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

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    hey guys when ethane is oxidised to ethanal isit distilled or reflux?
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    (Original post by touran22)
    hey guys when ethane is oxidised to ethanal isit distilled or reflux?
    I think it's distilled.

    I think you get ethanoic acid with reflux.

    I could be wrong - probably am - so I've essentially confused you further. :cool:

    I fail at this. xD
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    (Original post by touran22)
    hey guys when ethane is oxidised to ethanal isit distilled or reflux?
    Distillation prevents full oxidation, so yes, it is ethanal. Ethanoic acid is formed under reflux, and when there is an excess of H+/Cr2O7 -2
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    (Original post by touran22)
    hey guys when ethane is oxidised to ethanal isit distilled or reflux?
    It's definitely distilled.

    Explanation (don't need to know, but just for interest): ethanal doesn't have hydrogen bonding, so it has a lower boiling point than ethanoic acid so it can be saparated by distillation.

    Though I think you mean ethanol in your original question
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    am i understanding this right....

    basically if u want to work out enthalpy change of combustion and ur given values for enthalpy change of formation then do u do products - reactents

    and also if u want to find enthalpy change of formation and ur given combustion values then do u do
    reactents-products

    ?
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    (Original post by kimmey)
    am i understanding this right....

    basically if u want to work out enthalpy change of combustion and ur given values for enthalpy change of formation then do u do products - reactents

    and also if u want to find enthalpy change of formation and ur given combustion values then do u do
    reactents-products

    ?
    Yep, that is correct
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    hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

    cheers!
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    (Original post by touran22)
    hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

    cheers!
    link
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    (Original post by SteveCrain)
    link
    here dude : http://www.4shared.com/document/-csn...JUN_10_QP.html
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    (Original post by touran22)
    hey you guys, you see in the June 2010 paper Question 1 E iii....how do you work out the moloes for c2h2? im completly baffled?!

    cheers!
    You have to use moles of gas= vol. of gas produced (given) / 24dmcubed.

    24dm cubed is the molar volume of any gas at room temperature and pressure.
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    (Original post by Killjoy-)
    You have to use moles of gas= vol. of gas produced (given) / 24dmcubed.

    24dm cubed is the molar volume of any gas at room temperature and pressure.
    ahhh yeah i see...i thought you had to divide by 24000 so i kept getting it confused

    Also on that same paper Question 1 D ii - i got +128 when the answer is -128.
    How come?

    cheers.
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    (Original post by touran22)
    ahhh yeah i see...i thought you had to divide by 24000 so i kept getting it confused

    Also on that same paper Question 1 D ii - i got +128 when the answer is -128.
    How come?

    cheers.
    You have been given changes of formation and you need to find the enthalpy change of reaction, so sum the changes of formation for the products and then subtract the changes of formation for the reactants (you need to 'reverse' the arrows in a Hess' Law enthalpy change cycle.)

    Remember to multiply the enthalpy change for the formation of H2O by two before subtracting.

    As for your answer, I think you subtracted in the wrong order?
    Mark scheme (if you don't have it) http://www.ocr.org.uk/download/ms_10...0_gce_f322.pdf

    Paper from OCR http://www.ocr.org.uk/download/pp_10...n_gce_f322.pdf
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    (Original post by Killjoy-)
    You have been given changes of formation and you need to find the enthalpy change of reaction, so sum the changes of formation for the products and then subtract the changes of formation for the reactants (you need to 'reverse' the arrows in a Hess' Law enthalpy change cycle.)

    Remember to multiply the enthalpy change for the formation of H2O by two before subtracting.

    As for your answer, I think you subtracted in the wrong order?
    Mark scheme (if you don't have it) http://www.ocr.org.uk/download/ms_10...0_gce_f322.pdf

    Paper from OCR http://www.ocr.org.uk/download/pp_10...n_gce_f322.pdf
    hi thanks for this...is there a certain rule we need to follow so if your givin Formation and they want Reaction its Products - Reactants

    What about Formation and Combustion or Reaction and Combustion or any other combination?
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    (Original post by touran22)
    hi thanks for this...is there a certain rule we need to follow so if your givin Formation and they want Reaction its Products - Reactants

    What about Formation and Combustion or Reaction and Combustion or any other combination?
    Correct, the general rule is to reverse the arrows, so if you are given enthalpy of formation (to calculate enathlpy change of reaction or combustion) you would have to reverse the arrows for the reactants.

    So its sum of enthalpy change of formation of products take way sum of enthalpy change of formations of reactants.

    And if you are given enthalpy change of combustion then you would have to do the opposite, look at the examiner tip boxes on pages 199 and 201 of the course book as it summarises it using notation that I cannot/do not know how to type out on a forum.

    Oh, and if you are calculating enthalpy change of reaction, then it is associated with the molar quantities in the equation- you may have to multiply out your answer if it is calculated for one mole of a substance that does not appear as one mole in the equation.
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    (Original post by touran22)
    hi thanks for this...is there a certain rule we need to follow so if your givin Formation and they want Reaction its Products - Reactants

    What about Formation and Combustion or Reaction and Combustion or any other combination?
    Yup - I'm far to lazy to draw out Hess' cycle, so:

    Enthalpy change of reaction = Sum of Enthalpy of formation (PRODUCTS) - Sum of Enthalpy of formation (REACTANTS)


    Enthalpy change of reaction = Sum of Enthalpy of Combustion (REACTANTS) - Sum of Enthalpy of combustion (PRODUCTS)


    Enthalpy change of reaction = Sum of Bond Enthalpy (REACTANTS) - Sum of Bond Enthalpy (PRODUCTS)




    For all of the 'sum of' bits, just add up the figure as it's given - include any negatives etc, and then use the formulae. It'll give you the value for Enthalpy Change and the correct sign.

    Hope this helps.
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    (Original post by warthogBE)
    Yup - I'm far to lazy to draw out Hess' cycle, so:

    Enthalpy change of reaction = Sum of Enthalpy of formation (PRODUCTS) - Sum of Enthalpy of formation (REACTANTS)


    Enthalpy change of reaction = Sum of Enthalpy of Combustion (REACTANTS) - Sum of Enthalpy of combustion (PRODUCTS)


    Enthalpy change of reaction = Sum of Bond Enthalpy (REACTANTS) - Sum of Bond Enthalpy (PRODUCTS)




    For all of the 'sum of' bits, just add up the figure as it's given - include any negatives etc, and then use the formulae. It'll give you the value for Enthalpy Change and the correct sign.

    Hope this helps.
    you sir are a life saver (Killjoy aswell!!)

    seriously helped me out! are you guys sitting this exam in next week?
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    (Original post by touran22)
    you sir are a life saver (Killjoy aswell!!)

    seriously helped me out! are you guys sitting this exam in next week?
    Haha - it is a bit depressing, especially the sheer amount of content - glad I could help though.

    Yep, OCR, next Thursday. Are you OCR too?
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    Does anyone know where i can get the mark scheme for the January 2011 paper, for spec a Chemistry unit 2? Sorry of this is the wrong thread...
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    (Original post by Jim Lee)
    Does anyone know where i can get the mark scheme for the January 2011 paper, for spec a Chemistry unit 2? Sorry of this is the wrong thread...
    OCR?
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    (Original post by warthogBE)
    OCR?
    oops...yes, ocr. sorry
 
 
 
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