In aqueous solutions, there is not only the possibility of the oxidation and reduction of the ions of the solute, there is also the possible oxidation and reduction of the solvent, water. There are usually two anode reactions possible and two cathode reactions possible. In the electrolysis of dilute NaCl, hydrogen gas is produced at the cathode and oxygen gas at the anode (these electrode reactions occur in almost all dilute salt solutions, not only NaCl). In concentrated NaCl, chlorine gas is produced at the anode and hydrogen gas at the cathode.

The possibility of redox occuring on the water instead of the actual electrolytes at either (or both) the anode and cathode is definitely something to keep in the back of your mind on the exam. The reduction with the highest reduction potential is the one that will occur at the cathode, given the concentration of reagents.

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