Phalange
Badges: 0
Rep:
?
#1
Report Thread starter 8 years ago
#1
Hi

What is the colour of hexaamminechromium 3+?

I have 3 different books for Edexcel chemistry. One says VIOLET, another GREEN and another YELLOW...

??? Why are they different?
Also one book says when you react hexaaquachromium3+ with EXCESS OH- you get [Cr(OH)6]3- and another says [Cr(H2O)4(OH)2]-

The green one involved chloride ions which is understandable. So I have violet and yellow to decide now.... could the yellow one involve chloride too?
0
reply
DCRichards
Badges: 0
Rep:
?
#2
Report 8 years ago
#2
(Original post by Phalange)
Hi

What is the colour of hexaamminechromium 3+?

I have 3 different books for Edexcel chemistry. One says VIOLET, another GREEN and another YELLOW...

??? Why are they different?
Also one book says when you react hexaaquachromium3+ with EXCESS OH- you get [Cr(OH)6]3- and another says [Cr(H2O)4(OH)2]-

The green one involved chloride ions which is understandable. So I have violet and yellow to decide now.... could the yellow one involve chloride too?
the colour of [Cr(NH3)6]3+ is apparently purple according to this: http://www.creative-chemistry.org.uk...s/N-ch5-07.pdf, which makes sense as the copper ammonia complex is a deep blue, whereas as hexaaquacopper(II) is a lighter blue and so purple would seem a logical colour for it to be.

As for the adding OH-, I guess it would depend on the extent of the ligand substitution. According to this http://en.wikipedia.org/wiki/Chromium , [Cr(OH)3] becomes [Cr(OH)6]3- in alkaline solution and [Cr(H2O)6]3+ in acidic solution, so I would guess that you get a mix of both H2O and OH in the complex. I'm not 100% though.
1
reply
Genocidal
Badges: 16
#3
Report 8 years ago
#3
Books will have conflicting answers, but don't worry because if you look at the mark schemes for the exams they tend to allow like three different colours anyway.
2
reply
fruitloops
Badges: 1
Rep:
?
#4
Report 8 years ago
#4
yeah, look at a mark scheme!!
0
reply
Phalange
Badges: 0
Rep:
?
#5
Report Thread starter 8 years ago
#5
(Original post by Genocidal)
Books will have conflicting answers, but don't worry because if you look at the mark schemes for the exams they tend to allow like three different colours anyway.

(Original post by fruitloops)
yeah, look at a mark scheme!!
Only got two papers as it is a new spec and neither have mentioned this
Thanks though
0
reply
Phalange
Badges: 0
Rep:
?
#6
Report Thread starter 8 years ago
#6
(Original post by DCRichards)
the colour of [Cr(NH3)6]3+ is apparently purple according to this: http://www.creative-chemistry.org.uk...s/N-ch5-07.pdf, which makes sense as the copper ammonia complex is a deep blue, whereas as hexaaquacopper(II) is a lighter blue and so purple would seem a logical colour for it to be.

As for the adding OH-, I guess it would depend on the extent of the ligand substitution. According to this http://en.wikipedia.org/wiki/Chromium , [Cr(OH)3] becomes [Cr(OH)6]3- in alkaline solution and [Cr(H2O)6]3+ in acidic solution, so I would guess that you get a mix of both H2O and OH in the complex. I'm not 100% though.
I think this whole situation is ridiculous... but thanks for looking!
0
reply
DCRichards
Badges: 0
Rep:
?
#7
Report 8 years ago
#7
(Original post by Phalange)
I think this whole situation is ridiculous... but thanks for looking!
No worries, sitting Salters F334 tomorrow so this was some good pseudo-revision. Complexes really are complex though (bad joke intended), like someone else said, use a recent mark scheme as that will give you the exam answer. Do you have to know all the colours and things for the exam, cus that sounds like massive hard work. We have to explain the colour in terms of electrons and d-subshell splitting etc, but it's unlikely we have to say what colour it is, unless it leads on to a question about colour.
0
reply
laurawaldram
Badges: 0
Rep:
?
#8
Report 8 years ago
#8
AQA book says purple!

