John paul III - 10-11-2019 at 07:16
Potassium is more electropositive than Calcium, so in theory if it didn't react with water it could be used to displace Calcium from a solution of
it's salt. But if you write down the reaction as, let's say, Ca(HCOO)2 + 2K -> Ca + 2KHCOO, you'll find that free energy change is positive.
What makes the reaction instead favourable in an aqueous solution? Is it the higher strength of hydrogen bonding of the potassium ion, or the lack of
lattice energy of the salt that otherwise needs to be overcome?
If the lattice energy is the major factor, shouldn't it be possible to displace Calcium with Potassium from any solution as long as the solvent is
unreactive towards both metals?
[Edited on 11-11-2019 by John paul III]
SWIM - 10-11-2019 at 16:26
I don't get how a reaction that results in elemental calcium could be favored in aqueous solution.
hodges - 10-11-2019 at 16:42
I think what John paul III is saying is that, if the potassium did not react with water, a piece of potassium dipped into an aqueous solution of a
calcium salt would cause calcium to replace the potassium. Much as dipping zinc into an aqueous solution of a copper salt causes to copper to replace
the zinc.
CaCl2 + 2K -> 2KCl + Ca
That reaction does occur, but of course in aqueous solution, the calcium is going to immediately react with water, so you can't get calcium in this
way. Maybe this reaction is possible in other solvents besides water though.
DraconicAcid - 10-11-2019 at 18:57
It's not going to happen in water, or a reducable solvent (no alcohols, no acetone, nothing halogenated). It might work in something like THF or
glyme, or a molten salt such as NaCl/KCl eutectic. However, you'd have to use a soluble calcium salt- calcium carbonate is unlikely to work.
vibbzlab - 10-11-2019 at 20:07
That would be interesting to extract calcium from its salt using an element of higher electropositivity.
John paul III - 11-11-2019 at 10:40
I have a lot of calcium acetate, I'll order some THF this week and test the solubilty. This will be easier than trying to find the data with google
If it dissolves ill drop in a piece of potassium metal and see what happens.
Wikipedia says that THF forms complexes with Mg2+ and Li+ so there some hope 
Are there other amateur friendly ways to complex Ca2+ so that it will dissolve in an inert solvent without blocking the reactivity too much?
DraconicAcid - 11-11-2019 at 11:38
I know from experience that calcium acetate forms a gel when you add isopropanol to an aqueous solution of it, so I wouldn't expect it to be
particularly soluble in organic solvents. It might dissolve better when it's anhydrous, but I'd expect the anhydrous chloride to be a better choice.