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Equilibria, effect on rate reactions watch

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    I dont know how to work out the units for calculations of equilibrias.

    Also, I just cannot get my head around the effect of concentration on rate graphs and just the whole of that topic.

    All help is appreciated. :dh:

    Thank you
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    Unit for equilibria - if i'm right, you just cancel them out

    So if you have 3A + B -> 2C + D And the units are in mol-1 dm3 for example.

    You do products over reactants:

    2A x B/3C x D

    And then cancel out:

    So, B and D cancel each other out because they are 1:1. Leaving you with:

    2A/3C

    3C cancels out 2A, leaving you with just C. Making your units, mol-1 dm3. Sorry for the really crappy explanation, hope it helps
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    (Original post by Irrelevance)
    Unit for equilibria - if i'm right, you just cancel them out

    So if you have 3A + B -> 2C + D And the units are in mol-1 dm3 for example.

    You do products over reactants:

    2A x B/3C x D

    And then cancel out:

    So, B and D cancel each other out because they are 1:1. Leaving you with:

    2A/3C

    3C cancels out 2A, leaving you with just C. Making your units, mol-1 dm3. Sorry for the really crappy explanation, hope it helps
    Its not crappy, thanks for replying.
    But, this is the bit Im really stuck on...cancelling the units down to get a final unit.
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    (Original post by dream123)
    Its not crappy, thanks for replying.
    But, this is the bit Im really stuck on...cancelling the units down to get a final unit.
    Units are all related to how many moles there are of it. So, 1 mole is equal to 1 unit. We'll use mol-1 dm3 again.

    If there's one mole i.e. it's by itself in the reaction equation, it's simply mol-1 dm3

    If there's 2 moles, there are two units, so it becomes, mol-2 dm6

    3 moles - mol-3 dm9 etc.

    Back in the example of 3A + B -> 2C + D. We have:

    A - which is 3 moles, so mol-3 dm9

    B - mol-1 dm3

    C - 2 moles, so mol-2 dm6

    D - mol-1 dm3. Essentially making our equation:

    (mol-2 dm6) x (mol-1 dm3)
    --------------------------
    (mol-3 dm9) x (mol-1 dm3)

    Now just divide away! Crossing out the ones which have cancelled each other out:

    Step 1: Clear out the 1:1 ratios -


    (mol-2 dm9) x (mol-1 dm3) (mol-2 dm9)
    -------------------------- Becomes ------------
    (mol-3 dm6) x (mol-1 dm3) (mol-3 dm6)

    mol-1 dm3 have cancelled each other out of the equation. Leaving us with the above numbers.

    Step 2: Cancel out what's left as far as you can

    (mol-2 dm9)
    ------------ becomes 1/mol-1 dm3 due to the mol-2 dm9 cancelling out two units
    (mol-3 dm6)

    3. Invert signs if 1/

    1/mol-1 dm3 becomes mol dm-3

    Hope that's a little clearer. Edited.
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    I thought its product over reactants?

    D - mol-1 dm3. Essentially making our equation:

    (mol-3 dm9) x (mol-1 dm3)
    --------------------------
    (mol-2 dm6) x (mol-1 dm3)


    So, wouldnt it be:

    (mol-2 dm6) x (mol-1 dm3)
    --------------------------
    (mol-3 dm9) x (mol-1 dm3)
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    (Original post by dream123)
    I thought its product over reactants?

    D - mol-1 dm3. Essentially making our equation:

    (mol-3 dm9) x (mol-1 dm3)
    --------------------------
    (mol-2 dm6) x (mol-1 dm3)


    So, wouldnt it be:

    (mol-2 dm6) x (mol-1 dm3)
    --------------------------
    (mol-3 dm9) x (mol-1 dm3)
    Yeah, products over reactants, i just wrote it the wrong way round lol.

    So actually it becomes

    1
    ------
    mol-1 dm3

    If it's ever 1 over something, you just invert the signs. Making it turn from mol-1 dm3 into mol dm3
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    So, from here...how does the cancelling go? and thanks for your help so far!

    (mol-2 dm6) x (mol-1 dm3)
    --------------------------
    (mol-3 dm9) x (mol-1 dm3)

    do you do:

    -2 * -1 and 6*3 (top half)
    -3 * 9 and -1 * 3 (bottom half)
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    (Original post by dream123)
    So, from here...how does the cancelling go? and thanks for your help so far!

    (mol-2 dm6) x (mol-1 dm3)
    --------------------------
    (mol-3 dm9) x (mol-1 dm3)

    do you do:

    -2 * -1 and 6*3 (top half)
    -3 * 9 and -1 * 3 (bottom half)
    If they're on the same side, they can't cancel each other out. So A can cancel out C or D but not B. Or, if you prefer, you can only cancel diagonally or downwards/upwards

    Cancelling out would go like:

    (mol-1 dm3) cancels out (mol-1 dm3) completely and removing these two from the units equation.

    We'll do the next on in two parts. First:

    (mol-2 dm6) cancels out (mol-3 dm9)

    to make

    (mol-1 dm3) and (mol-2 dm6)

    And then again, because you need to completely cancel out 1 side or all sides to determine rates:

    (mol-1 dm3). And no probs about the help, as you can see, chemistry isn't really my forte lol
 
 
 
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