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Edexcel - Chemistry Unit 2 - 4 June 2013 Watch

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    (Original post by James A)
    Is when large groups prevents a species (-OH for instance) from attacking a central atom.

    That's the best I can come up with, Harry Williams and the rest of the crew can help us :cool:
    That seems fairly accurate. :holmes:

    (Original post by newyork newyork)
    what does it stand for?
    Do a quick Google search, I value my TSR membership.
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    Which of these reactions is not a redox reaction?
    A Mg(NO3)2(s) → MgO(s) + 2NO2(g) + 1⁄2O2(g)
    B HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    C Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    D Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

    Which is it and why?
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    (Original post by James A)
    Is when large groups prevents a species (-OH for instance) from attacking a central atom.

    That's the best I can come up with, Harry Williams and the rest of the crew can help us :cool:
    haha thanks guys
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    (Original post by MSamuel)
    can someone give me a definition of steric hindrance pleeeeeeeeeeeeeeease
    The word "steric" has something to do with "stereo" as in stereoisomerism.

    Steric hindrance most likely means that large molecules are attached to an atom, leaving no room for attack by other species.
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    please please help me! i am unsure about why group 1 carbonates (aside from lithium) do not decompose, but nitrates do??
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    (Original post by Maximan12)
    Which of these reactions is not a redox reaction?
    A Mg(NO3)2(s) → MgO(s) + 2NO2(g) + 1⁄2O2(g)
    B HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    C Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    D Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

    Which is it and why?
    What do you think before someone jumps in?
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    (Original post by posthumus)
    Chlorine is best oxidizing agent & iodide (not iodine) is the best reducing agents. I'm guessing because chlorine is far more electronegative it attracts the electron to a greater extent. Whereas iodine with it's big radius is more likely to be oxidized itself.




    Increasing the concentration on one side, will always shift equilibrium in the opposite direction I believe. Ignore any solids.
    okay now i get it thanks ,,, one more thing , could u please explain shortly about titration of iodine and stuff , im totally confused about what should i put where "apparatus" and what do we need to find , i know the whole calculation part but the rest is confusing me , thanks in advance
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    (Original post by Hi, How are you ?)
    Can anyone explain a hydrogen bond please =)
    Firstly hydrogen bonding will only occur with a very electronegative element attached to the H( nitrogen oxygen or fluorine, i hope you know what electronegativity is)
    and secondly occur if the electronegative element has a lone pair of electrons on it ( eg water, ammonia, HF ) .

    The hydrogen bond is the bonding between the hydrogen atom which is very d+ due to the electronegative element and the lone pair of electrons on another element with an lone pair of electrons.

    For example if they asked to draw hydrogen bonding in ethanol do something like this

    Note : To be safe put the lone pairs of electrons on the oxygen molecules to show the H bond is going to the lone pair on the oxygen.
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    guys really quick q...

    consider this reaction: K2Cr2O7 + 3SO2 + H2SO4 goes to K2SO4 + Cr2(SO4)3 + water

    what is the change in oxidation number of chromium??

    it says it goes down by 3, but i swear you need to divide by 2 to get the oxidation number and not keep it as Cr2 if that makes sense?
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    (Original post by HarryMWilliams)
    What do you think before someone jumps in?
    I would guess B because I can't see a transfer of electrons between any of the atoms
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    (Original post by Maximan12)
    Which of these reactions is not a redox reaction?
    A Mg(NO3)2(s) → MgO(s) + 2NO2(g) + 1⁄2O2(g)
    B HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    C Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    D Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

    Which is it and why?
    B because no change occurs
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    (Original post by Maximan12)
    Which of these reactions is not a redox reaction?
    A Mg(NO3)2(s) → MgO(s) + 2NO2(g) + 1⁄2O2(g)
    B HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
    C Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
    D Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

    Which is it and why?
    A redox reaction occurs when there is a change in OXIDATION STATE of an element. The best way to work it out is to write on top of each element its oxidation state ( eg H +1, O -2 etc ) and see which reaction has no change in oxidation states.

