<Tr> <Td> Cathode (reduction): </Td> <Td> 2 H (aq) + 2e </Td> <Td> → </Td> <Td> H (g) </Td> <Td> </Td> <Td> E = 0.00 V </Td> </Tr> <P> Thus, the standard potential of the water electrolysis cell (E = E − E) is − 1.23 V at 25 ° C at pH 0 ((H) = 1.0 M). At 25 ° C with pH 7 ((H) = 1.0 × 10 M), the potential is unchanged based on the Nernst equation . The thermodynamic standard cell potential can be obtained from standard - state free energy calculations to find ΔG ° and then using the equation: ΔG ° = - nFE ° (where E ° is the cell potential). In practice when an electrochemical cell is "driven" toward completion by applying reasonable potential, it is kinetically controlled . Therefore, activation energy, ion mobility (diffusion) and concentration, wire resistance, surface hindrance including bubble formation (causes electrode area blockage), and entropy, require a greater applied potential to overcome these factors . The amount of increase in potential required is termed the overpotential . </P> <P> If the above described processes occur in pure water, H cations will be consumed / reduced at the cathode and OH anions will consumed / oxidised at the anode . This can be verified by adding a pH indicator to the water: the water near the cathode is basic while the water near the anode is acidic . The negative hydroxide ions that approach the anode mostly combine with the positive hydronium ions (H O) to form water . The positive hydronium ions that approach the cathode mostly combine with negative hydroxide ions to form water . Relatively few hydronium / hydroxide ions reach the cathode / anode . This can cause a concentration overpotential at both electrodes . </P> <P> Pure water is a fairly good insulator since it has a low autoionization, K = 1.0 × 10 at room temperature and thus pure water conducts current poorly, 0.055 μS cm . Unless a very large potential is applied to cause an increase in the autoionization of water the electrolysis of pure water proceeds very slowly limited by the overall conductivity . </P>

With the help of an activity explain decomposition of h2o