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    Ok, here is the deal:

    What cobalt compound is it that is a green solid that is insoluble in water and will react with dilute H2SO4 to form the pink [Co(H2O)6]2+ ??

    Secondly,

    When [Co(H2O)6]2+ reacts with dilute NaOH it forms [Co(H20)4(OH)2] (blue ppt) right? Thats what I thought. Now, what happens if it reacts with Concentrated NaOH, it says on my question paper a blue solution, what formula?

    Ionic equations and the type of the reactions for the above would be most helpful too!

    Thank You. (Fingers crossed for Eier Von Statan)
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    (Original post by Hedgehunter)
    Ok, here is the deal:

    What cobalt compound is it that is a green solid that is insoluble in water and will react with dilute H2SO4 to form the pink [Co(H2O)6]2+ ??

    Secondly,

    When [Co(H2O)6]2+ reacts with dilute NaOH it forms [Co(H20)4(OH)2] (blue ppt) right? Thats what I thought. Now, what happens if it reacts with Concentrated NaOH, it says on my question paper a blue solution, what formula?

    Ionic equations and the type of the reactions for the above would be most helpful too!

    Thank You. (Fingers crossed for Eier Von Statan)
    The green compound is probably hydrated cobalt(III) salt. As Co2+ is more stable than Co3+ in aqueous solution(refer to standard electrode potential value), so aqua Co2+ is formed.

    Addition of OH- to Co2+ solution gives Co(OH)2 which may be pink or blue depending on the conditions. Only the pink form is stable. It is amphoteric and dissolves in conc. OH- to give a deep blue solution containing [Co(OH)4]2- ions from which crystalline salts can be obtained.

    Reference: Cotton & wilkinson advanced inorganic chem.
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    (Original post by shengoc)
    The green compound is probably hydrated cobalt(III) salt. As Co2+ is more stable than Co3+ in aqueous solution(refer to standard electrode potential value), so aqua Co2+ is formed.

    Addition of OH- to Co2+ solution gives Co(OH)2 which may be pink or blue depending on the conditions. Only the pink form is stable. It is amphoteric and dissolves in conc. OH- to give a deep blue solution containing [Co(OH)4]2- ions from which crystalline salts can be obtained.

    Reference: Cotton & wilkinson advanced inorganic chem.
    Thanks for your help. Though surely by adding Sulphuric its H+ that is involved and also, though I had looked at Co(OH)2, there is nothing about it in my text book, so I dont know whether it is related to A2 style questions. Anyway, I'm talking to my teacher about it tomorrow.
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    (Original post by Hedgehunter)
    Thanks for your help. Though surely by adding Sulphuric its H+ that is involved and also, though I had looked at Co(OH)2, there is nothing about it in my text book, so I dont know whether it is related to A2 style questions. Anyway, I'm talking to my teacher about it tomorrow.
    Co(OH)2 in solution would means Co would bind to water ligand available too, octahedral coordination is common for most TM. so you are right with Co(OH)2[H2O]4
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    After talking to my teacher I have put down Cobalt Oxide for the green solid as he said that in H2So4 it would form CoSO4 first then in the water molecule around, since CoSo4 is soluble would then form the hexaquacobalt. I think this works fine.
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    (Original post by Hedgehunter)
    After talking to my teacher I have put down Cobalt Oxide for the green solid as he said that in H2So4 it would form CoSO4 first then in the water molecule around, since CoSo4 is soluble would then form the hexaquacobalt. I think this works fine.
    haha, just one thing to note, it is very important to include oxidation state when you are dealing with transition metal, because they have variational oxidation state. ie if you just say iron oxide, it could be Fe(II) or Fe(III).
 
 
 
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