Electrolysis with H2 gas at anode!

[woelen]

I did an experiment in which I electroysed a plain table salt (NaCl) solution with different metal electrodes. Among the experiments there was an experiment with magnesium ribbon electrodes (both anode and cathode).

 

When a sufficiently high voltage is applied, then a gas is evolved at the cathode (hydrogen gas) and the anode dissolves slowly and falls apart (formation of MgCl2). But, I also get a gas at the anode (approximately 1/3 of the amount at the cathode) and the really funny thing is, this gas is hydrogen!

 

I collected gas from both electrodes and both can be confirmed to be hydrogen gas (they burn with kind of whoosh sound).

 

The result of this experiment really startles me. I never expected the formation of H2-gas at the anode. I simply expected to dissolve the magnesium metal to form some MgCl2.

[jsatan]

lol. Nice find,

I've not a clue.

[insane_alien]

maybe some magnesium hydride is formed in a side reaction. this would form hydrogen gas at the anode

[jsatan]

maybe some magnesium hydride is formed in a side reaction. this would form hydrogen gas at the anode

I was thinking that, but wouldnt changing the voltage not change the rate of this?

[jdurg]

There's a very good chance that the resistance to the flow of electricity has heated up the surface of your magnesium ribbon. Mg WILL react with water to form hydrogen gas at elevated temperatures. With the electrolysis going on, there's a very good possibility that any oxide coating on the Mg was removed causing it to react much quicker than expected. I'm willing to bet that the hydrogen you got was a result of the Mg reacting with the water, or some acids that were forming where the Cl was coming out.

[woelen]

There's a very good chance that the resistance to the flow of electricity has heated up the surface of your magnesium ribbon. Mg WILL react with water to form hydrogen gas at elevated temperatures. With the electrolysis going on, there's a very good possibility that any oxide coating on the Mg was removed causing it to react much quicker than expected. I'm willing to bet that the hydrogen you got was a result of the Mg reacting with the water, or some acids that were forming where the Cl was coming out.

If the power supply is disconnected, then the production of gas stops immediately, both at the anode and at the cathode. If the explanation were formation of H2, due to some acidic compounds formed, then the production of H2 would continue for at least several seconds or minutes, because this acidic stuff would be spread around throughout the liquid. In fact, I believe that the liquid is more alkaline instead of acidic, because of the fact that flocculent Mg(OH)2 is formed as well (not immediately, but after a few minutes, when hydroxide formed at the cathode has moved towards the anode region as well).

 

A similar problem I have with the heat formation. If heat is the cause of formation of H2, then I also would expect the formation of H2 to continue for a while after disconnecting the power supply. Any heat, produced, will take some time before it has spread sufficiently through the liquid.