and it also says excess OH- gives you [Cr(OH)6]3-

hope this helps!
0
reply
Phalange
Badges: 0
Rep:
?
#9
Report Thread starter 8 years ago
#9
(Original post by DCRichards)
No worries, sitting Salters F334 tomorrow so this was some good pseudo-revision. Complexes really are complex though (bad joke intended), like someone else said, use a recent mark scheme as that will give you the exam answer. Do you have to know all the colours and things for the exam, cus that sounds like massive hard work. We have to explain the colour in terms of electrons and d-subshell splitting etc, but it's unlikely we have to say what colour it is, unless it leads on to a question about colour.
We have to know it for Cr, Mn, FeII, FeIII, Ni, Cu and Zn
Hexaaqua versions with OH, NH3 and excess of both. Quite annoying lol...

Yes we have to explain the colour too! Best of luck tomorrow

(Original post by laurawaldram)
AQA book says purple!

and it also says excess OH- gives you [Cr(OH)6]3-

hope this helps!
Thanks, I hope this applies to Edexcel too!
0
reply
DCRichards
Badges: 0
Rep:
?
#10
Report 8 years ago
#10
(Original post by Phalange)
We have to know it for Cr, Mn, FeII, FeIII, Ni, Cu and Zn
Hexaaqua versions with OH, NH3 and excess of both. Quite annoying lol...

Yes we have to explain the colour too! Best of luck tomorrow
Woah what a riot. Thank you, good look for your exam too (I assume this is for an exam)!
0
reply
porkstein
Badges: 9
Rep:
?
#11
Report 8 years ago
#11
(Original post by Phalange)
...
Hexaaquachromium(III) is a kind of violet-blue-grey colour. However, it is very common for chloride or sulfate ions to be present in a solution containing chromium(III), and these can undergo a ligand exchange reaction with the hexaaquachromium(III). As far as I know, multiple ions can be exchanged, but only one is required to give the green colour commonly associated with chromium(III). Chromium(III) might form yellow complexes with other ligands, but I've never come across one. Chromate(VI) is yellow, however.

As for your textbooks: both are correct. Remember that adding excess hydroxide is not a ligand exchange reaction. It's an acid-base reaction. As with most acid-base reactions (and for that matter, ligand exchange reactions also), there is an equilibrium set up. In this instance, there are several interacting equilibria between compounds in which varying numbers of water molecules have been deprotonated, and hence both the tetrahydroxo- and the hexahydroxo- compound will be present.

Hope this helps.
1
reply
Phalange
Badges: 0
Rep:
?
#12
Report Thread starter 8 years ago
#12
(Original post by porkstein)
Hexaaquachromium(III) is a kind of violet-blue-grey colour. However, it is very common for chloride or sulfate ions to be present in a solution containing chromium(III), and these can undergo a ligand exchange reaction with the hexaaquachromium(III). As far as I know, multiple ions can be exchanged, but only one is required to give the green colour commonly associated with chromium(III). Chromium(III) might form yellow complexes with other ligands, but I've never come across one. Chromate(VI) is yellow, however.

As for your textbooks: both are correct. Remember that adding excess hydroxide is not a ligand exchange reaction. It's an acid-base reaction. As with most acid-base reactions (and for that matter, ligand exchange reactions also), there is an equilibrium set up. In this instance, there are several interacting equilibria between compounds in which varying numbers of water molecules have been deprotonated, and hence both the tetrahydroxo- and the hexahydroxo- compound will be present.

Hope this helps.
Thanks! But I've already sat my exam :P
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Sheffield Hallam University
    Get into Teaching in South Yorkshire Undergraduate
    Wed, 26 Feb '20
  • The University of Law
    Solicitor Series: Assessing Trainee Skills – LPC, GDL and MA Law - London Moorgate campus Postgraduate
    Wed, 26 Feb '20
  • University of East Anglia
    PGCE Open day Postgraduate
    Sat, 29 Feb '20

Do you get study leave?

Yes- I like it (50)
63.29%
Yes- I don't like it (2)
2.53%
No- I want it (23)
29.11%
No- I don't want it (4)
5.06%

Watched Threads

View All