Electrolysis – Half Equations and Ionic Equations

Half Equations

Electrons are either gained or lost in redox reactions and redox reactions occur during electrolysis.

  • When positively charged metal ions (cations) reach the negative cathode, they gain electrons and form neutral atoms. This process is called reduction.
  • When negatively charged ions (anions) reach the positive anode, they lose electrons and form neutral atoms. This process is called oxidation.

In redox reactions both oxidation and reduction take place at the same time. However, half equations focus on each reaction separately. A half-equation shows either oxidation or reduction on its own, and it is part of a larger reaction. With electrolysis, half equations show what happens at each electrode.

In half equations:

These guidelines should be followed when writing half-equations:

  • The symbol for electrons is e⁻.
  • The number of atoms on both sides of the equation must be equal.
  • The overall charge on each side of the equation must be the same (usually zero).

Writing half equations

1. Write down the ion and the product that will form at the electrode, with an arrow in between them. For example:

Cl → Cl2

2. Use coefficients to balance the atoms on both sides of the equation.

2Cl⁻ → Cl2

3. Use electrons to balance the charges on both sides of the equation.

2Cl → Cl2 + 2e⁻

  • Remember that some non-metal ions form diatomic molecules

Some other examples of half equations are:

Cu²⁺ + 2e⁻ → Cu

2H⁺ + 2e⁻ → H2

Ionic Equations

An ionic equation shows the overall reaction by combining half equations. For example, in the electrolysis of copper chloride, there are two half equations:

At the negative cathode:

Cu²⁺ + 2e⁻ → Cu

At the positive anode:

2Cl → Cl2 + 2e⁻

By combining these two equations, we get:

Cu²⁺ + 2e⁻ + 2Cl⁻ Cu + Cl2 + 2e⁻

As the electrons on both sides cancel, the final ionic equation is:

Cu²⁺ + 2Cl⁻ Cu + Cl2

When writing an ionic equation, it is important to check that the charges and atoms on both sides are balanced. It is also worth noting that in some cases, spectator ions (ions that do not participate in the reaction) can be removed from the equation to simplify it.