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    Anybody has the Heinemann OCR Chemistry Orange & Black book???? bcz i really need ur help, i have lost my CD, and i want to get the Answers to the stretch and challenge questions, so can someone plz send me the word file from the CD that has the answers. thankssssss
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    why does h2o have a bond angle of 104.5 and h20 ligands in [Cu(h20)6] 2+ have bond angles of 107.5
    shouldnt the h20 have a bigger bond angle due to the repulsion from both elecron lone pairs?
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    (Original post by viksta1000)
    dunno what the second equation's all about but all you need to know is that the buffer is H2CO3 <---> H+ + HCO3-

    There's lots of weak acid because it hardly dissociates and lots of its conjugate base but not much H+

    When you add acid...the concentration of H+ increases, the added H+ reacts with the conjugate base producing more weak acid thereby shifting equilibrium to the left and minimising pH change (not increasing or decreasing it...but *minimising* the change)

    When you add alkiali, the concentration of OH- increase, the added OH- reacts with the H+ in the buffer producing H2O...the weak acids has to dissociate more to replace the reacted H+ ions, it does this by shifting equilibrium to the right hence minimising the pH change
    Thankk youu!
    And yeah that second equation is in the book and it threw me rriight off, something to do when glucose is oxidised? And how H+ reacts with hydrogencarbonate ions, causing the equilibrium to be shifted to the left so carbon dioxide can be carried to the lungs and breathed out?
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    (Original post by TsunKit)
    Thanks but I said excluding June 2010/Jan 2011.
    Oh yeah, my bad haha
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    (Original post by Pandit Bandit)
    why does h2o have a bond angle of 104.5 and h20 ligands in [Cu(h20)6] 2+ have bond angles of 107.5
    shouldnt the h20 have a bigger bond angle due to the repulsion from both elecron lone pairs?
    H2O ligands now have 3 bp and 1lp
    In water lp-lp repulsion pushed the bond angles together. As bp-lp repulsion < lp-lp repulsion the bonds can now move further away
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    (Original post by Pandit Bandit)
    why does h2o have a bond angle of 104.5 and h20 ligands in [Cu(h20)6] 2+ have bond angles of 107.5
    shouldnt the h20 have a bigger bond angle due to the repulsion from both elecron lone pairs?
    Water has two lone pairs of electrons, so there is greater repulsion, decreasing the bond angle. Whereas, in the complex, one of its lone pairs is used to coordinatley bond to the transition metal, so there is less repulsion and the bond angle is increased to 107.5. Something along those lines, i think?
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    Can someone please explain the rate determining step??
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    Hey, In the June 2003 paper question 5, unifying concepts paper...Can someone explain to me how to work how many calcium ions present?
    Please..why did they times RMM by 2???
    http://pastpapers.org/A2/chemistry/u...ne_2816_01.pdf
    answer:
    mass of Ca in kidney stone = 2 x 31.3/100 = 0.626 g !
    moles of Ca in kidney stone = 0.626/40.1 = 0.0156 mol !
    number of Ca2+ ions removed = 6.02 x 1023 x 0.0156 = 9.39 x 1021 ions !
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    (Original post by sickofexamz)
    Can someone please explain the rate determining step??
    its the slowest step in a multi-step reaction mechanism and you'll find how many molecules that are involved in the rate equation:
    Rate: K(No2)2
    means there are 2 molecules of No2 in the slowest step
    I think
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    (Original post by sickofexamz)
    Can someone please explain the rate determining step??
    The slowest step in a multistep mechanism.

    The total number of moles of reactants before and in the RDS equals their order in the rate equation.
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    Feeling good after that geography exam- most stuff I revised well came up
    Now for a bit of chem... Any tips on balancing equations using oxidation numbers? I keep getting them wrong :/
    And what is this balancing charges business?
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    (Original post by student777)
    Feeling good after that geography exam- most stuff I revised well came up
    Now for a bit of chem... Any tips on balancing equations using oxidation numbers? I keep getting them wrong :/
    And what is this balancing charges business?
    Always make sure your charges balance i.e. if it's 5+ on one side, make sure it's 5+ on the other side too!
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    How much, for the january 2011 exam, was a raw mark of 83? 65 = 120, so the scale is 120/65... But that gives me over 150 UMS?
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    right in cell potential- the species with the most negative "e-nor" value is that the more powerful reducing agent or oxidising agent?
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    (Original post by Hydro.ZX)
    How much, for the january 2011 exam, was a raw mark of 83? 65 = 120, so the scale is 120/65... But that gives me over 150 UMS?
    Apparently nobody got above 80 raw marks in January so 83 will give you full UMS
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    (Original post by cws121)
    right in cell potential- the species with the most negative "e-nor" value is that the more powerful reducing agent or oxidising agent?
    If a half cell has the more negative standard electrode potential it means it has a tendency to lose electrons. Losing electrons means it has been oxidised and it is therefore a reducing agent.
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    (Original post by cws121)
    right in cell potential- the species with the most negative "e-nor" value is that the more powerful reducing agent or oxidising agent?
    The species with the more negative electrode potential is a more powerful reducing agent.
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    oh ffs, what's an oxidising/reducing agent again?
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    (Original post by haydyb123)
    Always make sure your charges balance i.e. if it's 5+ on one side, make sure it's 5+ on the other side too!
    But surely you can just get that from oxidation numbers? Eg if it's Cl- it has oxisation state -1

    And what about this question, from june 2010 question 2:

    BrO3- + ....Br- + ....H+ --> ....Br2 + ....H2O

    There aren't any charges on the RHS?
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    (Original post by hellosarah)
    oh ffs, what's an oxidising/reducing agent again?
    A reducing agent gives up its electrons, so it is oxidised by losing electrons then those electrons are give to what ever is reduced

    An oxidising agent gains electrons, so it self is reduced by picking up electrons from species that are oxidised

    I think that is correct
 
 
 
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