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Aqa chem 4/ chem 5 june 2016 thread

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Original post by zahraaj1
doesnt the solubility increase? or does it decrease?..



The reaction was exothermic so if temperature increased, the reaction would shift to the left so solubility would decrease :smile:
for percentage purity i got around 94.3%
Original post by amphoterism
H -196 kJ mol-1
S -189 K J mol-1
G -135 kJ mol-1

Btw what is the sign from answer -189 K J mol-1 indicates about the product relative to the reactants?


i put product is more ordered
Original post by BushDiddit
I screwed up big time


Same and I know where I went wrong as well. So many mistakes I could have not made...
Reply 2424
everything was easy on that paper except that percentage one. can someone quickly explain it to me.
Original post by amphoterism
What is the colour of (Cr(H2O)6)2+? And how to get it?


Blue, as Cr2+ ions are blue. I nearly put ruby red like a muppet!!
Original post by amphoterism
What is the colour of (Cr(H2O)6)2+? And how to get it?


Blue cus cr2+ ions are blue ??
Original post by Clarinet
The reaction was exothermic so if temperature increased, the reaction would shift to the left so solubility would decrease :smile:


The question stated that solubility is defined as the amount of solid ready to dissolve in a solvent ( in this case, water).
Hence as more MgCl2(s) is being produced, solubility increases.

Aqa were sly with this one like Q1A
Original post by Clarinet
The reaction was exothermic so if temperature increased, the reaction would shift to the left so solubility would decrease :smile:


Oh did no one write it in terms of entropy? I thought solubility increased because I worked it out with the Gibbs free energy
Reply 2429
Original post by Lazarus26
for percentage purity i got around 94%


how
Original post by Clarinet
The reaction was exothermic so if temperature increased, the reaction would shift to the left so solubility would decrease :smile:



oh but arent dissociation reactions endothermic?
Original post by Guls
everything was easy on that paper except that percentage one. can someone quickly explain it to me.


So you get your 5:2 oxalate to manganate ion ratio by combining half-cell equations.

Then you divide moles by three because it was a trimolar potassium iron oxalate compound reacting.

Then you get mass by multiplying by Mr.

Then you do % purity and get 94.4%.
Were we meant to put P and Na or P4O10 and Na2O??

I didn't realise if it said oxide or element. If so, what were the other answers in this reaction? Like the reaction in water, was that with the oxide or the element. And what was the reaction for the equation with P?
Reply 2433
mno4- and c2o4^2- are negatively charged so repel and the reaction has a high activation energy. 3 points for 3 marks.
Reply 2434
Original post by ravichauhan11
anyone put Zn and HCl , blue

for Cr3+ to Cr2+ ???


Original post by badaman
Yeah Zn and HCl.
Cr[H2O]6^2+ is Blue solution.


I wrote H+ ions not HCl, do I still get the mark?
Original post by Cadherin
So you get your 5:2 oxalate to manganate ion ratio by combining half-cell equations.

Then you divide moles by three because it was a trimolar potassium iron oxalate compound reacting.

Then you get mass by multiplying by Mr.

Then you do % purity and get 94.4%.


Yessss
Original post by zahraaj1
oh but arent dissociation reactions endothermic?

yeah but it was enthalpy of solution i think which is usually exothermic, if u worked it out it was exo for me anyways

i think it wanted you to talk about equilibrium like it mentioned in the question
Original post by RefusedAccess
The question stated that solubility is defined as the amount of solid ready to dissolve in a solvent ( in this case, water).
Hence as more MgCl2(s) is being produced, solubility increases.

Aqa were sly with this one like Q1A


no , eq shifts to the left, so not favoring the dissociation of MgCl2, therefore solubilty decreases. very similar question came up a few years a go
Reply 2438
Original post by Cadherin
So you get your 5:2 oxalate to manganate ion ratio by combining half-cell equations.

Then you divide moles by three because it was a trimolar potassium iron oxalate compound reacting.

Then you get mass by multiplying by Mr.

Then you do % purity and get 94.4%.


well done. how did you find the rest of the paper?
i think it went well, but i know i lost marks on some parts for sure --- just hoping unit 4 wasnt as terrible as i originally thought and the grade boundaries even out

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