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OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD Watch

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    Took me a while to understand from markscheme (obvs didnt get in first go), hope it makes sense, reply again if it doesnt.

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    (Original post by zirak46)
    Took me a while to understand from markscheme (obvs didnt get in first go), hope it makes sense, reply again if it doesnt.

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    Makes me panic that you're finding and answering these insane questions and I wouldn't even know where to start with that despite finishing learning the content :'(

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    I understand upto getting the conc of h2o2 but dont understand why we times by Mr?
    Is there a formula that im missing?
    n=cv, n=v/24, n=mass/mr
    Dont understand why we do c=mass/mr

    Shouldnt it mean by that logic 0.469 x 34 x (0.025)
    Because cv = mass/mr

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    (Original post by Hunnybeebee)
    Makes me panic that you're finding and answering these insane questions and I wouldn't even know where to start with that despite finishing learning the content :'(

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    Im at same stage as you, i literally started past papers today and i was so confused! Its just trial and error, do past papers and get them wrong, thats all good because this is just practise. Just learn from mark scheme and if it doesnt make sense, post them here, thats what im doing, posted 2 above so dont panic, we have time!

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    (Original post by zirak46)
    Im at same stage as you, i literally started past papers today and i was so confused! Its just trial and error, do past papers and get them wrong, thats all good because this is just practise. Just learn from mark scheme and if it doesnt make sense, post them here, thats what im doing, posted 2 above so dont panic, we have time!

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    I've only done questions by topic with physicsandmathstutor.com but need to hit the real past papers now. The panic is real. Got 6 weeks to polish things up :'(

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    (Original post by Hunnybeebee)
    I've only done questions by topic with physicsandmathstutor.com but need to hit the real past papers now. The panic is real. Got 6 weeks to polish things up :'(

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    Chill fam, take it steady, practise and go through your past papers every other day
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    (Original post by zirak46)
    Chill fam, take it steady, practise and go through your past papers every other day
    Thank you! Hopefully things look better soon

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    Value of Kc increases if forward reaction is endo and temp is increased. Whereas if forward reaction is exothermic and temp increases, value of Kc decreases. This I understand.

    However I flip to the next page and it says 'kc is altered by changes in temperature, the value of kc is unaffected by changes in conc and pressure' and below it gives examples of n2o4 <-> n2o2 and kc value changes. Anyone with the text book want to explain? Is it because by le chateliers principle it will shift the position of equilibrium to produce more reactant/product to restore the concentrations?



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    (Original post by Hunnybeebee)
    Value of Kc increases if forward reaction is endo and temp is increased. Whereas if forward reaction is exothermic and temp increases, value of Kc decreases. This I understand.

    However I flip to the next page and it says 'kc is altered by changes in temperature, the value of kc is unaffected by changes in conc and pressure' and below it gives examples of n2o4 <-> n2o2 and kc value changes. Anyone with the text book want to explain? Is it because by le chateliers principle it will shift the position of equilibrium to produce more reactant/product to restore the concentrations?



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    From what i understand:

    Kc is only affected by temp, thats it.
    But when pressure or conc changes, kc is altered and so each of the top and bottom of kc expression either increase or decrease until Kc is reached again.

    What page in the book were you takking about? Is this the Cambridge Ocr Chem book? If so, what page and i can try to help more?
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    (Original post by zirak46)
    I understand upto getting the conc of h2o2 but dont understand why we times by Mr?
    Is there a formula that im missing?
    n=cv, n=v/24, n=mass/mr
    Dont understand why we do c=mass/mr

    Shouldnt it mean by that logic 0.469 x 34 x (0.025)
    Because cv = mass/mr

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    Because the units are in g mol-3 not in mol dm-3 so you have to multiply the concentration by mr to get it to g dm-3.
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    (Original post by Hunnybeebee)
    Is it because by le chateliers principle it will shift the position of equilibrium to produce more reactant/product to restore the concentrations?



