ElectrolysisIB MYP Chemistry: Revision notes

Electrolysis is the breaking down of an ionic compound using electricity. Solid ionic compounds do not conduct because their ions are fixed in place. When the compound is molten or dissolved in water, the ions are free to move and can carry the current.

• The liquid that conducts and is broken down is the electrolyte.
• The cathode is the negative electrode. Positive ions (cations) move to it and gain electrons (reduction).
• The anode is the positive electrode. Negative ions (anions) move to it and lose electrons (oxidation).

Electrons travel round the wires, not through the electrolyte. The ions carry the charge through the liquid.

A binary compound contains just two elements. When one is molten and electrolysed, the products are easy to predict:

• The metal forms at the cathode.
• The non-metal forms at the anode.

For molten lead(II) bromide: lead forms at the cathode, Pb²⁺ + 2e⁻ → Pb, and bromine forms at the anode, 2Br⁻ → Br₂ + 2e⁻.

For molten sodium chloride: sodium forms at the cathode, Na⁺ + e⁻ → Na, and chlorine forms at the anode, 2Cl⁻ → Cl₂ + 2e⁻.

Water in a solution contains a few H⁺ and OH⁻ ions as well as the ions of the dissolved compound. So there is a choice of ions at each electrode.

• At the cathode: if the metal is more reactive than hydrogen, hydrogen forms: 2H⁺ + 2e⁻ → H₂. If the metal is less reactive than hydrogen (such as copper), the metal forms.
• At the anode: if a halide ion (Cl⁻, Br⁻, I⁻) is present, the halogen forms. Otherwise oxygen forms: 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Examples:

• Sodium chloride solution: hydrogen at the cathode, chlorine at the anode. Na⁺ and OH⁻ ions are left, so sodium hydroxide forms.
• Copper(II) sulfate solution (inert electrodes): copper at the cathode, oxygen at the anode. The blue colour fades as Cu²⁺ ions are used up.
• Water (with a little acid): hydrogen at the cathode and oxygen at the anode, in a 2 : 1 volume ratio.

Tests: hydrogen gives a squeaky pop, oxygen relights a glowing splint, chlorine bleaches damp litmus paper.

A half-equation shows what happens at one electrode. Check that the atoms and the charges balance. Electrons are on the left at the cathode (gained) and on the right at the anode (lost).

• Cathode: Cu²⁺ + 2e⁻ → Cu
• Cathode: 2H⁺ + 2e⁻ → H₂
• Anode: 2Cl⁻ → Cl₂ + 2e⁻
• Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻

Worked check for 2Cl⁻ → Cl₂ + 2e⁻: the left has charge 2−, and the right has 2e⁻ so also 2−. The charges balance.

Electroplating coats an object with a thin layer of metal, to protect it from corrosion or to make it look better.

• The object to be plated is the cathode.
• The anode is made of the plating metal.
• The electrolyte is a solution containing ions of the plating metal.

To silver plate a spoon: Ag⁺ + e⁻ → Ag at the spoon. At the anode, the silver dissolves, Ag → Ag⁺ + e⁻, so the solution stays at the same concentration. The object should be clean and a direct current is used.

Electrolysis of concentrated sodium chloride solution (brine) is done on a huge scale. It is called the chlor-alkali process because it makes chlorine and an alkali, sodium hydroxide.

• Chlorine (anode): used to kill bacteria in water, and to make bleach and plastics such as PVC.
• Hydrogen (cathode): used as a fuel, and to make ammonia and margarine.
• Sodium hydroxide solution (left in the cell): used to make soap, paper and bleach.

Electrolysis is also used to extract reactive metals such as aluminium and to plate objects.