Example 2.10

Example . Products of electrolysis depend on the nature of material being electrolysed and the type of electrodes being used. If the electrode is inert (e.g., platinum or gold), it does not participate in the chemical reaction and acts only as source or sink for electrons. On the other hand, if the electrode is reactive, it participates in the electrode reaction. Thus, the products of electrolysis may be different for reactive and inert . . Products of Electrolysis electrodes.The products of electrolysis depend on the different oxidising and reducing species present in the electrolytic cell and their standard electrode potentials. Moreover, some of the electrochemical processes although feasible, are so slow kinetically that at lower voltages these do not seem to take place and extra potential (called overpotential ) has to be applied, which makes such process more difficult to occur. For example, if we use molten NaCl, the products of electrolysis are sodium metal and Cl gas. Here we have only one cation (Na + ) which is reduced at the cathode (Na + + e – ® Na) and one anion (Cl – ) which is oxidised at the anode (Cl – ® ½Cl + e – ) . During the electrolysis of aqueous sodium chloride solution, the products are NaOH, Cl and H . In this case besides Na + and Cl – ions we also have H + and OH – ions along with the solvent molecules, H O. At the cathode there is competition between the following reduction reactions: Na + (aq) + e – ® Na (s) = – . V H + (aq) + e – ® ½ H (g) = . V The reaction with higher value of E o is preferred and therefore, the reaction at the cathode during electrolysis is: H + (aq) + e – ® ½ H (g) ( . ) but H + (aq) is produced by the dissociation of H O, i.e., H O ( l ) ® H + (aq) + OH – (aq) ( . ) Therefore, the net