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Aqa chem 4/ chem 5 june 2016 thread

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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 said no co2 produced for environmental advantage as only h20 produced
Reply 2601
Avatar for tbh12
tbh12
9
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 put the same as you for SA.

Environmental advantage - no fossil fuels burnt there no CO2(g) released into atmosphere.

Why not carbon neutral - I agree with your answer.
what did you guy get for enthalpy value and entropy values?
Original post by cherry1997
what did you guy get for enthalpy value and entropy values?


-189 i think
Original post by m_ermaidmotel
I got 50.7% for the purity, how many marks would I have dropped here ??????


3 or 4 I'd say
Original post by lahigueraxxx
I put for the environmental advantage that it doesn't release CO2 :frown:


Same, it should be right though ?
Original post by Suits101
Bingo!



Finally someone who agrees.


Can't believe people fell for that ahahah
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.
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.
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
Ahh how many marks would you lose for not dividing by 2 in the enthalpy change/entropy change questions?
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 ...
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.

Posted from TSR Mobile


Finally someone else who did the same as me! Do u think we are right or the others are?
Original post by Suits101
Only worth 3 marks.


was that the only question after the 7marker on calculations?
Reply 2614
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Bob78
1
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..
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
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?


Posted from TSR Mobile


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.
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
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
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

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