Kemics
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Hi,

I wonder if anyone could clarify for me the colour of Chromium chemistry (specifically with regards to AQA A-level spec answers!)

What colour is chromium 3+ as an aqua ion i.e [Cr(H2O)6]3+ ?

and what colour will the precipitate be after it's reacted with NH3, OH- or [CO3]2- (should be the same for all 3 i think?) should end up as [Cr(H20)3(OH)3](S)

Many Thanks!
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LadyEcliptic
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It's usually violet in the aqua complex
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RoseBrilliante
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(Original post by Kemics)
Hi,

I wonder if anyone could clarify for me the colour of Chromium chemistry (specifically with regards to AQA A-level spec answers!)

What colour is chromium 3+ as an aqua ion i.e [Cr(H2O)6]3+ ?

and what colour will the precipitate be after it's reacted with NH3, OH- or [CO3]2- (should be the same for all 3 i think?) should end up as [Cr(H20)3(OH)3](S)

Many Thanks!
(Original post by LadyEcliptic)
It's usually violet in the aqua complex
Hmm I've got [Cr(H20)6]3+ as a green ppt. and with the addition of OH- it first becomes a grey-green ppt. of [Cr(OH)3(H20)3] and then with excess OH- it becomes a green solution of [Cr(OH)6]3-
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Borek
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Color of the precipitate often depends on the anion involved, so there is no easy answer. Adding carbonate will precipitate carbonate, adding OH- or ammonia will initially precipitate hydroxide - but excess hydroxide (and excess ammonia) can dissolve the precipitate.

Compare http://www.docbrown.info/page07/transition04Cr.htm
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LadyEcliptic
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(Original post by RoseBrilliante)
Hmm I've got [Cr(H20)6]3+ as a green ppt. and with the addition of OH- it first becomes a grey-green ppt. of [Cr(OH)3(H20)3] and then with excess OH- it becomes a green solution of [Cr(OH)6]3-
Sorry - I do CCEA. We've learned it as a violet crystal for the aqua complex and green for any others, and on addition of NaOH/nh3 it's a green blue ppt.
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joeyjoeymc
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I've got it down as ruby for [Cr(H2O)6]^3+ on AQA. But I believe the mark scheme accept a mixture of other colours too, including violet. With a base it will form a grey-green precipitate of [Cr(H2O)3(OH)3]. Excess hydroxide produces a dark green solution of [Cr(OH)6]^3- and excess ammonia produces a purple solution of [Cr(NH3)6]^3+.
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username913907
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The colour of metal ligand complexes depends upon the metal AND the ligand. Consider the metal to have a range of colours available... which point within that scale is determined by which ligands are coordinated.
The colour of a complex is determined by the energy gaps between the energy levels. These are dictated by the electronic properties of both the metal AND the ligands hence the colour also depends upon the ligand.
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