The Student Room Group

Chemistry transition metals

In the equations of the aqueous ion complexes with ammonia, we add 2nh3 at first and then for excess ammonia we add 4nh3. Why 4? Why can’t 3nh3 be excess ammonia?
Reply 1
Hi, do you mind providing example equations? I'm not 100% sure what you mean...
Original post by laazzzydinosaur
In the equations of the aqueous ion complexes with ammonia, we add 2nh3 at first and then for excess ammonia we add 4nh3. Why 4? Why can’t 3nh3 be excess ammonia?

Are you thinking of copper(II), as in

[Cu(H2O)6]^2+ + 2NH3 -> Cu(OH)2(H2O)4 + 2NH4^+

Cu(OH)2(H2O)4 + 4NH3 —> [Cu(NH3)4(H2O)2]^2+ + 2H2O + 2OH^-

In the first reaction, two ammonias react because each one removes a H^+ and you need to remove two H^+ ions to form the neutrally charged hydroxide.

In the second, four ammonias react, because the most stable copper ammonia complex has four ammonia ligands around it (and two waters). If you study chemistry at uni level later, you will understand why copper specifically prefers this complex and not others - the reasons are far too complicated for A level.
Original post by UtterlyUseless69
Are you thinking of copper(II), as in
[Cu(H2O)6]^2+ + 2NH3 -> Cu(OH)2(H2O)4 + 2NH4^+
Cu(OH)2(H2O)4 + 4NH3 —> [Cu(NH3)4(H2O)2]^2+ + 2H2O + 2OH^-
In the first reaction, two ammonias react because each one removes a H^+ and you need to remove two H^+ ions to form the neutrally charged hydroxide.
In the second, four ammonias react, because the most stable copper ammonia complex has four ammonia ligands around it (and two waters). If you study chemistry at uni level later, you will understand why copper specifically prefers this complex and not others - the reasons are far too complicated for A level.

Ah yes this is exactly what i wanted to know. Thank you !
Original post by Methene
Hi, do you mind providing example equations? I'm not 100% sure what you mean...

Thank you ! But @UtterlyUseless69 has answered it, the equations i was referring to are in their answer.

Quick Reply

Latest