Electrolysis

Spontaneous redox reactions in voltaic cells convert chemical energy into electrical energy. Alternatively, external electrical energy drives nonspontaneous redox reactions in a process called electrolysis, taking place in an electrolytic cell.

An electrolytic cell has an anode for oxidation and a cathode for reduction. However, unlike the voltaic cell, where the negatively charged anode is the source of electrons, the electrolytic cell has an external source of electrons like a battery.

The battery draws electrons from the anode connected to its positive terminal and drives electrons towards the cathode, which is connected to its negative terminal. Thus electrons flow from the positive to the negative terminal of the external power source.

Electrolysis is used to separate ionic compounds into their constituent elements. In their molten or aqueous state, the constituent ions are free to conduct electricity.

Consider potassium bromide that melts at a temperature of 734 °C. Here, bromide and potassium ions are present at their lowest or highest oxidation state, respectively. Under power, the bromide ions are oxidized to bromine gas at the anode, and potassium ions are reduced to solid potassium at the cathode.

In molten ionic mixtures like sodium iodide and potassium bromide, standard electrode potentials are used to predict which substance will react. Generally, anions with a lower electrode potential are oxidized, and cations with a more positive electrode potential are reduced.

Interestingly, electrolysis of aqueous potassium bromide produces different products from molten salts due to the possible electrolysis of water.

Thus, two reactions could occur at each electrode: the oxidation of bromide ions or water at the anode and the reduction of potassium ions or water at the cathode.

Given the lower electrode potential, oxidation of bromide ions to bromine is favored, while the more positive electrode potential of water drives the reduction to hydrogen gas.

Stoichiometry of the half-reactions can be used to calculate the amount of substances produced or consumed during electrolysis.

For example, electrolysis of molten potassium bromide uses one electron to form a potassium atom. Thus, every mole of electrons, or 96,485 coulombs of charge, passed through the cell produces one mole or nearly 39 grams of solid potassium.