Halfreactions

Halfreactions, commonly written as half-reactions, are one half of a redox reaction in which electrons are either produced or consumed. In a redox couple, one species is oxidized (loses electrons) and another is reduced (gains electrons); separating these processes yields the oxidation and reduction half-reactions. Each half-reaction shows the species as it exists on one side of the electron transfer and explicitly includes electrons to balance charge.

To balance a redox equation, chemists balance each half-reaction separately and then combine them to cancel electrons. In acidic solution, oxygen atoms are balanced with water, hydrogen with H+, and the remaining charge is balanced with electrons. In basic solution, hydrogen ions are replaced by water and hydroxide to maintain charge and mass balance. The two half-reactions are added together so that the electrons cancel, giving the overall redox equation.

Common half-reactions include reduction of copper(II) ions: Cu2+ + 2 e- -> Cu(s); and oxidation of zinc metal: Zn(s) -> Zn2+ + 2 e-. When combined, Zn(s) + Cu2+ -> Zn2+ + Cu(s). Half-reactions underpin understanding of galvanic and electrolytic cells, standard electrode potentials, and redox balancing via the Nernst equation.

See also: standard reduction potential, electrochemistry, redox reaction.