Cambridge O Level Chemistry · Syllabus 5070 · Electrochemistry
Ionic half-equation
What is Ionic half-equation?
An equation showing the change at one electrode only, written with the electrons transferred included as \(e^-\). Electrons appear on the left when a species gains them and is reduced, and on the right when a species loses them and is oxidised; a correct half-equation balances both the atoms of each element and the total electrical charge on the two sides.
This definition is part of the Electrochemistry chapter in Cambridge O Level Chemistry.
Questions students ask about Ionic half-equation
How do you write an ionic half-equation for an electrode?
Write the ion on the left and the product on the right, then add the electrons on the correct side: on the left when the ion gains them (reduction at the cathode, for example \(\mathrm{Pb^{2+}} + 2e^- \rightarrow \mathrm{Pb}\)) and on the right when the ion loses them (oxidation at the anode, for example \(2\mathrm{Br^-} \rightarrow \mathrm{Br_2} + 2e^-\)). Then make two checks: count the atoms of each element on both sides, and add up the total charge on each side counting each \(e^-\) as \(-1\). Both must match.