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# Aqa chem 4/ chem 5 june 2016 thread Watch

1. (Original post by ahsan_ijaz)
Hi thanks,

I predicted a hard chem 4 paper ages ago lol - btw another Q - in AG= AH-TAS expression why is the entropy the one /1000 and not enthalpy as the enthalpy is kj/mol
Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol
2. (Original post by rolla01)
Does anyone have a diagram with the colours ions we need to learn for CHEM 5
https://getrevising.co.uk/resources/...d-observations
3. (Original post by Suits101)
Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol
does a reaction become spontaneous when AG is negative or positive?
Thanks
4. (Original post by 12284)
does a reaction become spontaneous when AG is negative or positive?
Thanks
Reaction is feasible if AG <= 0

(where <= means less than or equal to)
5. does anyone have any predictions for chem 5?
6. (Original post by ahsan_ijaz)
Hi thanks,

I predicted a hard chem 4 paper ages ago lol - btw another Q - in AG= AH-TAS expression why is the entropy the one /1000 and not enthalpy as the enthalpy is kj/mol
My answer is what I think so it might not be right. I can ask my teacher and see what they say about it.

The reason you have to divide it by 1000 is so that it is in KJ mol-1 K-1 and not J mol-1 K-1 (You're converting the Joules into kilo-joules).

Think of it like taking away a measurement that's in metres from a measurement that's in kilometres. Converting both units to kilometres (or both to metres) makes the calculation easier. You could just do it keeping the units in the form they are in, but it will be quite fiddly.

Also it's important to note KJ/Mol is a standard unit so most answers are in this form. Gibbs free energy is usually in KJ/mol.
7. (Original post by Suits101)
Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol
Just a quick note. ΔS is not measured in J/K/Mol it's measured in J / K mol. Just being a bit of a neat freak here xD
8. (Original post by faizah007)
does anyone have any predictions for chem 5?
You can't really predict what's going to be on the paper (especially since what chem4 was like). Go through all of it. Leave nothing out.
9. Getting stressed now :///
10. (Original post by rolla01)
Does anyone have a diagram with the colours ions we need to learn for CHEM 5
Will this help?

11. (Original post by rolla01)
Does anyone have a diagram with the colours ions we need to learn for CHEM 5
AQA Revision guide page 95 has a class table
12. http://filestore.aqa.org.uk/subjects...5-QP-JUN12.PDF

I dont understand how Gibbs free energy change is represented on a graph
13. (Original post by SirRaza97)
Will this help?

yes thank you
14. (Original post by chzm)
http://filestore.aqa.org.uk/subjects...5-QP-JUN12.PDF

I dont understand how Gibbs free energy change is represented on a graph
2(b) The straight line equation y = mx + c can be used to represent the the gibbs free energy equation where y = ΔG, m = -ΔS x = T and c = ΔH. To calculate the gradient you just do the change in y divided by the change in x. So take two points on the line and find the corresponding y an x values and do the same for a second point and do this calculation:

y1 - y2 / x1 - x2

The slope is -ΔS so the units are J K-1 Mol-1

2(d) is understanding that the rate of change of entropy changes for an element in it's different states. If the rate of change of entropy has changed then the slope (gradient) of your line has changed. So the ammonia has turned to liquid as the temperature is going lower.
15. Do we need to know all the reactions of the different cells in the nelson thornes book in the redox equilibria chapter?
16. If I said 'reactants' instead of 'reagents' would I lose a mark??
Do we need to know all the reactions of the different cells in the nelson thornes book in the redox equilibria chapter?
Only the hudrogen fuel cell
18. (Original post by lahigueraxxx)
If I said 'reactants' instead of 'reagents' would I lose a mark??
No, they are synonymous.

What context are you talking about?
No, they are synonymous.

What context are you talking about?
http://filestore.aqa.org.uk/subjects...W-QP-JUN11.PDF

http://filestore.aqa.org.uk/subjects...W-MS-JUN11.PDF

5fi and 5fiii (thank you btw )
20. No problem

Unless the examiner is a Vietnamese child marking your paper (which I wouldn't put past AQA), they will accept both 'reagents' and 'reactants' as synonymous.

Besides, my chem teacher (and I think many others) refer to 'reagents' and not 'reactants'.

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