[vanderwaals]

I think this question has been asked here before, but I still don't exactly understand it... Why is chlorine gas produced at the anode during the electrolysis of HCl? The standard oxidation potential for Cl- is -1.36, and the standard oxidation potential of water to form oxygen gas is -1.23, which is more positive. I thought that the equation with the more positive potential was more likely to occur. Does this depend on the concentration of HCl?

[AWK]

Such a process (electrolysis of HCl) is true for HCl with concentration over ~20 % (in this case the standard potential for Cl^- is useless).
For diluted HCl as you pointed out - electrolysis of water proceeds (you may use the standard potential table).
( H₂O + e^- => H₂ + OH^-; H₂O => O₂ + H₃O⁺ + e- ) ---- unbalanced)

[vanderwaals]

That makes a lot more sense. Thanks

[Borek]

The reason behind is kinetics. Standard potential tables deal with equilibrium potentials, they don't care about how fast the reaction is. It happens that most reactions involving oxygen are quite slow. That means, to make them reasonably fast you need to apply substantial overvoltage. That in turn means other systems present in the solution can react before the oxygen, even if that goes against the standard potential table.