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# Ligand Exchange reactions

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1. [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
2. 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
3. (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.
4. 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)

Thanks
5. (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)

Thanks
I think so, as it shows that 3+ ions are more acidic than 2+ ions, and that 3+ ions go to 2+ ions
6. (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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