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Aqa chem 4/ chem 5 june 2016 thread

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Original post by crillieswillies
Any idea how many marks I'd lose for getting delta G as -135000? I multiplied delta H by 1000 so delta H and delta S were in the same units :'(


You lose none thats correct :smile:
did delta G have to be 3 sig figures???
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
(edited 7 years ago)
everyone remember to include that deltaH was approx 0 because same amount and type of bonds being broken and formed? for the thermo stability Q
Anybody making an unofficial mark scheme like for chem 4?


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That was the nicest paper in the god damn world. Apart from the 7 marker calculation question it was so easy. Definite A*
Original post by ravichauhan11
everyone remember to include that deltaH was approx 0 because same amount and type of bonds being broken and formed? for the thermo stability Q


Yup I put this down.


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Original post by cowie
I drew two lines and wrote a caption for each..
One was horizontal and I mentioned fuel was constantly sullied
The other was a curve with decreasing gradient and I mentioned this was when fuel was not constantly suplied


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You can't draw two lines can you?
Reply 2508
Original post by Whitney997
What equations did you guys write for each electrode in the hydrogen-oxygen fuel cell question?


+ve electrode: O2 + 4e + 2H+ = 2OH-
-ve electrode: H2 + 2OH- = 2H2O + 4e
full equation: H2 + 02 + 2H+ = 2H2O
I'm pretty sure this is wrong though
What did everyone else get?
Original post by crillieswillies
Any idea how many marks I'd lose for getting delta G as -135000? I multiplied delta H by 1000 so delta H and delta S were in the same units :'(

- 135000 is right as long as you wrote J mol^-1 as units
Original post by ravichauhan11
everyone remember to include that deltaH was approx 0 because same amount and type of bonds being broken and formed? for the thermo stability Q


No - the starting reactant was [Co(H2O)6]2+ and the product was [Co(h2nch2ch2nh2)3]2+ not same type of bonds are broken as are formed but same number yes


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What did everyone get for the characteristics of chromium chemistry question?
I sat last years paper and this year's and I think they're about the same in difficulty although the calculation on this year's may have tripped up more people so I think the grade boundaries will be about the same if not one ums less for each grade boundary compared to last year
Original post by Cadherin
5/7 with ALL correct working shown.
-A1 for correct answer
-M4 for dividing by 3


Did you say catalyst for last quesiton was Mn2+ and tick H2SO4?
Original post by Pippy12345
What did everyone get for the characteristics of chromium chemistry question?

Coloured complexes
Variable oxidation states
Compex ions
And i gave examples for each :/


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Original post by 12284
+ve electrode: O2 + 4e + 2H+ = 2OH-
-ve electrode: H2 + 2OH- = 2H2O + 4e
full equation: H2 + 02 + 2H+ = 2H2O
I'm pretty sure this is wrong though
What did everyone else get?


+ve was O2 + 2 H20 + 4e -> 4OH-

The conditions were in alkaline so you can't have H+ ions in your equation.
Original post by BigWillow
You lose none thats correct :smile:


Phew! Thank you.
Original post by Suits101
Did you say catalyst for last quesiton was Mn2+ and tick H2SO4?


I had Mn2+ and H2SO4
Original post by randoms132
Coloured complexes
Variable oxidation states
Compex ions
And i gave examples for each :/


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Did it ask for examples?
Original post by crillieswillies
Any idea how many marks I'd lose for getting delta G as -135000? I multiplied delta H by 1000 so delta H and delta S were in the same units :'(


Did you write the units of deltaG as Jmol-1? If you did then you should get all the marks I think!

Then if you did deltaS divided by 1000 instead you'd have to give units of deltaG to be kJmol-1 right?
(edited 7 years ago)

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