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metalresearcher
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Quote: Originally posted by AndersHoveland  | If you intend to reduce a calcium salt, it may be much easier to use the chloride salt.
Calcium oxide has fairly high entropy of formation, due to higher lattice energy, and also not unlikely due to some mild covalent character between
the calcium and oxide ions. What many may not realize here is that reduction potentials can greatly change depending on the anion.
Metallic sodium could easily reduce molten CaCl2. Aluminum certainly would not be able to reduce it. The reactivities of these 3 metallic elements
would probably be the reverse if it was the oxide rather than the chloride.
To give you some idea of the general trends:
6 KOH + 2 Al --> Al2O3 + 6 K + 3 H2
K + NaCl --> KCl + Na
AlCl3 + 3 Na --> 3 NaCl + Al
CaCl2 + 2 Na --> 2 NaCl + Ca
Aluminum is not really as powerful of a reducing agent, it just has a strong affinity for oxygen.
While this may seem inexplicable, it can actually be understood in terms of Lewis "acidity" and "basicity" of various ions. Bare Al3+ ions
are actually quite acidic, whereas O2- ions are basic. So the bonding has a very covalent character.
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Exactly eleven years later ...
Reducing CaCl2 with Na metal is not an easy task as Na is near its boiling point : CaCl2 melts at 772 C and Na boils at 889 C,
so at the melting point of CaCl2 the Na is very volatile.
And reducing K from KCl using (the less un-noble/reactive) Na ?
Reducing K with Al and KOH does work, I have done it last week and with, but that is thanks to the low boiling point of K so vapor can be distilled
off from the (airtight) retort.
The same I did successfully with NaOH + Al lathings.
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clearly_not_atara
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CaCl2 has a mp of 770 C, but a 52/48 mol% NaCl/CaCl2 eutectic has a mp of about 512 C. It may be difficult to stir at these temperatures but I wonder
if you could form cocrystals from a solution of the two salts and then melt these. Alternatively you can melt the salts at higher temperatures and
then cool the mixture down a little before adding Na.
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metalresearcher
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Might be worth a trial.
Melting the salts and keep it at about 550 C and then add a few Na chunks.
However, the problem is that Ca metal floats on the salt solution as it is, despite heavier than Na, but still very light and can thus easily burn in
the ambient air.
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