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    Literally thought the same thing till this morning actually but no, Kc never changes overall. Changing conc and pressure means the kc system is no longer at equilibrium. So both the bottom and top of expression will either each inc or dec till Kc is reached.

    Meaning overall no change due to the expressions in Kc change

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    I dont understand though, sorry if i sound silly

    Conc x vol = mols

    Mass / Mr = mols

    We found conc by mols / vol , but how does conc relate to mass / mr ?

    Is it a case of g dm-3 means mass / vol, which is equivalent to conc x mr?

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    (Original post by AqsaMx)
    Because the units are in g mol-3 not in mol dm-3 so you have to multiply the concentration by mr to get it to g dm-3.
    Omg i get it thank you, just replying made me realise haha, thanks for the help! Hope my explanation helped you too?

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    (Original post by zirak46)
    Omg i get it thank you, just replying made me realise haha, thanks for the help! Hope my explanation helped you too?

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    Yes thank you it did. But you know how the second product has a -S-S- bond reformed, how do you know the disulphides bond is made again when it doesn't tell you?
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    (Original post by zirak46)
    Literally thought the same thing till this morning actually but no, Kc never changes overall. Changing conc and pressure means the kc system is no longer at equilibrium. So both the bottom and top of expression will either each inc or dec till Kc is reached.

    Meaning overall no change due to the expressions in Kc change

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    Okay so...

    Say I had 2NO2 <-> 2NO + O2

    Kc = [NO]^2[O2]
    [NO2]^2

    And I increased the conc of 2NO2 by x2, that means there is more reactant than product (bottom of expression increased). It is no longer in equilibrium so position of equilibrium shifts from left to the right to increase conc of NO and O2 and to decrease conc NO2 in the expression so Kc value can be restored?
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    (Original post by zirak46)
    From what i understand:

    Kc is only affected by temp, thats it.
    But when pressure or conc changes, kc is altered and so each of the top and bottom of kc expression either increase or decrease until Kc is reached again.

    What page in the book were you takking about? Is this the Cambridge Ocr Chem book? If so, what page and i can try to help more?
    What do you mean by kc is 'altered'? as in the equation in itself not the value? And noo the other one, black and blue

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    Just remember guys, questions like that disulfide bond are the most challenging ones. That one was only 2 marks out of 100. If you get a similar situation in the real one just move on and try to score best on the other 98 marks. You're better off getting 5 easy marks on a buffer solution than spending a long time on a 2 mark question.
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    (Original post by Hunnybeebee)
    Okay so...

    Say I had 2NO2 2NO + O2

    Kc = [NO]^2[O2]
    [NO2]^2

    And I increased the conc of 2NO2 by x2, that means there is more reactant than product (bottom of expression increased). It is no longer in equilibrium so position of equilibrium shifts from left to the right to increase conc of NO and O2 and to decrease conc NO2 in the expression so Kc value can be restored?
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    That all seems right!
    Just to make sure, recap of what happens from what I understand:

    -conc of NO2 has been increased
    -meaning system no longer in equilibrium
    -Kc isnt affected and therefore doesnt change
    -this is because top of kc expression will increase until the original value of Kc is reached
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    (Original post by Hunnybeebee)
    What do you mean by kc is 'altered'? as in the equation in itself not the value? And noo the other one, black and blue

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    From my understanding:

    Kc is conc of products / conc of reactants
    Changing temp will change VALUE of Kc
    Changing conc or pressure will change CONC of prod or reactants
    Meaning Kc is altered
    But we know kc never changes when change in pressure and conc
    Thats because the prod (bottom of expression) or/and reactants (top of expression) will change till Kc is reached

    Hope that makes sense and if im wrong, please someone do correct me!
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    (Original post by rory58824)
    Just remember guys, questions like that disulfide bond are the most challenging ones. That one was only 2 marks out of 100. If you get a similar situation in the real one just move on and try to score best on the other 98 marks. You're better off getting 5 easy marks on a buffer solution than spending a long time on a 2 mark question.
    Thats a very good point, took me good 10 mins to the question and i got no marks xD
 
 
 
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