Electrolysis is a process whereby the passage of an electrical current through a liquid induces a chemical change. The liquid, or electrolyte, may be a pure molten salt, a molten mixture of salts, or a solution of one or more salts in a liquid solvent. Although electrolysis was discovered within months of the invention of the electric battery by the Italian physicist Alessandro Volta in 1800, some twenty years passed before the British chemist Michael Faraday formulated mathematical laws to describe electrolytic processes.
For a liquid to function as an electrolyte, it must contain electrically charged particles, called ions. When electrodes are dipped into an electrolyte, both types of ions are attracted to the oppositely charged electrodes: the positive ions drift toward the negative electrode, while the negative ions drift toward the positive electrode. In this way, both types of ions contribute to the passage of current as they move. When common salt (sodium chloride, NaCl) dissolves in water, for example, sodium ions (Na+) and chloride ions (Cl–) break free from the rigid structure of solid salt and become able to carry current. Molten sodium chloride also conducts electricity.
Electrolysis with inert electrodes
The chemical changes that occur in electrolysis happen when ions reach oppositely charged electrodes. When the electrodes are made of inert materials, such as graphite or platinum, their only function is to provide or accept electrons, depending on the charge of the electrode. In the electrolysis of sodium chloride solution, for example, the anode, or positive electrode, accepts one electron from each chloride ion that reaches it. The neutral chlorine atoms so formed immediately pair up to form chlorine molecules (Cl2):
The cathode reaction in the electrolysis of sodium chloride is slightly more complex. Instead of sodium ions accepting an electron each to form sodium atoms, water molecules accept the extra electrons, forming hydroxide ions (OH–) and molecules of hydrogen gas (H2):
The overall reaction is obtained by combining the two electrode reactions. If sodium ions are added to both sides of the equation, it can be seen that the products of the electrolysis of brine, or sodium chloride solution, are hydrogen and chlorine gases and sodium hydroxide solution. This process was one of the earliest adopted on a large scale by the heavy-chemicals industry, since its products are important raw materials for numerous products, such as bleach and plastics:
The overall reaction is obtained by combining the two electrode reactions. If sodium ions are added to both sides of the equation, it can be seen that the products of the electrolysis of brine, or sodium chloride solution, are hydrogen and chlorine gases and sodium hydroxide solution. This process was one of the earliest adopted on a large scale by the heavy-chemicals industry, since its products are important raw materials for numerous products, such as bleach and plastics:
Hydrogen gas collects over the cathode, while chlorine gas collects over the anode; at the same time, the concentration of sodium chloride gradually diminishes in the electrolyte, while the concentration of sodium hydroxide increases.
