt (co2 released) in the transport of H2. I think your answer should be valid as well.
4. Did anyone get +209 for entropy? And what did you say for the increase in temperature in regards to solubility, because logic tells me it increases. But my original entropy was -155
5. Ahh how many marks would you lose for not dividing by 2 in the enthalpy change/entropy change questions?
6. (Original post by RME11)
Ratio of C2O4^2- to MnO4^1 was 2:5,

There were 26.4cm3 of 0.02 moldm3 MnO4- = 0.000528 mols MnO4 therefore there was 0.00132 moles C2O4^2- in 25cm3 therefore 0.0132 moles in 250cm3,

There was 3 C2O4^2- per one mole of the salt therefore there were 4.4x10-3 moles salt

Mr K3(Fe(c2o4)3).3h20 = 491.1, Mass = moles x mr = 4.4x10-3 x 491.1 = 2.16g

Original mass = 2.22 therefore 2.16/2.22 * 100 = 94.4% purity.
But you added 250 cm3 water to the solution ...
7. (Original post by Azzer11)
I got - 67.5 because I halved my values beforehand as 2 moles of product were being formed. Standard enthalpy of formation by definition is the formation of 1 mole of a compound.

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Finally someone else who did the same as me! Do u think we are right or the others are?
8. (Original post by Suits101)
Only worth 3 marks.
was that the only question after the 7marker on calculations?
9. Anyone get 94.4% for percentage purity?Also, would i get a mark for saying the reagents were Zn/H2So4 when cr2+ ions are formed..
10. hey guys how many marks do u think il lose if i got an answer of -155 and then put 155 and then for the next part wrote that the solubility increases. do u think ill still get marks for error carried forward
11. (Original post by cowie)
I SWEAR FUEL CELLS AREN'T REVERSIBLE THOUGH?! Which would mean equilibrium wouldn't shift as there isn't one and the emf would increase due to increase in pressure increasing rate of reaction?

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The reduction potentials are reversible tho, if you increase the conc on the left hand side you are gonna push the eq. to the right, and as this is the side that determines the reduction potential the E(Cathode) will increase and as EMF = E(Cathode)-E(Anode) this means the value of the EMF will increase.
12. (Original post by Suits101)
For redox questions:

Weakest OA: Zn2+
Conventional cell diagram: Zn | Zn2+ || Co2+ | Co
Half equation: Zn -> Zn2+ + 2e-
Cobalt species: Co3+, oxidation product is oxygen, then reaction

Negative electrode: H2 + 2OH- -> 2H2O + 2e-
Positive electrode: 4e- + O2 + 2H2O -> 4OH-
Overall reaction: 2H2 + O2 -> 2H2O
Graph of emf: just a straight horizontal line
Negative electrode: check cgp brown book pg 185
its the other way round
13. (Original post by SirRaza97)
I genuinely don't know. I find chem5 easier than chem4. If I was estimating:

67/68 - B
76/78 - A
83/85 - A*
yh me too - especially this year!!! it will be a bit precarious for me if the grade boundaries reach 90 marks for an A* because i made several stupid mistakes
14. (Original post by Aerosmith)
Can't believe people fell for that ahahah
(Original post by Kay Fearn)
Ahh how many marks would you lose for not dividing by 2 in the enthalpy change/entropy change questions?
In the previous paper where u had to divide it by 2 the question for delta H and delta S were both worth 3marks but today they were both worth 2marks each so I don't think you have to divide it by 2
15. (Original post by Mvpmb)
But you added 250 cm3 water to the solution ...
You dissolved the salt THEN added 250cm3 of water, you took 25cm3 of water from this - I almost made this mistake.
16. (Original post by DDan123)
I hope so, just seems so sneaky ffs lol

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So annoyed at myself for that - every past paper I've divided by 2 when needed to but I think I was just so happy with how the exam was going that it just went totally over my head

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17. (Original post by girl :D)
hey guys how many marks do u think il lose if i got an answer of -155 and then put 155 and then for the next part wrote that the solubility increases. do u think ill still get marks for error carried forward
Yea most probably you will still get marks for ECF if you explained your answer
18. (Original post by RME11)
You get the same answer regardless, if they were to be so cruel you'd only lose a mark.
What are you on about? You don't reach the same answers from the equation they gave you. I was trying to tell people what the correct reasoning was as a number of people appeared to be confused
19. (Original post by Agent R5)
was that the only question after the 7marker on calculations?
Yes.
20. any one doing biology5 tomorrow

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