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# Aqa chem 4/ chem 5 june 2016 thread watch

1. (Original post by EverydayHell)
Thanks you might be right there the question threw me off a bit because it the said the oxide dissolved in water, but only the mark scheme will tell.
I honeslty don't know you might be right I'm just saying that what i wrote. That first question was basically just an engish test with them trying to trick us
2. (Original post by RME11)
"Standard enthalpy change of reaction, ΔH°r - The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state."

^ The question asked for the standard enthapy change of the reaction given and you were given the correct ratio for the reaction stated, which was 2 SO2 + O2 -> 2SO3 and the standard enthalpy of formation values for these substances so you were expected to use D.H = D.H(Products) - D.H(Reactants) to work out the enthalpy change for the reaction given. The D.H calculated would give you the standard enthalpy change of such a reaction under standard conditions, which is what the question asked for.

Then you worked out the entropy change for the reaction given and put the figures into the gibbs free energy equation to work out the free energy change
for the reaction given.

tl;dr: there was no reason to divide by two, if you didn't you are correct.
Yay!
3. (Original post by Aethrell)
You could draw a skeletal formula for the [Co(H2NCh2Ch2Nh2)3]2+, right?
yes
4. Honourable mention to Suits101 for telling me about the chelate effect :3
5. (Original post by Aethrell)
You could draw a skeletal formula for the [Co(H2NCh2Ch2Nh2)3]2+, right?
what would that even look like? - not saying its wrong just dont know how you would do it
6. (Original post by JCleggy)

Standard enthalpy change of reaction, ΔH°r
'The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state.'

Alevelchemistry notes say this:
'The standard enthalpy change for a reaction is the heat energy change measured under standard conditions: 100 kPa and a stated temperature (usually 298K).'

and then goes on to further clarify:

'Given a reaction: A + 3B -> 2C + 4D

The standard enthalpy change for this reaction is taken to be the enthalpy change under standard conditions when one mole of A reacts with three moles of B to give two moles of C and four moles of D.'

From this I'm now certain that dividing by 2 was not required for the marks in this question.

(It is also mentioned in these notes that it is convention to leave the answer in kJmol^-1 which means those who did divide by two to get these units have a strong case for also gaining full marks.)
you would be right if it said that but it said formation there was delta Hf not delta Hr
Hey guys, will they accept [Ar] 3d7 4s2 and [Ar] 3d7 for the electron configurations or did you have to do the full ones??
It didn't ask for the full config so you should be ok.
8. I didn't put a 2+ or put brackets around the complex when drawing the compound with ethane1-2diamine - do you think I'll lose marks?

Posted from TSR Mobile
9. (Original post by 26december)
I didn't put a 2+ or put brackets around the complex when drawing the compound with ethane1-2diamine - do you think I'll lose marks?

Posted from TSR Mobile
You do usually lose a mark so no charge I think.
10. (Original post by Aerosmith)
Hilarious how people just cannot accept that the question required you to have an appreciation of the fact that 1 mol is formed when standard 😂😂
You're wrong.

The question asked 'for this reaction' - this is the standard reaction that occurs when OXYGEN is in its standard state (O2) is the one that was given.

Halving the values would be meaningless and would not be for the standard reaction when reagents and products are in their standard states.
11. (Original post by EverydayHell)
It didn't ask for the full config so you should be ok.
I did the shorthand cos effort :P
12. Can anyone link me to chem4 unofficial mark scheme? Thank you!!
13. (Original post by Ohnis)
Honourable mention to Suits101 for telling me about the chelate effect :3
It came up!!!
14. (Original post by koolgurl14)
I honeslty don't know you might be right I'm just saying that what i wrote. That first question was basically just an engish test with them trying to trick us
Hahaha, that's AQA!
15. (Original post by Ginpls)
Q8. First equation was just reacting it with 6 water moleculesP was Cr(H2O)3(OH)3Reagent was anything with OH- ions

NH3 can be used as a reagent here too!

Reagent for cr3+ to cr2+ is Zn and H+

Hydrogen fuel cell half eqn was

O2 + 4e- + 2H2O ---> 4OH- postive electrode

H2 + 2OH- ---> 2H2O + 2e- negative electrode

Overall: o2 + 2H2 ---> 2H2O

No effect if pressure increases as you are only increasing the pressure of oxygen so the same number of electron are released so same number of redox reactions??? (Not sure though)

Graph was a straight horizontal line

No effect on emf for increase in surface area of pt

Environmental advantage was h2o doesnt contribute to global warming as much as co2 and/or acid rain
For the reagents of Cr3+ to 2+, could you put HCl instead of H+?
16. (Original post by _newbe)
you would be right if it said that but it said formation there was delta Hf not delta Hr
The reaction in the contact process is not delta Hf, SO2 and O2 are not 'elements'
It is delta Hr, there is no need to divide by 2, in fact technically it is wrong to divide by 2

As a simple analogy:
For this reaction A + B -> C I'm sure you'd be fine with this being standard

That's where the argument of 1 mol breaks down. In fact, standard enthalpy of reaction specifically does not specify 1 mol for that very reason - because it's incorrect
17. (Original post by _newbe)
you would be right if it said that but it said formation there was delta Hf not delta Hr
I'd rather not argue the toss but it said neither of those things, The question said, word for word: 'Find the standard enthalpy change for this reaction'
18. (Original post by RME11)
"Standard enthalpy change of reaction, ΔH°r - The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state."

^ The question asked for the standard enthapy change of the reaction given and you were given the correct ratio for the reaction stated, which was 2 SO2 + O2 -> 2SO3 and the standard enthalpy of formation values for these substances so you were expected to use D.H = D.H(Products) - D.H(Reactants) to work out the enthalpy change for the reaction given. The D.H calculated would give you the standard enthalpy change of such a reaction under standard conditions, which is what the question asked for.

Then you worked out the entropy change for the reaction given and put the figures into the gibbs free energy equation to work out the free energy change
for the reaction given.

tl;dr: there was no reason to divide by two, if you didn't you are correct.
Thank you master wizard.
19. (Original post by 26december)
I didn't put a 2+ or put brackets around the complex when drawing the compound with ethane1-2diamine - do you think I'll lose marks?

Posted from TSR Mobile
You'll be fine.

.
Hey just wanted to confirm something:

Does SA effect emf of cell?

Would saying increase in pressure increase emf value, equilibrium shifts to RHS be ok for 2 marks do you think?

Thanks.
20. (Original post by Suits101)
I divided by 2 and put units as kJ/mol.

Definition when I googled standard enthalpy change:

The standard enthalpy of reaction (denoted ΔHr) is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions.

I don't know anymore! Haha
Nice back-peddle, I suggest you read up on your definitions because that is wrong:

http://chemwiki.ucdavis.edu/Core/Phy...ed_Definitions
http://www.chemguide.co.uk/physical/...finitions.html

If it wanted the DH for one mole, it would have asked for it.

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