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OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

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    (Original post by Lindizya)
    Can Someone please explain how to determine the rate determining step I don't understand it
    Firstly the rate determining step is the slowest step in the mechanism.

    The reactants for the rate determining step are those (and only those) that feature in the rate equation. If a reactant is 2nd order, then two molecules of it will be on the left, if 1st order then one.

    These combine together to make an intermediate (just make the left balance with the right, marking schemes are normally very flexible about what your products are on this step) - This is your rate determining step.

    Then whatever reactants from your chemical equation were not used above combine in a second (faster) step with your intermediate to make the product.

    See the below attached example.

    Click image for larger version. 

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    I hope that helps clear it up a bit
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    Does anyone have any tips on how to remember all the equations to work out things like ph, ka etc.. .
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    (Original post by silver0)
    Does anyone have any tips on how to remember all the equations to work out things like ph, ka etc.. .
    Afraid you just need to learn them, but Ka is just like Kc, it's (concentration of products) over (concentration of reactants) for the acid dissociation equilibrium.

    I recommend you summarise them on one page, they will seem a lot less!
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    2CH3OH(l) + 3O2(g) goes to 2CO2(g) + 4H2O(l) - This question was in the June 2011 paper. I dont understand why there is a decrease in entropy? Any help?. Thanks.
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    (Original post by moh.alt)
    2CH3OH(l) + 3O2(g) goes to 2CO2(g) + 4H2O(l) - This question was in the June 2011 paper. I dont understand why there is a decrease in entropy? Any help?. Thanks.
    As the no of moles of gas is decreasing. This is more important than the overall increase in moles.
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    (Original post by clownfish)
    As the no of moles of gas is decreasing. This is more important than the overall increase in moles.




    Oh yess! Ah didnt think of that, i just looked at the increase in the moles and not the decrease in gas moles. Thanks a lot.
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    If Anyone wants all the JAN 12 papers for biology and chemistry OCR A and B, heres the link:

    http://www.thestudentroom.co.uk/show....php?t=1943168
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    Born haber cycle calculations practice?
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    Not particularly enjoying this section so far. Much preferred F324. Anyone got any good revision materials/practice questions?
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    (Original post by Tom172)
    Not particularly enjoying this section so far. Much preferred F324. Anyone got any good revision materials/practice questions?
    http://www.a-levelchemistry.co.uk/
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    Hey how is everyone revising for this?? Im thinking past exam papers obviously and learning the content, but that did not work for f324, so anoyone got any advice? Much appreciated
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    Subscribing
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    Needing 125 in this unit, but I much prefer organic chemistry.. Ah well, I have Easter to try write some notes.
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    I'm starting to panic about finishing the spec, we're about halfway through now...
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    (Original post by SpringNicht)
    I'm starting to panic about finishing the spec, we're about halfway through now...
    If you need help I don't mind

    I've done a lot of it but need to brush up on Transition Metals and start papers. Oh and cover Electrodes :eek: Need to finish this unit very quickly myself.
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    hey wondering what resources (in terms of books not the internet) to use?

    can anyone reccommend the jimclark calculations book as i think both the ocr textbooks are dismal for calculations?

    thanks
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    (Original post by flowerpower1410)
    hey wondering what resources (in terms of books not the internet) to use?

    can anyone reccommend the jimclark calculations book as i think both the ocr textbooks are dismal for calculations?

    thanks
    Jim Clarke Calculations book are fantastic! It's the same guy who created ChemGuide.
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    (Original post by ManPowa)
    If Anyone wants all the JAN 12 papers for biology and chemistry OCR A and B, heres the link:

    http://www.thestudentroom.co.uk/show....php?t=1943168
    you're awesome!
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    I really hate this unit. Just like I hated ABG (unit 1) which I'm resitting in May. A summer filled of (mainly) inorganic chemistry topics is not my idea of fun. Want to just sit RPA 4 times :P
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    do we need to know how to calculate ph of a weak base (kb etc.)? its not completely clear on the spec...

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