6.5 - Half Equations
Combining half equations into a redox equation
A half equation (also called an ion-electron equation) shows one half of a redox reaction: either the substance being oxidised or the substance being reduced.
Na → Na+ + e- (oxidation: electrons lost, written on the right)
2H+ + 2e- → H2 (reduction: electrons gained, written on the left)
Adding two half equations together
- In an overall reaction, the number of electrons lost must equal the number of electrons gained.
- Multiply one or both half equations so they contain the same number of electrons, then add them together so the electrons cancel out completely.
Worked example - Combining two half equations
Potassium reacts with a dilute acid, the potassium being oxidised and the hydrogen ions reduced. Combine the two half equations into one redox equation.
Step 1 - Write the two halves and count the electrons
K → K+ + e-
2H+ + 2e- → H2
The potassium equation has one electron and the hydrogen equation has two, so they won't cancel as they stand.
Step 2 - Multiply so the electron counts match
Doubling everything in the potassium equation gives two electrons on each side:
2K → 2K+ + 2e-
2H+ + 2e- → H2
Step 3 - Add the two equations together
Everything on the left of both arrows goes left; everything on the right goes right:
2K + 2H+ + 2e- → 2K+ + 2e- + H2
Step 4 - Cancel the electrons
They now appear in the same number on both sides:
2K + 2H+ → 2K+ + H2
Check it: two potassiums and two hydrogens on each side, and a total charge of 2+ on each side.
Step 5 - Interpretation
The potassium lost electrons, so it was oxidised, and the hydrogen ions gained them, so they were reduced. Cancelling the electrons is simply the record that every electron the potassium lost was taken by a hydrogen ion. If the electrons won't cancel, step 2 wasn't finished.