Metal deposition on passing electricity [closed]

A question asks about the results on electrolysis involving metals, and the answer says that Mg and Al would be deposited on the cathode.

Aren't their reduction potentials lower than that of water? Aluminium has a reduction potential of a bout -1.66, and water's is about -0.8.

How does that work out?

• It should not work, unless it is molten salt, or there is a mercury cathode. Jul 2 '20 at 15:34
• Got it; can you specify why water seems to have two different electrode potentials for oxidation and reduction? www1.lsbu.ac.uk/water/electrolysis.html says it's -0.828 V at the cathode and 1.229 at the anode. Jul 2 '20 at 15:40
• Isn't one usually the negative of the other? (Sorry if this's really dumb.) Jul 2 '20 at 15:44
• Opposite potentials for opposite half reactions. These are not the opposites. Oxidation of H2O to O2 is not the opposite to reduction of H2O to H2. The opposite of reduction of H2O to H2 is oxidation of H2 to H2O. Jul 2 '20 at 15:45
• Got it. Thanks. Jul 2 '20 at 15:51

Mentioned deposition of metallic magnesium or aluminium does not work, unless it is molten salt, or there is a mercury cathode.

Opposite potentials are for the mutually opposite half reactions. Water oxidation and water reduction are not the opposite half reactions. Oxidation of $$\ce{H2O}$$ to $$\ce{O2}$$ is not the opposite to reduction of $$\ce{H2O}$$ to $$\ce{H2}$$. The opposite of reduction of $$\ce{H2O}$$ to $$\ce{H2}$$ is oxidation of $$\ce{H2}$$ to $$\ce{H2O}$$.