[Vert1go]

This is a reaction I've wanted to set up for a while now, and I just want to check if there are any steps I missed or things I should change in this method that might make it not work.
Target reaction: [Co(H₂O)₆]²⁺(aq)+ 4Cl^-(aq) ⇌ [CoCl₄]^2-(aq) + 6H₂O(l)
Proposed Method:
1. Add CoO to HCl until there is solid CoO at the bottom. CoO + 2HCl → CoCl₂ + H₂O
2. Filter out the excess CoO, then leave the solution for the water to evaporate, leaving crystals of CoCl₂.
3. Add concentrated HCl to the crystals, leaving CoCl₄ in solution. CoCl₂ + 2HCl → CoCl₄ + H₂.
4. This should leave [CoCl₄]^2- and water which should react giving the target reaction. A mixture that will change colour when heated or cooled due to the different colours of the cobalt species.
Will this method work? If not, why? Please let me know any steps I need to do that I didn't mention.

[Hunter2]

No. 3 is wrong.
It should be CoCl2 + 2 HCl => 2 H+ +  [CoCl4]2-
There will be no hydrogen development.

You will have the chlorido complex directly after dissolving CoO in HCl., because you need HCl in excess to dissolve.
CoO + 4HCl  => 2H+ + [CoCl4]2- + H2O

[Borek]

This should leave [CoCl₄]^2- and water which should react giving the target reaction. A mixture that will change colour when heated or cooled due to the different colours of the cobalt species.

I know you can shift the equilibrium of a blue CoCl₄^2- solution by dilution, are you sure it will change color also because of the temperature changes?

Dry cobalt chloride changes color because of the hydration changes and is sensitive to humidity/temperature, but it won't work the same way in water.

[Vert1go]

This should leave [CoCl₄]^2- and water which should react giving the target reaction. A mixture that will change colour when heated or cooled due to the different colours of the cobalt species.

I know you can shift the equilibrium of a blue CoCl₄^2- solution by dilution, are you sure it will change color also because of the temperature changes?

Dry cobalt chloride changes color because of the hydration changes and is sensitive to humidity/temperature, but it won't work the same way in water.

https://www.youtube.com/watch?v=WFdjJ_aS234
This is a video of the reaction working, but I am still confused about that part you mentioned.. I'll let you know if I find an explanation for it.

[Borek]

Nice, I didn't know

I mean - nothing surprising about the equilibrium shift for the exo- or endothermic reaction when the temperature changes, that's basic thermodynamics, but it is rarely so pronounced and so nicely visible.