Chrome Alum reaction

Hey,

What do you observe when Chrome Alum ( KCr(SO₄ )₂ .12H₂ O ) is reacted with Barium Chloride solution?

I know it will form a white barium sulphate precipitate but will the solution be green or purple?
I have always known Chromium ions to be described as green-violet in solution so what would you choose in a multiple choice?

Thanks

Reply 1

CrazyChemist

Hmmm - you can ignore the .12H₂O...the Barium and Sulphate ions are taken out of solution leaving K⁺, Cr³⁺ and Cl^- in solution. Well Cl^- is very pale green in solution (more or less clear, so negligible), and K⁺ forms a clear solution (i think...) so the only colouring ion in solution is Cr³⁺, which is violet. Ergo the answer is violet (and if K⁺ forms a purple-ish solution, then it just adds to the violet-ness of the solution - handy)

Btw, Cr⁶⁺ is green. Remember that different oxidation states have different colours. Important point.

Reply 2

Hawk

OP

1

CrazyChemist

Hmmm - you can ignore the .12H₂O...the Barium and Sulphate ions are taken out of solution leaving K⁺, Cr³⁺ and Cl^- in solution. Well Cl^- is very pale green in solution (more or less clear, so negligible), and K⁺ forms a clear solution (i think...) so the only colouring ion in solution is Cr³⁺, which is violet. Ergo the answer is violet (and if K⁺ forms a purple-ish solution, then it just adds to the violet-ness of the solution - handy)

Btw, Cr⁶⁺ is green. Remember that different oxidation states have different colours. Important point.

SO out of these choices,
a) A white precipitate in a green solution
b) A white precipitate in a purple soloution

you would say b? Thats what I put, it was CCEA Chem exam today.

Reply 3

Hawk

OP

1

emm are you sure Cr⁶⁺ isn't yellow-orange?

Reply 4

CrazyChemist

Hawk

emm are you sure Cr⁶⁺ isn't yellow-orange?

Ye - i've just been revising a bit more there seem to be contradictions in the textbook. Looks like Cr³⁺ is probably green, but i can't vouch for Cr⁶⁺. I'm afraid the answer looks to be the green solution, but i wouldn't trust me yet...

Reply 5

Hawk

OP

1

Yeh every tiem I look somewhere I get a different answer. I always had it as violet-green in my notes and my teacher was abit stumped as well. He thinks they might end up having to accept both answers, hopefully anyway :smile:

Reply 6

Borek

4

Cr(3+ ) can have different colours, depending on what it is complexed with. If with water it is green, if water is replaced by Cl- it gets violet. Please remember that when Cr3+ is dissolved it is already complexed by water, regardless of the anions present in the solution, and that it can be complexed by both Cl- and SO4(2-) to make chemistry of this solution more complicated.

I am not sure about details, but any inorganic chemistry textbook should have this information. There is always some equilibrium between water complexed and chloride complexed forms, so exact color of the solution will depend on the temperature and reagents concentrations.

Best,
Borek
--

Reply 7

Hawk

OP

1

Borek

Cr(3+ ) can have different colours, depending on what it is complexed with. If with water it is green, if water is replaced by Cl- it gets violet. Please remember that when Cr3+ is dissolved it is already complexed by water, regardless of the anions present in the solution, and that it can be complexed by both Cl- and SO4(2-) to make chemistry of this solution more complicated.

I am not sure about details, but any inorganic chemistry textbook should have this information. There is always some equilibrium between water complexed and chloride complexed forms, so exact color of the solution will depend on the temperature and reagents concentrations.

Best,
Borek

But we weren't given any of them details. All we were told was,
What is observed when Chrome Alum is reacted with a solution of Barium Chloride?
A) Cream precipitate in green solution
B) Cream precipitate in purple solution
C) White precipitate in green solution
D) White precipitate in purple solution

Think it was a badly set question