Chem 5:Transition Metal Colours

OP

I have CuCl(4)(2-) as green, [Fe(H20)6]3+ as yellow/orange :frown:

Hm well AQA syllabus just says colours of complexes in this module..but I think it's

colour of
all the vanadium ox.states,
chromate,dichromate
iron(II), iron (III), cobalt, aluminium, copper, chromium aqeous reactions w/Na(2)CO(3), HCl, NH3, NaOH & colours in excess
silver reactions w/thio, NaOH and NH3

anyone got anymore we need?

Reply 3

OP

Ok can someone just confirm this:

CuCO(3) = blue or green?
[Cr(H2O)6]3+ + conc. HCl ---> [CrCl(6)]3- or [CrO(4)]2- (then [Cr(2)O(7)]2-) ?

Thanks :biggrin:

Reply 4

ShOcKzZ

7

Asphyxiating

Ok can someone just confirm this:

CuCO(3) = blue or green?
[Cr(H2O)6]3+ + conc. HCl ---> [CrCl(6)]3- or [CrO(4)]2- (then [Cr(2)O(7)]2-) ?

Thanks :biggrin:

CuCO3 is a 'blue-green' ppt not 'blue or green'

not sure about the second..never come accross it. shud i be worried? lol.

Reply 5

Definitely not [CrO(4)]2-

You will need to put [Cr(H2O)6]3+ with H2O2 and OH- to do that.

I think [CrCl(6)]3- is also green...?

Reply 6

OP

selkie222

Definitely not [CrO(4)]2-

You will need to put [Cr(H2O)6]3+ with H2O2 and OH- to do that.

I think [CrCl(6)]3- is also green...?

Ok so it makes [CrCl(6)]3-?
Is [Fe(H20)(6)]3+ yellow or brown?

Thanks v much :biggrin:

p.s. yes [CrCl(6)]3- is green

Reply 7

OP

ShOcKzZ

CuCO3 is a 'blue-green' ppt not 'blue or green'

not sure about the second..never come accross it. shud i be worried? lol.

hm argh so i have blue and green and blue-green, I take it it's close then so they might accept any of those answers? Did it say blue-green in the mark scheme of a past paper somewhere?

Reply 8

ShOcKzZ

7

Asphyxiating

Ok so it makes [CrCl(6)]3-?
Is [Fe(H20)(6)]3+ yellow or brown?

Thanks v much :biggrin:

p.s. yes [CrCl(6)]3- is green

where are you getting these complexes from..ive just got:

[Fe(H2O)6]2+
[Co(H2O)6]2+
[Cu(H2O)6]2+

[Fe(H2O)6]3+
[Al(H2O)6]3+
[Cr(H2O)6]3+

with ..OH- (little), OH- (excess) NH3 (little) NH3 (excess) and Carbonate..

where do all the Cl's come into it?? :confused:

Reply 9

Actually not sure if it makes anything... [CrCl(6)]3- sounds like a reasonable choice between the two but you'll need quite concentrated HCl since Cl- is a weaker ligand than H2O.

In my notes there is a partially cholosubstituted species [CrCl(2)H2O(4)]+ which sounds more possible.

In the module book (I think... just checked, yes) it says that [Fe(H2O)(6)]3+ is a (scary!) pale violet(!!!) but appears yellow due to hydrolysis.

It is brown when precipitated as [Fe(H2O)(3)(OH)(3)]

Mark schemes are usually somewhat flexible on these points, I understand.

I might have seen a pale violet iron(III) crystal once, but I have never seen it pale violet in solution.

selkie

Reply 10

ShOcKzZ

where are you getting these complexes from..ive just got:

[Fe(H2O)6]2+
[Co(H2O)6]2+
[Cu(H2O)6]2+

[Fe(H2O)6]3+
[Al(H2O)6]3+
[Cr(H2O)6]3+

with ..OH- (little), OH- (excess) NH3 (little) NH3 (excess) and Carbonate..

where do all the Cl's come into it?? :confused:

Vanadium?

Yes all those above are pretty much what you need. But for several random ones--- Co, Cu they do expect you to know the colour of the tetrachloro ion.

[CoCl4]2- is blue
[CuCl4]2- is yellow-green (interesting little equilibrium, because if you add more water, it goes back to blue due to H2O being a stronger ligand)

Apart from those, you might have some di-substituted chloro ions ith Cr(III) and V(III), for which a good guess is green.

And by the way anhydrous CoCl2 is blue--- it is part of a test for water (blue to pink)

That's all I think...

selkie

Reply 11

OP

selkie222

Vanadium?

Yes all those above are pretty much what you need. But for several random ones--- Co, Cu they do expect you to know the colour of the tetrachloro ion.

[CoCl4]2- is blue
[CuCl4]2- is yellow-green (interesting little equilibrium, because if you add more water, it goes back to blue due to H2O being a stronger ligand)

Apart from those, you might have some di-substituted chloro ions ith Cr(III) and V(III), for which a good guess is green.

And by the way anhydrous CoCl2 is blue--- it is part of a test for water (blue to pink)

That's all I think...

selkie

Thanks, yeap I'm getting there now, slowly! Thanks so much, you've been a real help :tongue:

Reply 12

OP

Ok one more.. :tongue:

When you add excess NaOH to [Cr(H2O)6]3+ do you get [Cr(OH)6]3- which is dark green soln or do you get [Cr(OH)4(H2O)2]- which is a green soln?

Also do we need to know that Co(H2O)4(OH)2 stands in air to become oxidised to beige ppt Co(OH)3 ?

Ok think i'm alright now thanks :smile:

Reply 13

kingpong

Where are all these 6 Cl- ligand complexes from?!?!

I was under the impression Cl- could only form 4 ligands due to size/charge etc.???

Reply 14

kingpong

Where are all these 6 Cl- ligand complexes from?!?!

I was under the impression Cl- could only form 2 ligands due to size/charge etc.???

Good point. Yes 6 Cl- ligand complexes probably won't happen (not to the metals we are studying anyways), but 4 Cl- complexes do happen to Cu and Co (which is yellow green and blue respectively).

Asphyxiating

When you add excess NaOH to [Cr(H2O)6]3+ do you get [Cr(OH)6]3- which is dark green soln or do you get [Cr(OH)4(H2O)2]- which is a green soln?

It seems to be possible for [Cr(OH)6]3- to get to 6OH-, partly maybe because OH- is a strong ligand... but there are reasons to do with chromium too, which we don't need to know.

I think the most important thing to know with [Cr(H2O)6]3+ is that it forms green ppt on adding a little base, and it dissolves on excess OH- to form a green solution.

And again [Cr(H2O)6]3+ is a (startling) "red-violet" or "ruby red" according to the module books--- however it appears green due to hydrolysis (which is what I usually see!)

selkie

Reply 15

OP