Reading the patent (thanks for that!) reinforces my notion that he patented an untested idea: I can't see the reduction of NaCl to be possible with
either lead, iron, magnesium or aluminium: the HoFs of the resulting chlorides are just too low. At high temperatures, entropy might push the
equilibrium somewhat toward forming sodium gas. In the case of Al, AlCl3 is highly volatile and a mixture of sodium gas and AlCl3 vapour would upon
cooling simply revert. So, heating any of these metals with NaCl at whatever temperature will not yield the metal, unless you manage to distil it off
at high temperature.
In fact, the first semi-industrial production of Al was by Woehler:
AlCl3 + 3 K ---> Al + 3 KCl with a highly negative ΔH ≈ - 605 kJ/mol Al
The HoF of NaCl is very close to that of KCl, so sodium metal could also be used to reduce AlCl3 to Al metal - ΔH ≈ - 527 kJ/mol Al (and
has been, IIRW). That's how it was made before Hall, apparently.
That is basically why I looked at fluorides: because of the higher HoF and thus generally smaller ΔH. |