    In this case it is B as:
    the H is always +1
    the Na is always +1
    the Cl is always -1
    the O is always -2

    If you look at C for example you can see the Fe changes from 0 to +2 as SO4 has a charge -2 and the CU changes from +2 to 0
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    (Original post by tahadamani)
    Occours when electronegative atoms (oxygen nitrogen and fluorine) bond to a hydrogen atom, this is a very strong intermolecular force and is much stronger then London forces. The high difference in electro negativity causes hydrogen bonds which require much higher energy to break, so molecules with hydrogen bonds have a much greater boiling point and melting point
    Hydrogen bonding:

    When hydrogen is covalently bonded to highly electronegative elements such as oxygen, fluorine or nitrogen, the attraction of the the atom for the electron pair in the bond is so high that the electron density shifts very close to the higher electronegative element. This makes the hydrogen nucleus unusally exposed. Lone pair of electrons from an atom in the adjacent molecule form bond with the hydrogen. This bonding is called hydrogen bonding.
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    what equations do we need to know (except the obvious)
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    (Original post by Mimi85)
    okay now i get it thanks ,,, one more thing , could u please explain shortly about titration of iodine and stuff , im totally confused about what should i put where "apparatus" and what do we need to find , i know the whole calculation part but the rest is confusing me , thanks in advance
    Sorry I'm not sure quite what you mean? The apparatus ? Burette,conical flask & pipette ?
    (Original post by charlieejobson)
    guys really quick q...

    consider this reaction: K2Cr2O7 + 3SO2 + H2SO4 goes to K2SO4 + Cr2(SO4)3 + water

    what is the change in oxidation number of chromium??

    it says it goes down by 3, but i swear you need to divide by 2 to get the oxidation number and not keep it as Cr2 if that makes sense?
    SO4 has charge of -2
    therefore Cr2 has +6 charge
    +6/2 = +3
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    (Original post by iwant)
    Firstly hydrogen bonding will only occur with a very electronegative element attached to the H( nitrogen oxygen or fluorine, i hope you know what electronegativity is)
    and secondly occur if the electronegative element has a lone pair of electrons on it ( eg water, ammonia, HF ) .

    The hydrogen bond is the bonding between the hydrogen atom which is very d+ due to the electronegative element and the lone pair of electrons on another element with an lone pair of electrons.

    For example if they asked to draw hydrogen bonding in ethanol do something like this

    Note : To be safe put the lone pairs of electrons on the oxygen molecules to show the H bond is going to the lone pair on the oxygen.
    So why is HF only able to for 1 hydrogen bond when it has 3 lone pair of e-?
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    Ah god freaking out. Can anyone explain why in tertiary halogenoalkanes you get an intermediate but In primary you don't get it (it all just happens in almost one step)?????????



    Posted from TSR Mobile
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    (Original post by Hi, How are you ?)
    So why is HF only able to for 1 hydrogen bond when it has 3 lone pair of e-?
    Because there is only one hydrogen atom. If there were two, there would have been 2 hydrogen bonding per molecule.
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    (Original post by Hi, How are you ?)
    So why is HF only able to for 1 hydrogen bond when it has 3 lone pair of e-?
    Because the ratio of H:F among H-F molecules is 1:1. If F forms hydrogen bonds to 3 H each, then as H can only form hydrogen bonds to 1 atom, only 1/3 of the F will get to form hydrogen bonds and the remaining 2/3 F will not have any. Of course this is not how reality works. So each F will only form 1 H bond.

    What does it have to do with lone electron pairs? :confused:
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    (Original post by Matterhorn)
    Ah god freaking out. Can anyone explain why in tertiary halogenoalkanes you get an intermediate but In primary you don't get it (it all just happens in almost one step)?????????



    Posted from TSR Mobile

    Hydroxide ion faces steric hindrance when it attacks the carbon atom with the halogen attached as it is largely crowded/blocked by alkyl groups.

    Primary halogenoalkanes do not form carbocations because the primary carbocations formed will be too unstable.
 
 
 
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