Electrolysis Explained: What Happens at Each Electrode

Electrolysis is splitting an ionic compound into its elements using electricity. The compound must be melted or dissolved so its ions can move. When a current flows, positive ions travel to the negative electrode (the cathode) and gain electrons, while negative ions travel to the positive electrode (the anode) and lose electrons. Electrolysing copper chloride solution, for example, coats the cathode with copper and gives off chlorine gas at the anode.

What do you need for electrolysis?

An electrolysis cell has three parts:

• An electrolyte: an ionic compound that is molten or dissolved in water, so its ions are free to move. A solid won't work, because its ions are locked in a lattice (see ionic vs covalent bonds).
• Two electrodes: rods that carry the current into the electrolyte. They are usually inert (graphite or platinum), so they don't react themselves.
• A DC power supply: it pushes electrons onto one electrode, making it negative, and pulls them off the other, making it positive.

The current in the wires is carried by electrons. In the electrolyte it is carried by the ions themselves, moving through the liquid.

What happens at the cathode and anode?

The rule is simple: opposites attract.

• At the cathode (−): positive ions (cations) arrive and gain electrons. Gaining electrons is reduction.
• At the anode (+): negative ions (anions) arrive and lose electrons. Losing electrons is oxidation.

A handy memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Another is "PANIC": Positive is Anode, Negative Is Cathode.

When an ion gains or loses electrons at an electrode it becomes a neutral atom or molecule and leaves the solution. Chemists say the ion has been discharged.

How to write half equations

A half equation shows what happens at one electrode, with the electrons written in. The charges must balance on both sides.

At the cathode, copper ions gain two electrons each:

Cu²⁺ + 2e⁻ → Cu

At the anode, chloride ions lose one electron each, and pair up into chlorine molecules:

2Cl⁻ → Cl₂ + 2e⁻

Check the charges. On the left of the cathode equation, +2 and two electrons (−2) make 0; on the right, a copper atom is 0. On the left of the anode equation, two Cl⁻ make −2; on the right, Cl₂ is 0 plus two electrons, −2. Balanced.

Half equations for other common ions:

At the cathode (reduction)	At the anode (oxidation)
Cu²⁺ + 2e⁻ → Cu	2Cl⁻ → Cl₂ + 2e⁻
2H⁺ + 2e⁻ → H₂	2Br⁻ → Br₂ + 2e⁻
Na⁺ + e⁻ → Na (molten only)	4OH⁻ → O₂ + 2H₂O + 4e⁻
Al³⁺ + 3e⁻ → Al (molten only)	2O²⁻ → O₂ + 4e⁻ (molten only)

Electrolysis of copper chloride

The electrolysis of copper chloride solution is the classic first experiment. Copper(II) chloride solution is blue because of its Cu²⁺ ions. With graphite electrodes:

• At the cathode, a pink-brown layer of copper builds up: Cu²⁺ + 2e⁻ → Cu.
• At the anode, bubbles of pale green chlorine are given off: 2Cl⁻ → Cl₂ + 2e⁻. It smells of swimming pools and bleaches damp blue litmus paper.

As the copper ions are used up, the blue colour of the solution fades.

What happens with solutions: the rules

A solution contains more than one kind of ion. Water itself provides a few H⁺ and OH⁻ ions, so at each electrode two ions compete, and only one is discharged.

At the cathode: the metal is made only if it is less reactive than hydrogen (copper, silver, gold). Otherwise hydrogen is given off.

At the anode: if the solution contains a halide (chloride, bromide, iodide), the halogen is given off. Otherwise oxygen is given off, from the hydroxide ions of the water.

That's why you can't make sodium by electrolysing salt water. Sodium is far more reactive than hydrogen, so hydrogen ions are discharged instead. To make sodium, you have to electrolyse molten sodium chloride, with no water present.

Electrolysis of brine

Brine is concentrated sodium chloride solution, and the electrolysis of brine is one of the biggest chemical industries in the world.

• At the cathode: hydrogen gas (sodium is too reactive to be made): 2H⁺ + 2e⁻ → H₂.
• At the anode: chlorine gas (chloride is a halide): 2Cl⁻ → Cl₂ + 2e⁻.
• Left in the solution: Na⁺ and OH⁻ ions, which is sodium hydroxide.

All three products are useful. Chlorine is used to make PVC and to keep drinking water safe, sodium hydroxide makes soap and paper, and hydrogen is a fuel.

Electrolysis of water

Pure water barely conducts, so a little sulfuric acid is added. Sulfate isn't a halide, so oxygen is made at the anode, and hydrogen at the cathode:

2H₂O → 2H₂ + O₂

Collect the gases in upturned test tubes and you get twice as much hydrogen as oxygen, because each water molecule has two hydrogen atoms for every oxygen atom. Test them: hydrogen burns with a squeaky pop, and oxygen relights a glowing splint.

How much is made? Faraday's laws

In 1833 Michael Faraday showed that the amount of substance made is proportional to the charge that flows: Q = I × t (coulombs = amps × seconds). One mole of electrons carries 96 485 C, a quantity now called the Faraday constant.

Example: 1.0 A for 30 minutes gives Q = 1.0 × 1800 = 1800 C, which is 1800 ÷ 96 485 = 0.0187 mol of electrons. Copper needs two electrons per atom, so 0.0093 mol of copper is deposited, which is 0.59 g.

The same idea is used to purify copper. With an impure copper anode and a pure copper cathode in copper sulfate solution, the anode dissolves (Cu → Cu²⁺ + 2e⁻) and pure copper builds up on the cathode, ready for electrical wires.

Common mistakes

• Mixing up the electrodes. The cathode is negative and attracts positive ions.
• Saying sodium forms at the cathode in brine. In a solution, hydrogen forms instead.
• Unbalanced half equations. The charges on each side must be equal; add electrons to fix them.
• Forgetting that ionic solids don't conduct. The ions must be free to move.
• Saying electrons flow through the solution. In the electrolyte, ions carry the charge.

Frequently asked questions

Why does electrolysis need a molten or dissolved compound?

In a solid ionic compound the ions are held tightly in a lattice and can't move, so they can't carry a current or reach the electrodes. Melting the compound or dissolving it in water breaks up the lattice, freeing the ions to move. Then positive ions can travel to the cathode and negative ions to the anode.

What is the difference between the anode and the cathode?

The anode is the positive electrode, connected to the positive terminal of the power supply. Negative ions go there and lose electrons (oxidation). The cathode is the negative electrode. Positive ions go there and gain electrons (reduction). Metals and hydrogen are always made at the cathode; non-metals like chlorine and oxygen at the anode.

Why are electrodes usually made of graphite?

Graphite conducts electricity, is cheap and doesn't react with most of the products, so it doesn't interfere with the reaction. Platinum is even more unreactive but very expensive. In the industrial electrolysis of aluminium oxide the graphite anodes do slowly burn away, because the oxygen made at them reacts with the hot carbon to form carbon dioxide.

Try it yourself

Open Electrolysis and watch blue copper ions drift to the cathode and green chloride ions to the anode, as copper builds up and chlorine bubbles into the test tube. Turn the voltage down until the current stops, then switch to brine and acidified water and compare what each electrode makes, including the 2:1 volumes of hydrogen and oxygen. The 🔬 Faraday experiment shows how mass grows with charge. For the energy side of reactions, see Chemical Energetics.