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Ligand Exchange reactions

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    [Cr(H2O)6]3+ ions react with water to form an acidic solution. Complete the equation for this reaction.

    [Cr(H2O)6]3+ (aq) + H2O (l) --> ? + ?


    Well, I guessed: [Cr(H2O)4(OH)2]+ as the main ion formed, but the markschemes main answer was: [Cr(H2O)5(OH)]2+ .

    My question is, how do you know how many ligands to exchange? Why was my answer incorrect?

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    Its to do with the amphoteric nature. And its pKw or 14 value meaning water has a very small chance of becoming OH- or H3O+ so its very unlikely for two Hydrogen's to be removed from he H2O's in the water.

    the fact is it becomes very slightly acidic
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    (Original post by lekha2611)
    [Cr(H2O)6]3+ ions react with water to form an acidic solution. Complete the equation for this reaction.

    [Cr(H2O)6]3+ (aq) + H2O (l) --> ? + ?


    Well, I guessed: [Cr(H2O)4(OH)2]+ as the main ion formed, but the markschemes main answer was: [Cr(H2O)5(OH)]2+ .

    My question is, how do you know how many ligands to exchange? Why was my answer incorrect?

    Thanks
    For this particular case, I just remember that chromium ions mainly exist as Cr6+, Cr3+, and Cr2+ ions. Hence, [Cr(H2O)4(OH)2]+ would not be formed

    Also, it is the same equation (except the TM ion) for [Fe(H2O)6]3+ ions being acidic, as they only lose 1 proton per complex ion.
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    I guess it's something I just have to remember.

    (Original post by thegodofgod)

    Also, it is the same equation (except the TM ion) for [Fe(H2O)6]3+ ions being acidic, as they only lose 1 proton per complex ion.
    Is this the case for most metal ions? (The losing one proton per complex ion idea)

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    (Original post by lekha2611)
    I guess it's something I just have to remember.



    Is this the case for most metal ions? (The losing one proton per complex ion idea)

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    I think so, as it shows that 3+ ions are more acidic than 2+ ions, and that 3+ ions go to 2+ ions
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    (Original post by thegodofgod)
    I think so, as it shows that 3+ ions are more acidic than 2+ ions, and that 3+ ions go to 2+ ions
    Cool, thank you

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Updated: May 24, 2012
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