Construct and combine oxidation and reduction half-equations to balance redox reactions.

What this dot point is asking

TASC expects you to construct balanced half-equations in acidic solution and combine them into a balanced full ionic equation, including disproportionation reactions.

Why half-equations help

A redox reaction is electron transfer: one species is oxidised (loses electrons) and another is reduced (gains electrons). Splitting the reaction into two halves lets you track the electrons explicitly and balance even complicated reactions reliably. It also makes the link to electrochemical cells direct, since each electrode reaction is a half-equation.

Balancing a half-equation in acidic solution

Use this order for each half-equation:

1. Balance the atoms being oxidised or reduced.
2. Balance oxygen by adding $\text{H}_2\text{O}$.
3. Balance hydrogen by adding $\text{H}^+$.
4. Balance charge by adding electrons $e^-$.

Electrons appear on the left of a reduction half-equation (gained) and on the right of an oxidation half-equation (lost).

Checking your answer

A correctly balanced redox equation has the same number of each atom on both sides and the same total charge on both sides. Always verify the charge balance; it is the step most often skipped. For a disproportionation, the same element appears in both halves, so check the atom totals of that element carefully.

Balancing in basic solution

Many TASC redox reactions occur in acidic solution, but some are in basic conditions. The simplest approach is to balance the equation first as if it were acidic (using $\text{H}^+$ and $\text{H}_2\text{O}$), then neutralise the $\text{H}^+$ by adding the same number of $\text{OH}^-$ to both sides. The $\text{H}^+$ and $\text{OH}^-$ on one side combine to form water, and you cancel any water that then appears on both sides. The result is a half-equation containing $\text{OH}^-$ and $\text{H}_2\text{O}$ but no $\text{H}^+$, appropriate for a basic medium.

Linking half-equations to cells

Each balanced half-equation is exactly the reaction that occurs at one electrode of an electrochemical cell: the oxidation half is the anode reaction and the reduction half is the cathode reaction. The number of electrons in the half-equation is the same number used in Faraday's law calculations and in finding standard cell potentials. Building the skill of writing clean half-equations therefore pays off directly in the electrochemistry and electrolysis questions later in Unit 3.

In the exam, write each half-equation clearly, follow the atom-then-charge order, scale for equal electrons, cancel them on combining, and finish by checking both atom and charge balance.