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# AQA Chemistry C3 - Unofficial Mark Scheme

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1. (Original post by bod247)
Its Rb because in group 1 (alkali metals), the Boiling point and Melting Point decrease as you go down. Rb was the last Alkali Metal so its the lowest Boiling Melting/Point.
its fluorine. Rubidium is a metal, fluorine is a gas
2. Fluorine is a gas at room temperature
3. Here is the bond energy once and for all:

ethanol + oxygen-->carbon dioxide + water
CH3CH2OH + 3O2 --> 2CO2 + 3H2O

on the left there are:
5*H-C bonds (each with an energy of 413) which totals 2065
1*C-C bonds at 347
1*C-O bonds at 358
1*O-H bonds at 467
3*O=O bonds (each with an energy of 495) which totals 1485
the total left hand side energy is the sum of these at 4722

on the right side there are:
4*C=O bonds (each with an energy of 799 [for CO2]) which totals 3196
6*O-H bonds (each with an energy of 467) which totals 2802
the total right hand side energy is the sum of these at 5998

4722-5998=-1276
it is left as a negative but a positive answer is fine at GCSE
For the energy change I divided 3444 by 0.8 and go 4305? ANYONE ELSE?
I got that as well
5. (Original post by TheAnon121)
I am not sure about the bond energy questions, some are saying -1623 and some -1276, which is the right answer? Could someone please say why the other one is wrong also.
I'm not exactly sure as to which is correct (I personally put -1623) but I know why people go different answers. Some people included the balance numbers and some didn't...
6. Thought on grade boundaries for both C2 and 3?
I think C2 - 51, C3 - 44?
7. For the last question I didn't refer to equilibrium would I still get the marks?

I said:
A low temperature increases the yield but slows down the rate of reaction
A high temperature would lower the yield
A low pressure increases the yield but is very costly to maintain and dangerous.
8. (Original post by Fr3dg)
what?? it was lowest melting point, not lowest on the table, rubidium is solid at room temperature, flourine is a gas
i meant Fluorine haha
9. (Original post by Jack 2016)
I got that as well
you dont need to find out the energy released per gram, it asked for the energy released, not the energy released per gram
10. (Original post by sunny181113)
Thought on grade boundaries for both C2 and 3?
I think C2 - 51, C3 - 44?
For an A or A*

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11. (Original post by Fr3dg)
Here is the bond energy once and for all:

ethanol + oxygen-->carbon dioxide + water
CH3CH2OH + 3O2 --> 2CO2 + 3H2O

on the left there are:
5*H-C bonds (each with an energy of 413) which totals 2065
1*C-C bonds at 347
1*C-O bonds at 358
1*O-H bonds at 467
3*O=O bonds (each with an energy of 495) which totals 1485
the total left hand side energy is the sum of these at 4722

on the right side there are:
4*C=O bonds (each with an energy of 799 [for CO2]) which totals 3196
6*O-H bonds (each with an energy of 467) which totals 2802
the total right hand side energy is the sum of these at 5998

4722-5998=-1276
it is left as a negative but a positive answer is fine at GCSE
This is what I got, hopefully this is right! That question was long for only 3 marks
12. (Original post by LiamCollins)
For the last question I didn't refer to equilibrium would I still get the marks?

I said:
A low temperature increases the yield but slows down the rate of reaction
A high temperature would lower the yield
A low pressure increases the yield but is very costly to maintain and dangerous.
could be hit or miss really, first mark is right, but you probably wanted to talk about it being a compromise, i think it migt be down to tghe marker?
13. (Original post by dausa99)
For an A or A*

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A*
14. (Original post by LiamCollins)
For the last question I didn't refer to equilibrium would I still get the marks?

I said:
A low temperature increases the yield but slows down the rate of reaction
A high temperature would lower the yield
A low pressure increases the yield but is very costly to maintain and dangerous.
I think, for 5 marks, you still have to refer to equilibria being shifted the wrong way if the temperature is too high.
15. (Original post by Fr3dg)
Here is the bond energy once and for all:

ethanol + oxygen-->carbon dioxide + water
CH3CH2OH + 3O2 --> 2CO2 + 3H2O

on the left there are:
5*H-C bonds (each with an energy of 413) which totals 2065
1*C-C bonds at 347
1*C-O bonds at 358
1*O-H bonds at 467
3*O=O bonds (each with an energy of 495) which totals 1485
the total left hand side energy is the sum of these at 4722

on the right side there are:
4*C=O bonds (each with an energy of 799 [for CO2]) which totals 3196
6*O-H bonds (each with an energy of 467) which totals 2802
the total right hand side energy is the sum of these at 5998

4722-5998=-1276
it is left as a negative but a positive answer is fine at GCSE
I got that but everyone else is saying it's wrong because it was an exothermic reaction and the energy went up

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16. What did every1 put for the fill in the grid question i put no change for all apart from a box on the top row where i put 'White precipitate' i think it was under calcium
17. I've just had a thought, for the energy change with specific heat capacity question, I put 3,444J but 4.2 is the SHC for water to change ONE gram of water, should 3,444 therefore be multiplied by 0.8 to scale it down to 0.8g from 1g?

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18. (Original post by romansholiday)
This is what I got, hopefully this is right! That question was long for only 3 marks
yeah its a bit of a piss take really, especially since there is wuite a high chance to get it wrong, i didnt even get the result i wrote down in the exam :[
19. (Original post by RayRay99)
Last question:

Low temp is needed for a higher yield of ammonia as the forward reaction is exothermic.
But this would decrease the rate of reaction too much so a comprise temp of 450'c is used.

A high pressure would also give a higher yield as there are fewer molecules in the right than on the left.
But a higher pressure costs too much money so a compromise pressure is used. (200atm)
How many marks could you pick up without referring to equilibrium and exothermic\endothermic reactions?
20. For the last question, what would happen if the pressure was too high?

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