10. Chemical Effects of Electric Current

When an electric current is passed through certain liquids (called electrolytes — solutions or molten substances that conduct electricity and undergo chemical decomposition in the process), it brings about chemical changes at the electrodes. This is known as the chemical effect of current, and the underlying process is called electrolysis.

Electrolysis — Basic Principles

In electrolysis, two electrodes (conductors, usually metal or graphite rods) are dipped into an electrolyte and connected to a source of direct current (DC). The electrode connected to the positive terminal of the battery is called the anode, and the electrode connected to the negative terminal is called the cathode. Under the influence of the electric field, the positive ions (cations) in the electrolyte migrate toward the cathode and the negative ions (anions) migrate toward the anode; at each electrode, these ions either gain or lose electrons and are converted into neutral atoms or molecules, which may be deposited on the electrode, released as gas, or otherwise altered.

Common exam-relevant applications and examples of electrolysis include: electroplating, in which a thin, even layer of a more valuable or corrosion-resistant metal (such as chromium, gold, silver, nickel, or zinc) is deposited onto a base metal object by making the object the cathode in an appropriate electrolyte bath — used for gold/silver-plated jewellery, chrome-plated automobile parts, and zinc-plated ('galvanised') iron to prevent rusting; the electrolysis of water, which decomposes water into hydrogen gas at the cathode and oxygen gas at the anode (produced in the ratio of 2:1 by volume, matching the formula H₂O); electrorefining of impure metals such as copper to obtain a highly pure form; and industrial extraction of reactive metals such as aluminium (from molten alumina) and sodium, which are too reactive to be extracted by ordinary chemical reduction and must instead be extracted using electrolysis. The two laws quantifying the amount of substance deposited or liberated during electrolysis were formulated by Michael Faraday and are known as Faraday's laws of electrolysis, distinct from his laws of electromagnetic induction covered later in this chapter.