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# AQA GCSE Chemistry - C2 & C3 (14th May 2015) watch

1. (Original post by zc555)
Hi, can someone go through comparing energy released by fuels, Energy transfers in solutions both using Q = mCT and Calculating Bond Energies formula for Unit 3 please? I seem to be having huge problems with maths in Chemistry, I'll be very grateful for your help!
Calculating bond energies:

This really is simpler that it might seem.
The question will give you a formula, for example CH4 + 2O2 -> CO2 + 2H2O
It will then show you the different types of bonds within the reactants and products, and it will give you values for the energy of each bond. You then need to simply add up the bonds for each part. For example take CH4. This contains 4 C-H bonds, each with an energy of 412kJ/mol. So, for this reactant, the energy will be 412*4=1648kJ/mol. Then, do this for the other parts. 2O2 contains 2 lots of a O=O bond. The calculation for this is 498*2=996kJ/mol. Now, the products. CO2 contains 2 lots of a C=O bond. Calculation = 743*2=1486kJ/mol. Finally, 2H20 has 4 lots of O-H bond (because in one molecule of H20 there are 2 O-H bonds). Calculation = 463*4=1852kJ/mol.

This means that on the reactant side (left side) of the equation, our energies are 1648 + 996 and on the products (right) it's 1486 + 1852. Simplified this gives 2644 -> 3338. To work out the overall energy change, simply do 2644-3338=(-)694kJ/mol.
2. (Original post by neil20143)
I can't imagine any of them being six-markers except perhaps electrolysis The rest could be 2-4 mark questions though? I think Nanoparticles or Catalysts in industry are very likely though.
Oh okay, and ooh really?

Better take someone else advice as I am just with the clouds all the time;DD
3. (Original post by fabsafxo)
Oh okay, and ooh really?

Better take someone else advice as I am just with the clouds all the time;DD
To be honest, they could spring anything on us!
4. What are lubricants because this question has appeared a couple of times to do with nano particles?!
5. Anyone got any simple steps for working out empirical formulas?
6. (Original post by imyleighx)
Anyone got any simple steps for working out empirical formulas?
Yes! MAR!
write down the masses, divide by the Ar's and then divide again by te smallest number to get a ratio
7. (Original post by imyleighx)
Anyone got any simple steps for working out empirical formulas?
Write down the mass/percentage.
Divide that by the elements' Ar
Use answers as ratio for elements
8. (Original post by Positivedani)
Yes! MAR!
write down the masses, divide by the Ar's and then divide again by te smallest number to get a ratio
Thankyou so much!!
9. I want the haber process for a 6 mark, nice and easy. It was a 4/5 mark on last years test so fingers crossed, AQA tend to move 4/5 marks into 6 marks.
10. (Original post by DrNippleson)
I want the haber process for a 6 mark, nice and easy. It was a 4/5 mark on last years test so fingers crossed, AQA tend to move 4/5 marks into 6 marks.

Outr teacher thinks there will be a big part of it in e paper as the examiner's report for the section last year was quite badly recieved (it hadn't been done as well).
12. (Original post by Positivedani)
Carboxylic acid+alcohol in the prescene of sulfuric acid catalyst with a water by product
Thanks
13. (Original post by amber.991)
Calculating bond energies:

This really is simpler that it might seem.
The question will give you a formula, for example CH4 + 2O2 -> CO2 + 2H2O
It will then show you the different types of bonds within the reactants and products, and it will give you values for the energy of each bond. You then need to simply add up the bonds for each part. For example take CH4. This contains 4 C-H bonds, each with an energy of 412kJ/mol. So, for this reactant, the energy will be 412*4=1648kJ/mol. Then, do this for the other parts. 2O2 contains 2 lots of a O=O bond. The calculation for this is 498*2=996kJ/mol. Now, the products. CO2 contains 2 lots of a C=O bond. Calculation = 743*2=1486kJ/mol. Finally, 2H20 has 4 lots of O-H bond (because in one molecule of H20 there are 2 O-H bonds). Calculation = 463*4=1852kJ/mol.

This means that on the reactant side (left side) of the equation, our energies are 1648 + 996 and on the products (right) it's 1486 + 1852. Simplified this gives 2644 -> 3338. To work out the overall energy change, simply do 2644-3338=(-)694kJ/mol.
Wow, thank you so much! I'll give it a try with some questions.
14. What definitions would they ask in unit 2 and 3? Would they ask like what is relative molecular mass?
15. can somebody please list the key points of the Haber Process? thanks very much!
16. (Original post by maxjackson5)
can somebody please list the key points of the Haber Process? thanks very much!
Haber process:

N2(g) + 3H2(g) = 2NH3(g)

This is a reversible reaction, meaning the products can themselves react the give the original reactants.
The nitrogen is obtained from the air (78% Nitrogen composition).
Hydrogen can be obtained from electrolysis of water or from hydrocarbons.
Ammonia is used as fertiliser or as bleach.

Conditions in reaction vessel: 200atm, 450 degrees Celsius, presence of an iron catalyst.

