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# OCR AS - Chemistry Unit F322 - Chains, energy and resource - REVISION! watch

1. (Original post by godknowsprerna)
hey can someone plz explain to me the equilibrium question when they ask you stuff like
predict and explain the condition tht would giv the max equilibrium yield of ethanol from ethene n steam q 5 b iii) from jan 2010 ... the answer is its an exothermic reaction so low temperature for high yield I DONT GET IT HOW DOES THIS WORK?

and what is the stuff about compromising
If temperature increases, the equilibrium will move in the endothermic direction.
If temperature decreases, the equilibrium will move in the exothermic direction.

So if you want it to do the forwards reaction, and the forwards reaction is exothermic, then you need it to move in the exothermic direction, so you need a low temperature.

5b(iii) January 2010
Using le Chatelier's principle, predict and explain the conditions that would give maximum equilibrium yield of ethanol from ethene and steam:
- high pressure would shift the equilibrium to the right where there are fewer moles of gas
- low temperature would move the equilibrium in favour of the exothermic direction

You have to compromise because
- too high of a pressure is expensive and potentially dangerous
- you want a high enough temperature to keep the rate of reaction fast enough without compromising the yield
2. (Original post by Dave_Beeston)
Don't put 25C I've seen that disallowed wrote because the definition specifically states 298K best to play it safe!
oh no really? i always write that ok i will have to remember it! thankyou, you may have just saved me a mark
3. (Original post by godknowsprerna)
hey can someone plz explain to me the equilibrium question when they ask you stuff like
predict and explain the condition tht would giv the max equilibrium yield of ethanol from ethene n steam q 5 b iii) from jan 2010 ... the answer is its an exothermic reaction so low temperature for high yield I DONT GET IT HOW DOES THIS WORK?

and what is the stuff about compromising
C2H4+ H20--> C2H5OH Enthalpy change: -46KJ

Firstly with pressure:

In this reaction there are 2 gaseous molecules on the left and just 1 on the right. If you increase the pressure, the system will increase the production of the chemicals with the lowest number of gas molecules in order to lower the pressure. High pressure will therefore favour the production of ethanol as it would lower the pressure, moving the position of equilibrium to the right. And vice versa for lower pressure.

With Temperature:

From this we can see that the forward reaction has an enthalpy change of -46. This indicates an exothermic reaction i.e. one that raises the temperature. By using lower temperatures, the system favours the forward reaction in order for it to produce more ethanol and raise the temperature of the system. Therefore a low temperature would be more favoured in this situation. A high temperature would favour the decomposition of ethanol back to its reactants.

Comprimising is where you don't use the ideal conditions due to too high costs or too slow reactions.
4. Where it doesn't specify how to draw a compound, shall I just draw the displayed formula?
5. Can someone quickly explain how to predict the major and minor products of Addition reactions of unsymmetrical Alkenes?

Thank you.
6. Can anyone explain the difference between percentage yield and atom economy? Thanks
7. (Original post by smithythedrummer)
c2h4+ h20--&gt; c2h5oh enthalpy change: -46kj

firstly with pressure:

In this reaction there are 2 gaseous molecules on the left and just 1 on the right. If you increase the pressure, the system will increase the production of the chemicals with the lowest number of gas molecules in order to lower the pressure. High pressure will therefore favour the production of ethanol as it would lower the pressure, moving the position of equilibrium to the right. And vice versa for lower pressure.

With temperature:

From this we can see that the forward reaction has an enthalpy change of -46. This indicates an exothermic reaction i.e. One that raises the temperature. By using lower temperatures, the system favours the forward reaction in order for it to produce more ethanol and raise the temperature of the system. Therefore a low temperature would be more favoured in this situation. A high temperature would favour the decomposition of ethanol back to its reactants.

Comprimising is where you don't use the ideal conditions due to too high costs or too slow reactions.
thank you soooo much
8. It won't really be a paper which is anything like past questions... probably will have a considerable amount of ORGANIC... GREEN CHEMISTRY has'nt really featured much at all in recent times either. Remember MECHANISMS with CURLY ARROWS must be EXTREMELY ACCURATE to satisfy the examiner!
9. (Original post by science7)
Can someone quickly explain how to predict the major and minor products of Addition reactions of unsymmetrical Alkenes?

Thank you.
We don't need to know this for AS. But just remember the major product always has the H bonded to the first carbon.
10. (Original post by medusa cascade)
I have genuinely learnt more in the few hours that I've been here than my chemistry teacher has taught me all year. Thank you so much everyone.
are you taking chemistry next year?
11. (Original post by lolo-x)
the average enthalpy change when one mole of a given type of bond is broken in the molecules of a gaseous species
SO the only differece between defi of bond en.. and average bond en.. is latter is average enthalpy
12. Can anyone explain the difference between percentage yield and atom economy? Thanks
13. (Original post by patra)
Can anyone explain the difference between percentage yield and atom economy? Thanks
% Yield = (Actual amount of product / theoretical amount of product) x 100

Atom economy = (Mr of desired product / Mr of all products) x 100
14. (Original post by patra)
Can anyone explain the difference between percentage yield and atom economy? Thanks
Percentage Yield: actual moles of product obtained divided by the expected moles of product all multiplied by 100

Atom economy: Mr of desired products divided by the sum of Mr of all products x 100

Percentage yield is really how efficiently the reaction is taking place - is the reaction operating at its maximum potential?

Atom economy shows you how much useful product you are getting out of all of the products you have got, so it shows you how wasteful your reaction is
15. (Original post by SmithytheDrummer)
The Octane Number is how efficient a fuel burns. The longer and less branched the hydrocarbon chain is, the lower the octane number and the lower the efficiency. This is why we use fuels with higher octane numbers so that less waste is produced and less energy is used to overcome the activation energy(I think).
Thanks man
16. Hess law defi !?
17. Enthalpy change: when is it products minus reactants and reactants minus products??
18. Can anyone explain the difference between percentage yield and atom economy? Thanks
high percentage yeild shows high conversion of reactants into product

High AE shows little waste produced!
19. (Original post by arvin_infinity)
SO the only differece between defi of bond en.. and average bond en.. is latter is average enthalpy
It seems that way to me

although another definition I have found for average bond enthalpy: The enthalpy change that occurs when 1 mole of a given type of covalent bond is broken, averaged across a range of compounds in the gaseous state

i would say
bond enthalpy: the enthalpy change that occurs when 1 mole of a given type of covalent bond is broken

average bond enthalpy: the enthalpy change that occurs when 1 mole of a given type of covalent bond is broken, averaged across a range of compounds in the gaseous state

to be on the safe side?
20. (Original post by arvin_infinity)
Hess law defi !?
enthalpy change of a reaction is independant of the route it takes provided the initial and final conditions are the same!

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