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positive metal ions are reduced to the element The electrodes must be made of an inert material, so they dont react with the electrolyte • At the anode, the negative non-metal ions are oxidised to the element 1.4.3.3 Using Electrolysis to Extract Metals • If a metal is too reactive to be reduced with carbon (1.4.1 Reactivity of metals > 1.4.1.3 Extraction of metals and reduction), then we use electrolysis to extract it. This method can be very expensive, as it requires a lot of energy to melt the ore and produce the current For Example: We can extract Aluminium from the ore Bauxite, using electrolysis: 1. The ore is mixed with cryolite to lower the boiling point, when it is molten, it will contain free ions that conduct electricity (Al203) To make it less expensive, compounds can be mixed with compounds like cryolite to lower the boiling point 2. The positive Al³+ ions will be attracted to the cathode, where they pick up 3 electrons and become neutral Aluminium atoms which sink to the bottom of the tank 3. The negative 02- lons will become attracted to the Anode, where they lose two electrons and become neutral oxygen molecules when they join up ● ● aluminium oxide → aluminium + oxygen 2Al₂O 30 4A + 30₂ (1) 1.4.3.4 Electrolysis of Aqueous Solutions However, when electrolysis happens in an aqueous solution, you have to factor in the ions from the water. 2(g) Hydroden ions - H+ Hydroxide ions - OH- The ions that discharge at the electrodes will depend on the relative reactivity of the ions in the solution The Cathode At the cathode, H+ ions will be present ● hydrogen gas will be produced if the metal ions form an elemental metal more reactive than hydrogen. Or, if they form a metal less reactive than hydrogen, a layer of the pure metal will be produced and coat the cathode The Anode At the anode, OH ions will be present if the OH and the halide ions (Cl-, Br, I) are present, the molecules of Chlorine, Bromine or lodine are produced. If there are no halide ions present, the OH- ions from the water are discharged and oxygen gas and water are produced 1.4.3.5 Representation of Reactions at Electrodes as Half Equations Half equations show the reactions at the electrodes You can combine half equations at each electrode to get the ionic equation of the overall reaction For the electrolysis of aqueous sodium chloride solution the half equations are: Negative Electrode: 2H + 2e → H₂ Positive Electrode: 2Cl → Cl₂ + 2e- (or 2Cl2e → Cl₂) You can combine these to get the ionic equation: 2H+ + 2Cl → H₂ + Cl₂ The electrons on each side of the half equations balance, so they cancel out in the full ionic equation.