I need help with Preferential Discharge Theory

[dihan8917]

Like the title says, the theory confuses me. All the online resources I've looked through simply gave me the definition of the theory and factors affecting which ions discharge at each electrode during electrolysis. My question is, why does only one type of cation and one type of anion get discharged at all? I've been trying to think of this with the classic example, aqueous NaCl solution with inert electrodes. I have Na⁺ and H⁺ cations in solution, and PD theory states that only H⁺ ions get discharged at the cathode. What I don't get is, Na⁺ ions collide with the cathode as well, right? Why have I been told that it doesn't get reduced at all? Now, my hypothesis is that the Na⁺ ions do get reduced, but the Na atoms that form almost immediately collide with and get oxidized by H⁺ ions, which then get reduced and escape as H₂ gas. Am I correct?

As for the anode, I've been taught that the anion that discharges is based on the concentration, because OH^- and Cl^- are so close to each other in the discharge series. If, for instance, Na⁺ ions were to sufficiently outnumber the H⁺ ions, would Na⁺ similarly be preferentially discharged? I just want to confirm that concentration of ions is always a factor.

As for my Chemistry knowledge, I'm a Cambridge IGCSE student, which is the equivalent of a high school student in America (I don't know which year, exactly). Help would be very appreciated.