Electrolysis of Water - Thermodynamics of The Process

Thermodynamics of The Process

Decomposition of pure water into hydrogen and oxygen at standard temperature and pressure is not favorable in thermodynamic terms.

Anode (oxidation): 2 H₂O(l) → O₂(g) + 4 H+(aq) + 4e− Eo
ox = -1.23 V (Eo
red = 1.23 ))

Cathode (reduction): 2 H+(aq) + 2e− → H₂(g) Eo
red = 0.00 V

Thus, the standard potential of the water electrolysis cell is -1.23 V at 25 °C at pH 0 (H+ = 1.0 M). The potential is changed to -0.82 V at 25 °C at pH 7 (H+ = 1.0×10−7 M) based on the Nernst Equation. However, electrolysis will not generally proceed at these voltages, as the electrical input must provide the full amount of enthalpy of the H₂-O₂ products (286 kJ per mol). This takes the theoretical and real observed threshold of electrolysis to (-)1.48 V.

The negative voltage indicates the Gibbs free energy for electrolysis of water is greater than zero for these reactions. This can be found using the G = -nFE equation from chemical kinetics, where n is the moles of electrons and F is the Faraday constant. The reaction cannot occur without adding necessary energy, usually supplied by an external electrical power source.