Electrolysis is the breaking of chemical bonds in compounds to produce elemental substances. The process involves passing electric current at specific voltage between two electrodes, through the compound (electrolyte) to be decomposed. The electrolytes are composed of positively charged (cations) and negatively charged anions). Positively charged anode attracts anions. Anions, on reaching anode, release, their electrons to it and get reduced to neutral atoms. Negatively charge cations, attract cations and discharge their positive charge to neutral atoms.
The voltage required for the process depends on the ability of the ions to receive or release electrons. This ability of several elements, molecules, and ions has been measured in aqueous solutions of them and available as a standard reduction potentials. Standard reduction potential is the potential (ability) in volts, of receiving electrons, at standard conditions of 1molar aqueous solution of the electrolyte, measured at 298K and one atmospheric pressure.
For example, the ability of cupric ions to take two electrons to become neutral copper atom is, given by a standard reduction potential of 0.34V
Cu2+ (aq) + 2e– → Cu (s) E° = 0.34 V
Oxidizing ability (losing electrons) of the substance is opposite of the reduction process. So, the oxidizing potential of a substance is generally of same magnitude but opposite in sign to the reduction potential. Oxidation potential of copper atom to cupric ion is -0.34V.
Higher the reduction potential, higher will be feasibility of reduction in aqueous solution. Similarly, higher the oxidation potential (smaller the reduction potential), higher the probability to get oxidized.
Electrolysis is a process involving reduction at cathode and oxidation at anode. So, feasibility of electrolysis depends upon the individual redox potentials of the electrolyte ions.
Moreover, in aqueous solutions, water also gets involved. Water can be, oxidized and reduced. Unlike other ions and their elemental atoms, redox potential is different. Redox reaction of water, also need a higher voltage than predicted by the thermodynamic values.