IGCSE Chemistry Electrochemistry notes

Electrolysis is the decomposition of an ionic compound, either molten or in aqueous solution, by the passage of electricity. The compound being broken down is called the electrolyte. An ionic solid will not work on its own: its ions are locked in a fixed lattice, so nothing can carry the current. Melting the compound, or dissolving it in water, frees the ions to move and the electrolyte can then conduct.

• Positive electrode

  Anode

  Negative ions (anions) are attracted here. Non-metals are released.

• Negative electrode

  Cathode

  Positive ions (cations) are attracted here. Metals or hydrogen are released.

• The compound

  Electrolyte

  The molten or dissolved ionic compound that is decomposed by the current.

Supplement: charge is carried in two different ways. In the metal wires of the external circuit, charge is carried by electrons. Inside the electrolyte, charge is carried by the ions moving to the electrodes: cations drift to the negative cathode and anions drift to the positive anode.

One rule covers almost every case: metals or hydrogen form at the cathode, and non-metals form at the anode. Positive metal ions and hydrogen ions gain electrons at the cathode; negative ions lose electrons at the anode and are released as non-metals such as oxygen, chlorine or bromine.

1. 1Write the ions

   Split the electrolyte into its positive and negative ions.

2. 2Send them to the electrodes

   Positive ions go to the cathode; negative ions go to the anode.

3. 3Name the products

   A metal or hydrogen forms at the cathode; a non-metal forms at the anode.

Supplement: you can show what happens at each electrode with an ionic half-equation. At the cathode lead ions are reduced: Pb²⁺ + 2e⁻ → Pb. At the anode bromide ions are oxidised: 2Br⁻ → Br₂ + 2e⁻. Notice that electrons are gained on the left for reduction and lost (produced) on the right for oxidation.

When the electrolyte is dissolved in water, the water itself provides H⁺ and OH⁻ ions. At the cathode, these compete with the metal ions. If the metal is less reactive than hydrogen (for example copper or silver), the metal is deposited. If the metal is more reactive than hydrogen (for example sodium or potassium), hydrogen gas is released instead, because the metal ion is too stable to be discharged from water.

At the anode, a similar choice appears. If the solution contains a halide (chloride, bromide or iodide) in reasonably concentrated solution, the halogen is released. Otherwise, oxygen is released, coming from the breakdown of water. Dilute solutions of metal sulfates and dilute sodium chloride therefore give oxygen at the anode.

Supplement: the half-equations change because water is now involved. In salt solutions that give hydrogen at the cathode, water molecules are reduced: 2H₂O + 2e⁻ → H₂ + 2OH⁻, which is why the solution becomes alkaline there. At an anode where oxygen forms, water molecules are oxidised: 2H₂O → O₂ + 4H⁺ + 4e⁻.

Electroplating is the use of electrolysis to put a thin layer of one metal onto the surface of another. It is done to improve appearance — a shiny, attractive finish — and to resist corrosion, protecting the metal underneath from rusting or tarnishing. Silver-plated jewellery and chromium-plated taps are everyday examples.

1. 1Make the object the cathode

   The object to be plated is connected to the negative terminal of the supply, so it is the cathode.

2. 2Use the plating metal as the anode

   A rod of the coating metal is connected to the positive terminal and slowly dissolves.

3. 3Choose an electrolyte of the plating metal

   The electrolyte is a solution containing ions of the plating metal, such as silver nitrate for silver plating.

• The anode gradually gets smaller as its metal atoms form ions that go into the electrolyte.
• The metal ions are attracted to the cathode, where they gain electrons and are deposited as a thin layer.
• The object becomes the cathode, which is the part candidates most often get the wrong way round.

A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity, and water is the only product: 2H₂ + O₂ → 2H₂O. Hydrogen is fed to one electrode and oxygen to the other, and they react with the help of a catalyst. Electrons flow through the external circuit as ions move through the electrolyte, so the cell keeps making electricity as long as the gases are supplied.

• The only chemical product is water, so no carbon dioxide or pollutant gases are released.
• A fuel cell keeps producing electricity as long as hydrogen and oxygen are supplied.
• It is efficient and quiet, because there is no burning and few moving parts.

Supplement: there are disadvantages too. Hydrogen is flammable and explosive, so it must be stored and transported carefully, often in heavy high-pressure or very cold tanks. Hydrogen is also expensive to produce and is often made using fossil fuels, and both the fuel cell and the refuelling infrastructure are costly to build.