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Ligand Exchange reactions

[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 :smile:
Reply 1
Avatar for Akbar2k7
Akbar2k7
20
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
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 :smile:


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 :smile:

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.
Reply 3
Avatar for lekha2611
lekha2611
OP
2
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
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 :redface:
Reply 5
Avatar for lekha2611
lekha2611
OP
2
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 :redface:


Cool, thank you :smile:

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