A 200atm pressure is used because high pressures favour the forward reaction, so increase yield of ammonia (because the products of the forward reaction have a lower volume, 2 ammonia molecules for every 4 reactant molecules). However the cost of constructing a vessel which could withstand pressures more extreme than this would be very high.

450 degrees temperature is used for the purpose of increasing the rate of reaction, not to increase yield. In fact, increasing the temperature actually favours the backward reaction which is endothermic. Here a compromise is made; a relatively high temperature is used to allow a fast reaction, while not so high that it has large impact on the yield of ammonia.

The iron catalyst is important because although it has no effect on yield, it increases the rate of reaction (by offering alternative reaction pathway, lower activation energy etc.). This means that equilibrium is reached sooner, and allows for a lower temperature to be used. Without the catalyst, temperature would have to be increased further which would decease the yield of ammonia further.
(Iron catalyst is usually found as trays of iron in the reaction vessel)

Ammonia is produced as a gas, but is liquefied in a condenser, and removed from the vessel. Unreacted nitrogen and hydrogen are recycled so that there is no waste.

I think that's about all! Let me know if I've missed anything anyone! Hope this helps
17. (Original post by pollyy)
Haber process:

N2(g) + 3H2(g) = 2NH3(g)

This is a reversible reaction, meaning the products can themselves react the give the original reactants.
The nitrogen is obtained from the air (78% Nitrogen composition).
Hydrogen can be obtained from electrolysis of water or from hydrocarbons.
Ammonia is used as fertiliser or as bleach.

Conditions in reaction vessel: 200atm, 450 degrees Celsius, presence of an iron catalyst.

A 200atm pressure is used because high pressures favour the forward reaction, so increase yield of ammonia (because the products of the forward reaction have a lower volume, 2 ammonia molecules for every 4 reactant molecules). However the cost of constructing a vessel which could withstand pressures more extreme than this would be very high.

450 degrees temperature is used for the purpose of increasing the rate of reaction, not to increase yield. In fact, increasing the temperature actually favours the backward reaction which is endothermic. Here a compromise is made; a relatively high temperature is used to allow a fast reaction, while not so high that it has large impact on the yield of ammonia.

The iron catalyst is important because although it has no effect on yield, it increases the rate of reaction (by offering alternative reaction pathway, lower activation energy etc.). This means that equilibrium is reached sooner, and allows for a lower temperature to be used. Without the catalyst, temperature would have to be increased further which would decease the yield of ammonia further.
(Iron catalyst is usually found as trays of iron in the reaction vessel)

Ammonia is produced as a gas, but is liquefied in a condenser, and removed from the vessel. Unreacted nitrogen and hydrogen are recycled so that there is no waste.

I think that's about all! Let me know if I've missed anything anyone! Hope this helps
Isn't there problems with increasing pressure to produce ammonia ? Is it generally unsafe to use at high pressures?

Posted from TSR Mobile
18. (Original post by pollyy)
Haber process:

N2(g) + 3H2(g) = 2NH3(g)

This is a reversible reaction, meaning the products can themselves react the give the original reactants.
The nitrogen is obtained from the air (78% Nitrogen composition).
Hydrogen can be obtained from electrolysis of water or from hydrocarbons.
Ammonia is used as fertiliser or as bleach.

Conditions in reaction vessel: 200atm, 450 degrees Celsius, presence of an iron catalyst.

A 200atm pressure is used because high pressures favour the forward reaction, so increase yield of ammonia (because the products of the forward reaction have a lower volume, 2 ammonia molecules for every 4 reactant molecules). However the cost of constructing a vessel which could withstand pressures more extreme than this would be very high.

450 degrees temperature is used for the purpose of increasing the rate of reaction, not to increase yield. In fact, increasing the temperature actually favours the backward reaction which is endothermic. Here a compromise is made; a relatively high temperature is used to allow a fast reaction, while not so high that it has large impact on the yield of ammonia.

The iron catalyst is important because although it has no effect on yield, it increases the rate of reaction (by offering alternative reaction pathway, lower activation energy etc.). This means that equilibrium is reached sooner, and allows for a lower temperature to be used. Without the catalyst, temperature would have to be increased further which would decease the yield of ammonia further.
(Iron catalyst is usually found as trays of iron in the reaction vessel)

Ammonia is produced as a gas, but is liquefied in a condenser, and removed from the vessel. Unreacted nitrogen and hydrogen are recycled so that there is no waste.

I think that's about all! Let me know if I've missed anything anyone! Hope this helps
wow, thank you! i will get remembering those facts tomorrow after a good nights sleep ahaha, thanks again
19. (Original post by maxjackson5)
wow, thank you! i will get remembering those facts tomorrow after a good nights sleep ahaha, thanks again
Hahah it's okay! Good luck on Thursday!
20. (Original post by Nai18)
Isn't there problems with increasing pressure to produce ammonia ? Is it generally unsafe to use at high pressures?

Posted from TSR Mobile
Yeah there is! Sorry I missed that, thank you!

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