Electrolysis of aqueous solutions – GCSE Combined Science Chemistry

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When you electrolyse a molten salt there is one positive ion and one negative ion, so there is no choice about what forms. In a solution there is a choice, because the water adds ions of its own. This lesson gives you the two rules that settle the choice.

It matters because a large part of the chemical industry is electrolysis of solutions. Chlorine for clean water, hydrogen as a fuel and sodium hydroxide for soap all come from salt solution. It is also a required practical, so you will meet it in an exam.

Course note: every Combined Science course has one required practical on electrolysis. In some courses it uses inert electrodes and several solutions. In others it uses copper sulfate and copper electrodes. This lesson teaches both. Half equations are Higher tier only.

The cell and the ions in a solution

The cell is a beaker of solution with two electrodes joined to a direct current supply. The electrode joined to the negative terminal is the cathode and the one joined to the positive terminal is the anode. Cations move to the cathode and anions move to the anode.

Now count the ions. Copper chloride solution has Cu²⁺ and Cl⁻ from the salt. Water adds H⁺ and OH⁻. So two cations compete at the cathode and two anions compete at the anode. Only one of each pair is discharged, which means it gains or loses electrons and leaves the solution.

Electrons travel only in the wires. In the solution, ions carry the charge. At the cathode the ions receive electrons, and at the anode ions give electrons up to the circuit.

The rule at the cathode

At the cathode, compare the metal in the salt with hydrogen in the reactivity series. If the metal is more reactive than hydrogen, hydrogen ions are discharged and hydrogen gas forms. The metal ions stay in solution. If the metal is less reactive than hydrogen, the metal is discharged and coats the cathode.

Sodium, potassium, calcium, magnesium, aluminium and zinc are above hydrogen, so these solutions give hydrogen. Copper, silver and gold are below hydrogen, so they give the metal.

Higher tier: the cathode reaction is always reduction. 2H⁺ + 2e⁻ → H₂ for hydrogen, and Cu²⁺ + 2e⁻ → Cu for copper. Electrons are on the left because they are gained.

The rule at the anode

At the anode, look for a halide ion: chloride, bromide or iodide. If the solution is concentrated and contains one, the halogen forms: chlorine, bromine or iodine. If there is no halide, hydroxide ions are discharged and oxygen forms.

Sulfate and nitrate ions are never discharged at this level. A solution of copper sulfate, sodium sulfate or silver nitrate therefore gives oxygen at the anode.

You can tell the halogens apart. Chlorine is a pale green gas that bleaches damp litmus paper. Bromine makes the solution orange-brown. Iodine makes it brown.

Higher tier: the anode reaction is oxidation. 2Cl⁻ → Cl₂ + 2e⁻ for chlorine, and 4OH⁻ → O₂ + 2H₂O + 4e⁻ for oxygen. Electrons are on the right because they are lost.

Water, brine and copper sulfate

Dilute sulfuric acid is a way of electrolysing water. Hydrogen forms at the cathode and oxygen at the anode, with volumes in the ratio 2 : 1, because water splits as 2H₂O → 2H₂ + O₂. The acid only adds ions so that the water conducts.

Concentrated sodium chloride solution is brine. Sodium is more reactive than hydrogen, so hydrogen forms at the cathode. Chloride is a halide, so chlorine forms at the anode. The Na⁺ and OH⁻ ions left behind make sodium hydroxide solution.

All three products of brine are useful. Hydrogen is used as a fuel and to make ammonia. Chlorine is used to disinfect water and to make bleach and plastics. Sodium hydroxide is used to make soap and paper.

Copper sulfate solution with inert electrodes gives copper at the cathode and oxygen at the anode. The blue colour fades because copper ions are removed. The solution slowly becomes more acidic, because hydroxide ions are used up and H⁺ ions remain.

Copper electrodes: purifying and plating

If the electrodes are copper, the anode is no longer inert. It dissolves: Cu → Cu²⁺ + 2e⁻. At the cathode, copper ions are discharged: Cu²⁺ + 2e⁻ → Cu. The anode loses mass and the cathode gains almost exactly the same mass.

This is how copper is purified. The impure copper is the anode, a thin sheet of pure copper is the cathode, and the electrolyte is copper sulfate solution. Pure copper builds up on the cathode. The impurities fall to the bottom as sludge. Pure copper conducts much better, which matters for wires.

Electroplating works the same way. The object to be coated is the cathode. The anode is the plating metal. The electrolyte contains ions of the plating metal. A thin layer of silver on a cheap ring is an example.

Required practical: electrolysing solutions

Inert electrodes. Pour enough solution into a beaker to cover the electrodes. Place two graphite electrodes in it, held apart so they do not touch. Connect them to a low voltage d.c. supply and switch on. Record what you see at each electrode: bubbles, a coating or a colour change.

Test any gas. Hydrogen burns with a squeaky pop at a lit splint. Oxygen relights a glowing splint. Chlorine bleaches damp blue litmus paper, which turns red first. Chlorine is toxic, so use a fume cupboard or a small volume for a short time.

Copper electrodes. Dry and weigh both electrodes. Run the cell for a measured time at a steady current. Rinse with distilled water, dry and weigh again. The anode loses mass and the cathode gains almost the same mass. The difference arises because some copper falls off the anode as sludge.

Control variables when comparing: volume and concentration of solution, the electrodes, the voltage and the time. Record results in a table with the headings solution, cathode, anode, products and explanation.

Exam technique and recap

Command words. State: one fact. Name: the substance. Explain: give a reason with because. Predict: apply the rules to a solution you have not seen. Describe: say what you would do or see.

For a prediction, work in order. List the ions, including H⁺ and OH⁻. At the cathode, compare the metal with hydrogen. At the anode, look for a halide. Name each product. For 6 marks add the gas test, what is left in solution and, at Higher tier, a half equation.

Recap. Water always adds H⁺ and OH⁻. Cathode: a metal below hydrogen forms the metal, otherwise hydrogen. Anode: a halide gives the halogen, otherwise oxygen. Sulfate and nitrate are never discharged. With copper electrodes the anode dissolves and the cathode grows.