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Aqa chem 4/ chem 5 june 2016 thread

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How much does everyone know about spectophotometry/colorimetry? I know it's not on the spec but it links into the aqua ions and titrations sections.
Chem 5 revision going well
What are all the types of reactions?

There's ligand substitution
There's acid base

Anymore?




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Original post by 12284
Wait I'm confused now, I thought a reaction was more likely to occur if AS is negative?


I think you're getting confused with delta G, a reaction would happen (feasible) if delta G is negative, but with entropy the more disordered the more positive it becomes.
Someone help with 3c and 4a pls

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-W-QP-JAN10.PDF
Does anyone have the Chem 5 2015 mark scheme pleaseee
Thanks in advance
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Original post by shiney101
I think you're getting confused with delta G, a reaction would happen (feasible) if delta G is negative, but with entropy the more disordered the more positive it becomes.


Ooohh thanks!
Reply 1907
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Original post by RedDevil1997
Does anyone have the Chem 5 2015 mark scheme pleaseee
Thanks in advance


Its online on the AQA website
What are all the types of reactions?

There's ligand substitution
There's acid base

Anymore?






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Reply 1909
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Anam
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Original post by Super199


4a the bond enthalpy is twice the atomisation enthalpy

3c im stuck on it too
Original post by joebush
How much does everyone know about spectophotometry/colorimetry? I know it's not on the spec but it links into the aqua ions and titrations sections.


colorimetry is on the spec .... 3.5.4 under formation of coloured ions : we need to know how to determine concentration from from relative absorption of light, using a calibration graph
Original post by Anam
4a the bond enthalpy is twice the atomisation enthalpy

3c im stuck on it too


Do you mind explaining why it is twice atomisation enthalpy?
Does anyone know the full equation for fe(h2o)4(oh)2 with oxygen to form fe(h20)3(oh)3?
Original post by 1/2(p^2-p)
colorimetry is on the spec .... 3.5.4 under formation of coloured ions : we need to know how to determine concentration from from relative absorption of light, using a calibration graph

Ah, thanks a lot. Luckily I had revised it, I was just hoping it wasn't in vain.
Can someone explain the colorimetry thing with the calibration curve?
Original post by Super199
Can someone explain the colorimetry thing with the calibration curve?


you plot a graph using known concentrations and known absorption's then use this graph to find the concentration of an unknown ion by reading of the calibration graph... so say you want to kno the conc of ion X you plot graph using known conc's of X and its relative absorbance then use this graph to find an the conc of a unknown sample of X when you kno its absorption
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Original post by shiney101
I think you're getting confused with delta G, a reaction would happen (feasible) if delta G is negative, but with entropy the more disordered the more positive it becomes.


what about enthalpy of solution? Is it more feasible if it is negative or positive?
Original post by thehollowcrown
Can someone help me with bidentate ligands and M3+ ions? I assumed they would react in the same way as M2+ ions but according to http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JAN12.PDF Q8ei they don't. Why is this?


I was confused when I did this paper too. You have to realise that en is going to act as a base. Aluminium doesn't undergo ligand exchange with ammonia:

[Al(H2O)6]3+ + 3NH3 --------> [Al(H2O)3(OH)3] + 3NH4+

So the same thing happens with en:

2[Al(H2O)6]3+ + 3(NH2CH2CH2NH2)--------> 3[Al(H2O)3(OH)3] + 3(NH2CH2CH2NH2)
Original post by youngar
I was confused when I did this paper too. You have to realise that en is going to act as a base. Aluminium doesn't undergo ligand exchange with ammonia:

[Al(H2O)6]3+ + 3NH3 --------> [Al(H2O)3(OH)3] + 3NH4+

So the same thing happens with en:

2[Al(H2O)6]3+ + 3(NH2CH2CH2NH2)--------> 3[Al(H2O)3(OH)3] + 3(NH2CH2CH2NH2)


Ohh, that makes so much more sense now, thank you!
Original post by Super199
Do you mind explaining why it is twice atomisation enthalpy?


Ok, so what I did was to look at the compound being formed, silver chloride. We all know that silver chloride has only one Cl- ion, and so during the atomisation of Cl2, the born haber cycle (an imaginative one) would already have 1/2 Cl2 to make one Cl, therefore the enthalpy shown in the table is only of one cl atom. So if Cl-Cl is broken, twice the amount of energy would be needed. I hope this helps.

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