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    Can someone please explain to me why Al2O3 has a higher melting point than SiO2?? Surely SiO2 should have the higher melting point as it's giant covalent?? Thanks in advance x
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    (Original post by Crankyla)
    I think it is supposed to be [Al(OH)4(H2O)2]-. At least, that's how I've always written it.

    Also, I'm unclear on the 'why' of substitution reactions. For example, [Cu(H2O)6]2+ reacts with an excess of ammonia, it forms [Cu(H2O)2(NH3)4]2+. When my class asked our Chemistry teacher why that happens and why aren't all the H2O ligands replaced, all she said was "It just does". Can someone explain it, please??
    I asked my teacher this as well and it basically doesn't matter. I can't remember what they said either - something about NH3 being a better ligand than water although that doesn't explain why some water remains. Here's a link but as you can see, its super complicated and way beyond the spec.
    http://chemistry.stackexchange.com/q...ns-and-ammonia

    (Original post by shiney101)
    When they ask why a coloured ion is formed do you say the d-electrons absorb visible light or white light? I've always said it's visible light but then in one of the 2010 papers it said white light so now i'm not sure.
    It absorbs a certain frequency of white light does it not? I'm pretty sure white light and visible light are the same thing but my physics isn't very strong. I wouldn't just say it absorbs white light though because then there will be no light remaining/ transmitted right?

    (Original post by GO97)
    Why are enthalpy changes approximate?
    is it because averages are used?
    What paper is this? If you're talking about enthalpy change calculated using mean bond enthalpy, then its because the values are averaged over a range of compounds.
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    (Original post by lahigueraxxx)
    Can someone please explain to me why Al2O3 has a higher melting point than SiO2?? Surely SiO2 should have the higher melting point as it's giant covalent?? Thanks in advance x
    Al2O3 is ionic and forms a giant ionic lattice.
    SiO2 has macro molecular structure but only has covalent bonds holding structure

    Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding


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    (Original post by nutcase13)
    aluminium oxide
    why?
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    (Original post by Lilly1234567890)
    why?
    idk, im just quoting the book, i think it may have something to do with the electronegativity difference between Al and Si, although that could be wrong
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    (Original post by 26december)
    Al2O3 is ionic and forms a giant ionic lattice.
    SiO2 has macro molecular structure but only has covalent bonds holding structure

    Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding


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    Oh I always thought that covalent bonding was stronger, especially if the structure is macromolecular Thanks!
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    When AS (entropy) is negative for a reaction, is there more order or more disorder?
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    (Original post by 26december)
    Al2O3 is ionic and forms a giant ionic lattice.
    SiO2 has macro molecular structure but only has covalent bonds holding structure

    Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding


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    Why is the melting point for Na2O less than SiO2?
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    (Original post by 12284)
    When AS (entropy) is negative for a reaction, is there more order or more disorder?
    More order, so basically less disorder if entropy is negative.
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    What are all the types of reactions?

    There's ligand substitution
    There's acid base

    Anymore?



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    (Original post by shiney101)
    More order, so basically less disorder if entropy is negative.
    Wait I'm confused now, I thought a reaction was more likely to occur if AS is negative?
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    (Original post by Manexopi)
    I asked my teacher this as well and it basically doesn't matter. I can't remember what they said either - something about NH3 being a better ligand than water although that doesn't explain why some water remains. Here's a link but as you can see, its super complicated and way beyond the spec.
    http://chemistry.stackexchange.com/q...ns-and-ammonia
    Okay, I think I understand it... kind of. It makes sense that the water ligands would be competing for the binding sites with ammonia, especially if it's in high concentration. Thanks so much
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    (Original post by shiney101)
    Why is the melting point for Na2O less than SiO2?
    Na2O is giant ionic. SiO2 is giant covalent. ionic is weaker than covalent
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    Guys can the born haber cycle get anymore complicated than Mgcl2 or NaCl??
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    How much does everyone know about spectophotometry/colorimetry? I know it's not on the spec but it links into the aqua ions and titrations sections.
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    Chem 5 revision going well
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    What are all the types of reactions?

    There's ligand substitution
    There's acid base

    Anymore?




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    (Original post by 12284)
    Wait I'm confused now, I thought a reaction was more likely to occur if AS is negative?
    I think you're getting confused with delta G, a reaction would happen (feasible) if delta G is negative, but with entropy the more disordered the more positive it becomes.
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    Someone help with 3c and 4a pls

    http://filestore.aqa.org.uk/subjects...W-QP-JAN10.PDF
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    Does anyone have the Chem 5 2015 mark scheme pleaseee
    Thanks in advance
 
 
 
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