Half equations for electrode reactions

A half equation represents the reaction at one electrode. It shows the particles that react, the particles produced and the electrons transferred. The side on which the electrons appear shows whether electrons are gained or lost.

Reduction half equation

In reduction, particles gain electrons. Electrons therefore appear on the left-hand side of the half equation, with the reacting particles. For example, hydrogen ions gain electrons to form hydrogen:

\ce{2H+ + 2e- -> H2}

There are two hydrogen atoms on each side. The total charge is also balanced: the two hydrogen ions have a charge of $$$+2$$$ and the two electrons have a charge of $$$-2$$$ , giving zero overall on the left. The hydrogen molecule on the right has no charge.

Oxidation half equation

In oxidation, particles lose electrons. Electrons therefore appear on the right-hand side of the half equation, with the products. For example, hydroxide ions can form oxygen and water:

\ce{4OH- -> O2 + 2H2O + 4e-}

There are four oxygen atoms and four hydrogen atoms on each side. The four hydroxide ions have a total charge of $$$-4$$$ on the left. On the right, oxygen and water have no charge, while the four electrons have a total charge of $$$-4$$$ .

Balancing half equations

A half equation must balance both the number of atoms of each element and the overall charge. Electrons have no atoms to balance, but their negative charge must be included when checking the total charge on each side.

The oxidation half equation can also be written by subtracting the electrons from the left-hand side:

\ce{4OH-} - \ce{4e-} \longrightarrow \ce{O2 + 2H2O}

This is an alternative way of expressing the same electron loss; it does not mean that electrons are gained. The charge is still balanced: subtracting the electrons’ charge of $$$-4$$$ from the hydroxide ions’ charge of $$$-4$$$ gives zero, matching the uncharged products.

Redox at the electrodes in electrolysis

During electrolysis, ions transfer electrons at the electrodes. The two electrodes have opposite charges, and the electron transfer at each one is different.

Cathode: reduction

The cathode is the negative electrode. It attracts positively charged ions, which can gain electrons from the electrode. Gaining electrons is called reduction, so the reaction at the cathode is a reduction.

For example, a copper ion can gain two electrons to form a copper atom:

\ce{Cu^{2+} + 2e- -> Cu}

Anode: oxidation

The anode is the positive electrode. It attracts negatively charged ions, which can lose electrons to the electrode. Losing electrons is called oxidation, so the reaction at the anode is an oxidation.

For example, two chloride ions can each lose an electron, forming a chlorine molecule:

\ce{2Cl- -> Cl2 + 2e-}