HCl

is it possible to use electrolysis to synthesise HCl from H2O and NaCl?

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No.

The Cl would evolve as a gas and the Na would recombine with the H2O to give Na(OH); NH4Cl + heat > NH3 + HCl; the problem is that both are gasses and recombine upon cooling to give a white powdery smoke. At one time electric trains had a heating element in the smoke stack and NH4Cl pills were place there to give clouds of NH4Cl by just such a reaction.

You can make HCl by mixing H2SO4 (sufuric acid/oil of vitriol) with CaCl2 or SrCl2 and filtring out the CaSO4 or SrSO4, but its cheaper and easier to just by it.

So how does electrolysis work with compounds and dissolved substances?

Electrolysis of CuSO4 or Ni salts works because pure Cu or Ni is not terribly reactive in water or even acid; because Al is so reactive, Davy was unable to even use molten Al salts to isolate the metal and the early procedures were quite complicated and expensive; then C. M. Hall in his garage lab hit upon dissolving Al2O3 in molten cryolite, sodium aluminum fluoride, with which Al doesn't react. The products are Al metal at the bottom of the vat and pure O2 fizzing off in clouds of pearly white steam-- I have a warm spot in my heart for Alcoa because they were the only mill in my childhood that didn't belch out clouds of red or black smoke.

Ionic stuff get electrolysed when in aqueous or liquid form.

Ionic compounds + Water electrolysis (use mercury as a cathode)---> Hydrogen (at cathode) + Oxygen and Original anion (at anode) + Metal (original cation) (inside cathode in form of amalgam)

eg.
2NaCl + 2H[sub]2[/sub]O electrolysis---> 2Na (amalgam) + 2H[sub]2[/sub] (gas) + O[sub]2[/sub] (gas) + Cl[sub]2[/sub] (gas)

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