Half-equation

A half-equation is an ionic equation that shows either the loss of electrons (oxidation) or the gain of electrons (reduction) at one electrode, or for one species, on its own. Malay: setengah persamaan · Chinese: 半反应式.

Because a redox reaction has two linked changes, it is often clearer to write each one separately as a half-equation and then combine them. A correct half-equation must balance for atoms and for charge, and the electrons appear explicitly. In an oxidation half-equation the electrons are written on the right, because they are lost; in a reduction half-equation they are written on the left, because they are gained. When you add the two halves, the electrons lost must equal the electrons gained, so you sometimes multiply one half-equation through before combining them.

Example. For the electrolysis of molten lead(II) bromide, the cathode reaction is Pb²⁺ plus 2e- to Pb (reduction) and the anode reaction is 2Br⁻ to Br₂ plus 2e- (oxidation). Each half-equation balances in atoms and charge, and both involve two electrons, so they combine directly into the overall change Pb²⁺ plus 2Br⁻ to Pb plus Br₂.

Confusion to avoid. Do not leave the electrons out, and do not put them on the wrong side; that single error reverses the meaning from oxidation to reduction. Also keep half-equations ionic, write the actual ions and atoms taking part, not full formula units, and make sure the total charge is the same on both sides once the electrons are included.

Combining the halves. To build the overall ionic equation, first make the number of electrons equal in both halves by multiplying, then add them so the electrons cancel out. For example, combining Fe²⁺ to Fe³⁺ plus e- with Cl₂ plus 2e- to 2Cl⁻ needs the iron half doubled, giving 2Fe²⁺ plus Cl₂ to 2Fe³⁺ plus 2Cl⁻. Practising this cancelling step is what makes writing full redox equations reliable under exam pressure.

Half-equations are the tool you use to describe reactions at the anode and cathode in electrolysis, in voltaic cells and in rusting, so mastering them underpins much of this chapter and they are marked strictly in SPM Chemistry.