help with a chemistry question

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Xenon
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#1
Report Thread starter 16 years ago
#1
In the trends and patterns paper 2002, there is this question:

Aqueous ammonia reacts with water like this

NH3 (aq)+ H2O(l) goes to NH4(aq) + OH-(aq)

When aqueous ammonia is added dropwise to aqueous copper(II) ions, a very pale blue ppt is observed which disappears in excess ammonia to give a deep blue solution.
Write equations to show the formation of aqueous copper (II) ions of

i) The pale blue precipitate
ii) The deep blu precipitate

I know that in ligand substitution, the pale blue is formed from [Cu(H2O)6]2+ and so on, but is this to do with ligand substittution. Can someone please help me?
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#2
Report 16 years ago
#2
pale blue ppt = [Cu(H2O)4(OH)2] - this is deprotonation

dark blue ppt = [Cu(NH3)4(H2O)2]2+ - ligand exchange
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Guest Rob
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#3
Report 16 years ago
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Yes, it is ligand substitution. The equations are:

i) [Cu(H2O)6]{2+} (aq) ----(-OH/NH4OH dropwise)---> Cu(H2O)4(OH)2 (s)

Goes from a pale blue solution to a neutral, pale blue ppt

ii) Cu(H2O)4(OH)2 (s) ----(Xs NH3/NH4OH)---> [Cu(NH3)4(H2O)2]{2+} (aq)

Pale blue ppt redissolves to a royal blue solution
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Xenon
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#4
Report Thread starter 16 years ago
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Thanks guys. Are these just fixed equations which you have to make sure you know for the exam, or could they throw something totally unusual at you?
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#5
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Originally posted by Xenon
Thanks guys. Are these just fixed equations which you have to make sure you know for the exam
basically
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