March 26, 2017

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Why if you dissolve cobalt chloride solution in distilled water and then pour acetone in it, the bottom part of the graduated cylinder is red and the top part is blue?

What do you mean by it has one color with water of crystallization?

CoCl2*6H2O is a purple solid.

I'm told it has something to do with Le Chatelier's principle and the equilibrium shifting to the right and hydrogen bonding, but I still don't understand.

CoCl2 is one of those salts that have one color with water of crystallization and another when it has no water of crystallization.
CoCl2*6H2O is pink (or red?)
CoCl2 is blue.
I'll let you think that through.

If you add Cobalt(II) chloride x. 6H2O to water, it dissolves to give Chloride ions and CobaltII ions in solution. CobaltII ions are red in water. That makes the bottom red. Now in the top (acetone has less density) the water is squeezed out, and the CoCL2 is blue in that solution.

There is no hydrogen bonding in the acetone, so the water stays in the bottom.

So there is cobalt and water on the bottom and CoCl2 and acetone on the top?

Am I mixed up on my colors? I thought the hydrated form (CoCl2*6H2O) was pink/red and the anhydrous form was blue/purple. At any rate, in the two solvents you have one form in one solvent and the other form in the other solvent. Did you have two layers?

Yeah, I had two layers. No, CoCl2*6H2O is purple. I don't really know what the anydrous stuff is. I'm pretty sure we didn't use that.

The following is a quote from The Merck Index, 12th edition, 1996.

"Cobaltous chloride. Pale-blue hydroscopic leaflets; colorless in very thin layers; turns pink on exposure to moist air."
"Hexahydrate, monoclinic crystals. Structure is reported to be [CoCl2(H2O)4].2H2O. Pink to red, slightly deliquescent, monoclinic prismatic crystals. On heating loses 4H2O at 52-56o forming the dihydrate, violet or blue crystals, stable unless exposed directly to moisture. Loses another H2O by 100o, giving the monohydrate, violet, hydroscopic, amorphorous solid or needles. Remaining water lost at 120-140o. The aq soln is pink to red but turns blue when heated or when HCl or H2SO4 is added."
So it has a bunch of colors, depending upon the amount of water the anhydrous crystal has taken up.

Umm, okay, umm, could you tell me what's on the top and what's on the bottom? Was I right?

In bottom part of the solution Co(II) ion is hydrated, therefore it has pink color. When you add acetone on it some part of Co(II) ions diffuse to acetone, loosing its water of hydration. Therefore color at the top of cylinder is blue.

  • chemistry - ,

    we had a lab session. when I added commercial acetone to cobalt chloride solution the system remained pink. it actually turned to a lighter pink color. why didn't it turned blue as its supposed to?

  • chemistry - ,

    Try adding more acetone... at least the same volume of the original solution or more... keep adding it and eventually you will see the top blue layer

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