Electrolysis of Aqueous Solutions

Electrolysis Basics

Electrolysis is the process of using an electric current to break down a compound into its elements or simpler compounds. This happens when an electric current passes through a substance that conducts electricity, called an electrolyte.

The setup includes two electrodes: the cathode (negative electrode) and the anode (positive electrode). When the current flows, ions in the electrolyte move towards the electrodes. Positive ions (cations) move to the cathode to gain electrons, and negative ions (anions) move to the anode to lose electrons.

This movement of ions and electron transfer causes chemical changes at the electrodes, producing new substances.

Electrolysis of Aqueous Solutions

When the electrolyte is an aqueous solution (a solution where water is the solvent), the presence of water affects the products formed during electrolysis. Water itself can ionise slightly into H⁺ and OH⁻ ions, which compete with the ions from the dissolved compound at the electrodes.

Because of this competition, the products formed depend on:

• The ions present in the solution
• The relative reactivity of these ions

More reactive ions are less likely to be discharged (gain or lose electrons) at the electrodes. Instead, less reactive ions or water molecules are discharged.

Products at Electrodes

At the cathode (negative electrode):

• If the solution contains metal ions less reactive than hydrogen (e.g., copper, silver), the metal is deposited at the cathode.
• If the metal ions are more reactive than hydrogen (e.g., sodium, potassium, calcium, magnesium, aluminium), hydrogen gas is produced instead.

At the anode (positive electrode):

• If the solution contains halide ions (chloride, bromide, iodide), the corresponding halogen gas is produced.
• If no halide ions are present, oxygen gas is produced from the hydroxide ions in water.

The reactivity series helps predict which ions will be discharged. For example, hydrogen is less reactive than sodium, so hydrogen gas is produced instead of sodium metal.

For instance, in aqueous copper(II) sulfate solution, copper ions (Cu²⁺) are less reactive than hydrogen ions (H⁺), so copper metal deposits at the cathode, and oxygen is produced at the anode from water.

Half Equations

Half equations show the reactions occurring at each electrode during electrolysis. They include the transfer of electrons and must be balanced for both atoms and charge.

At the cathode, reduction occurs (gain of electrons). At the anode, oxidation occurs (loss of electrons).

Examples of half equations:

• Hydrogen ion reduced at cathode:
  $$2H^+ + 2e^- \rightarrow H_2$$
• Metal ion reduced at cathode (e.g., copper):
  $$Cu^{2+} + 2e^- \rightarrow Cu$$
• Hydroxide ion oxidised at anode:
  $$4OH^- \rightarrow O_2 + 2H_2O + 4e^-$$
• Halide ion oxidised at anode (e.g., chloride):
  $$2Cl^- \rightarrow Cl_2 + 2e^-$$

Balancing charges and atoms in half equations is essential to correctly represent the electrode reactions.

For example, when water is oxidised at the anode, four hydroxide ions lose four electrons to form one molecule of oxygen gas and two molecules of water.

Understanding half equations helps predict and explain the products of electrolysis in aqueous solutions.

For example, in the electrolysis of aqueous sodium chloride solution (brine), chlorine gas is produced at the anode, hydrogen gas at the cathode, and sodium hydroxide remains in solution. (See Electrolysis of Brine notes for more detail.)

For aqueous copper sulfate:

At the cathode, copper ions gain electrons and deposit as copper metal:

$$Cu^{2+} + 2e^- \rightarrow Cu$$

At the anode, hydroxide ions lose electrons to form oxygen gas and water:

$$4OH^- \rightarrow O_2 + 2H_2O + 4e^-$$

For aqueous sodium sulfate (no halide ions):

At the cathode, hydrogen ions are reduced to hydrogen gas:

$$2H^+ + 2e^- \rightarrow H_2$$

At the anode, hydroxide ions are oxidised to oxygen gas:

$$4OH^- \rightarrow O_2 + 2H_2O + 4e^-$$

For aqueous potassium bromide:

At the cathode, hydrogen gas is produced (potassium is more reactive than hydrogen):

$$2H^+ + 2e^- \rightarrow H_2$$

At the anode, bromide ions are oxidised to bromine gas:

$$2Br^- \rightarrow Br_2 + 2e^-$$

For aqueous copper chloride:

At the cathode, copper metal deposits:

$$Cu^{2+} + 2e^- \rightarrow Cu$$

At the anode, chloride ions are oxidised to chlorine gas:

$$2Cl^- \rightarrow Cl_2 + 2e^-$$

These examples show how the products depend on the ions present and their reactivity.

For instance, in aqueous sodium sulfate, hydrogen and oxygen gases are produced instead of sodium metal or sulfate gas because water ions are discharged instead.

This is because sodium is more reactive than hydrogen, so hydrogen ions are discharged at the cathode, and sulfate ions are less easily discharged than hydroxide ions, so oxygen is formed at the anode.

For example, if you electrolyse aqueous copper sulfate, copper metal forms at the cathode and oxygen gas forms at the anode.

An example: Electrolysis of aqueous copper sulfate produces copper metal at the cathode and oxygen gas at the anode.