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# Aqa chem 4/ chem 5 june 2016 thread Watch

1. (Original post by 12284)
I wrote H+ ions not HCl, do I still get the mark?
not sure, think you had to specify with the acid
2. (Original post by ravichauhan11)
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
I agree

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3. (Original post by olivia7001)
Oh did no one write it in terms of entropy? I thought solubility increased because I worked it out with the Gibbs free energy
I talked about gibbs free energy too, but said it increased when temperature increases so it becomes less soluble?

(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
What?? So solubility increases? I don't understand what you mean...?
4. What was the equation for the solution of MgCl2? I couldn't remember solution so i put MgCl2(s) ---> Mg2+(aq) + 2Cl-(aq)
5. (Original post by DDan123)
What did people put down for the last question about colorimetry?

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Plot a graph of concentrations and absorption rates using known concentrations of KMnO4
Then set up colorimeter and place unknown sample and see what it's absorption rate is
Use the absorption rate on the graph and go across until you find the concentration

Something like that haha

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6. (Original post by Lazarus26)
What was the equation for the solution of MgCl2? I couldn't remember solution so i put MgCl2(s) ---> Mg2+(aq) + 2Cl-(aq)
That's fine.
7. If u right p4o10 rather than jus p is it rong
8. (Original post by olivia7001)
Oh did no one write it in terms of entropy? I thought solubility increased because I worked it out with the Gibbs free energy
I did this too. I thought we had to use the values of the previous part but I think we might have misinterpreted it? Idk

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9. (Original post by DDan123)
What did people put down for the last question about colorimetry?

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first said about how MnO4- ions are coloured and the rest arent ( just as an intro). make up solutions of known conc. Measure absorption. plot graph of results, then use the absorption for the unknown conc one to work out the concentration ( extrapolation) .
10. (Original post by sockface)
If u right p4o10 rather than jus p is it rong
was the second one Na and not Na2O then?
11. (Original post by Guls)
mno4- and c2o4^2- are negatively charged so repel and the reaction has a high activation energy. 3 points for 3 marks.
little Mn2+ formed initially, so reaction is uncatalysed
Mn2+ lowers Ea as allows for reactions between positive and negative ions.

*just* for safety :P
12. was the enthalpy of solution positive or negative
13. i didnt do 90x2 for atomisation of potassium oh no how many marks was this question for and how many marks will i get if i did not do that ahhh
14. so whats everyones grade boundary predictions for chem 4 and them 5 then??
15. My dumb ass did mno4 with fe 2+ smh rip 7marks LOL...Ive failed

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16. (Original post by 26december)
Plot a graph of concentrations and absorption rates using known concentrations of KMnO4
Then set up colorimeter and place unknown sample and see what it's absorption rate is
Use the absorption rate on the graph and go across until you find the concentration

Something like that haha

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Yessss I basically wrote that, just wasn't sure whether that's what they were looking for at the time aha.

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17. (Original post by zahraaj1)
was the enthalpy of solution positive or negative
Negative something like -155 kjmol-1

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18. (Original post by olivia7001)
Oh did no one write it in terms of entropy? I thought solubility increased because I worked it out with the Gibbs free energy
Yeah i did as well :/ but i think it's wrong because the delta g value doesn't affect the rate of the reaction ? idk
19. what did everyone's graph look like for the hydrogen fuel cell?
20. What was the mass people got for the C2O42- ions in 250cm3 solution?

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