Chemistry 3.27 - 3.29 - Electrolysis half-equations

Half-equations and redox

Higher tier only

A half-equation represents the change at one electrode and shows the electrons transferred. It must conserve both the number of each type of atom and the total charge.

In every electrolysis reaction:

• reduction occurs at the cathode because the reacting species gains electrons;
• oxidation occurs at the anode because the reacting species loses electrons.

To write a half-equation:

1. write the reacting ion or species on the left and the product on the right;
2. balance the atoms, remembering that chlorine and bromine form $\ce{Cl2}$ and $\ce{Br2}$;
3. add electrons to balance the total charge;
4. check atoms and charge on both sides.

Balancing molten half-equations

Worked example 1: lead at the cathode

Start with $\ce{Pb^2+ -> Pb}$. The left side has charge $+2$ and the right side has charge $0$. Adding two electrons to the left makes the total charge zero on both sides:

$\ce{Pb^2+(l) + 2e- -> Pb(l)}$

The electrons are on the left, so lead ions gain electrons. This is reduction at the cathode.

Worked example 2: bromine at the anode

Start with $\ce{Br- -> Br2}$. First balance bromine atoms: $\ce{2Br- -> Br2}$. The left side has total charge $-2$, so add two electrons to the right:

$\ce{2Br-(l) -> Br2(g) + 2e-}$

The electrons are on the right, so bromide ions lose electrons. This is oxidation at the anode.

Half-equations in aqueous solutions

These patterns cover the named products: