Electrolysis: Encyclopedia II - Electrolysis - Overview

Electrolysis - Overview

An ionic compound is dissolved with an appropriate solvent, or otherwise melted by heat, so that its ions are available in the liquid. An electrical current is applied between a pair of metal electrodes immersed in the liquid. The negatively charged electrode is called the cathode, and the positively charged one the anode. Each electrode attracts ions which are of the opposite charge. Therefore, positively charged ions (called cations) move towards the cathode, while negatively charged ions (termed anions) move toward the anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the probes, electrons are absorbed or released by the ions, forming a collection of the desired element or compound. For example, when water is electrolyzed, hydrogen gas (H₂) bubbles at the cathode, and oxygen gas (O₂) rises at the anode. This effect was first discovered by William Nicholson, an English chemist, in 1800.

The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. It is thus impossible to create a perpetual motion machine by combining the two processes. See water fuel cell for an example of such an attempt.)

The following technologies are related to electrolysis:

• Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process.
• Gel electrophoresis is an electrolysis where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.

Adapted from the Wikipedia article "Overview", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki