Electrolysis when multiple ions present

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jd12345
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What are the products of electrolysis of brine solution?
At cathode i know it will be hydrogen

But i am confused about the products at anode:-
Either water will get oxidised or chlorine. Considering the oxidation potential values - water should get converted to oxygen but practically chlorine is obtained. My book says it is because of overpotential of oxygen( what does overpotential means?)

Also what has concentration related to the products obtained - example :- when electrolysis of sulphuric acid is done - water is oxidised to oxygen if solution is dilute and SO4 2- is oxidised to S2O82- if it is concentrated. Why?
Even if SO4 2- is available easily then also the most favoured reaction should proceed right? - which is oxidation of water
 

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Potential of the reaction depends on the concentrations (see Nernst equation).

In concentrated solutions transport of the substance can be disturbed, which means there is not enough substance near the electrode, then the first thing present will get reduced/oxidized.

If the electrode reaction is slow, it doesn't go as expected on the electrode, even at its own redox potential. To speed it up we need to use higher potential - this is called overpotential. Trick is, if there is a competing fast reaction it may occur at some intermediate potentials before our main reaction gains speed. That's the case with water electrolysis, production of oxygen is notoriously slow.
 
thank you - this site and people here are awesome...why didnt i find it earlier?