Electrolysis of Liquid Zinc Chloride

This demonstration involves significant hazards and should not be replicated.  

Zinc chloride melts slightly under 300 °C.  Here this is achieved using a meker burner and a hot plate.  The molten zinc chloride boils and produces smoke that is small clumps of the solid.  Graphite electrodes are used with two 9V batteries in series to conduct an electrolysis reaction where zinc metal is produced at the cathode and chlorine gas at the anode.  The chlorine gas causes slight bubbling to occur and the zinc metal produced is confirmed by dipping the cathode into 6M hydrochloric acid where hydrogen gas is produced and the bubbling is evident.  In order to simplify the electrolysis reaction, it was helpful to shift the crucible with the molten salt to a hot plate where placement of the electrodes while heating continued allowed for the reaction to proceed without the liquid zinc chloride freezing.  This allowed the electrolysis to continue for any length of time as long as care was taken to avoid having the wiring substantially isolated from the hot plate to avoid damage to the plastic insulation.  Using the meker burner required the flame to be shut off to perform the reaction and this allowed only a minute or so before the salt began freezing especially at the site of the electrodes.  

This demonstration is great to see because most of the electrolysis reactions we see in education focus on aqueous salt solutions.  Meanwhile many industrial productions of metal, especially reactive metals, involve electrolysis in the molten or liquid state.  The aqueous reaction can be more difficult to understand because there is then a competition for the water itself to undergo oxidation or reduction.  This is confusing for students to understand especially without having had physics and a strong understanding of the complex issues that go with electricity and magnetism.  With a molten salt, there is no water to be oxidized or reduced so as long as the battery supplies a sufficient voltage to push the electrons with enough force to make the reaction successful.  In this particular demonstration, 18 volts is used to exceed the amount of force needed by a lot in the event that the conductivity of the ionic liquid is poor and this resistance impacts the voltage at the electrodes where the reaction is occurring.  

This demonstration does also show aqueous zinc chloride undergoing electrolysis.  In this reaction zinc metal is formed, but it is not clear whether the gas produced is chlorine, oxygen or a mixture of the two.  

2H2O(l) → O2(g) + 4H+(aq) + 4e- E° = -1.23 V

2Cl-(aq) → Cl2(g) + 2e- E° = -1.36 V

The physical interpretation of these two half reactions is that it is easier to remove electrons from water than from chloride ions by a small margin.  These half reactions assume standard conditions including 1M solutions which means that the chloride reaction could become more favorable as the concentration of chloride ions increases.  Neither product was tested for and I believe a flame test for oxygen would be insufficient as chlorine gas might support a similar combustion reaction and you may also have a mixture of the two gases.  Testing for chlorine would have to occur.