Electrolytic cell half reaction equations?

[Jack Litmus]

Hi everyone, I was just wondering how I would choose which species in an electrolytic cell to use as the cathode and which to use as the anode.

For example, for aqueous aluminum chloride (inert electrodes), H2O (-0.83V) and H2O (+1.23V) will be used as the anode and cathode for the half reactions, but while I think I understand why H2O (-0.83V) is chosen has the anode over Al (everything @ Al and below will not be deposited or something if water is present), I don't understand why H2O(+1.23V) is chosen over Cl2 (+1.36) as the cathode.

Would someone please explain how to pick the anode and cathode species? Thank you!

 

[AGNuke]

Electrolytic cells prefer species at anode with high Standard Reduction Potential at Anode and low SRP at Cathode. But nonetheless, Cl₂ will be liberated at the cathode along side Oxygen.

 

[JohnRC]

Electrolytic cells prefer species at anode with high Standard Reduction Potential at Anode and low SRP at Cathode. But nonetheless, Cl₂ will be liberated at the cathode along side Oxygen.

There are a lot of complicated issues in the chlorine/oxygen competition at an anode. Two of the main considerations are
(1) that an electrolyte is not always at 1 M activity (standard state), and any departure from this affects the potential value (Nernst equation)
(2) that chloride ion is much more hydrophobic than hydroxyl, and will tend to adsorb at the surface of any physical electrode, and therefore have a greater availability to the electrode for electron removal. Or another way of looking at it, a much greater local concentration in the immediate region of the anode surface than in the bulk electrolyte.