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neurologyYou need to know

• Electrolysis is the decomposition of an ionic compound by passing a direct electric current through a molten or dissolved electrolyte.
• An electrolyte is a molten or dissolved ionic substance that can conduct electricity because it contains mobile ions.
• Electricity is the flow of electrical charge; electrons carry charge in metal wires and ions carry charge through electrolytes.
• Solid ionic compounds do not conduct electricity because their ions are fixed in a lattice and cannot move to carry charge.
• Ionic compounds must be molten or dissolved for electrolysis because their ions become free to move to the electrodes.
• The cathode is the negative electrode, and positively charged ions called cations move towards it.
• The anode is the positive electrode, and negatively charged ions called anions move towards it.
• Reduction happens at the cathode because cations gain electrons there.
• Oxidation happens at the anode because anions lose electrons there.
• During electrolysis, metal ions are discharged at the cathode by gaining electrons to form metal atoms.
• During electrolysis, non-metal ions are discharged at the anode by losing electrons to form non-metal atoms or molecules.
• Metals more reactive than carbon, such as aluminium, are extracted by electrolysis rather than by reduction with carbon.
• Metals less reactive than carbon can usually be extracted from their ores by reduction using carbon or carbon monoxide.
• Aluminium is extracted by electrolysing aluminium oxide dissolved in molten cryolite.
• Cryolite lowers the melting point of aluminium oxide and improves the conductivity of the electrolyte, reducing the energy cost of extraction.
• In aluminium extraction, aluminium ions gain electrons at the cathode to form molten aluminium: Al³⁺ + 3e⁻ → Al.
• In aluminium extraction, oxide ions lose electrons at the anode to form oxygen gas: 2O²⁻ → O₂ + 4e⁻.
• The positive carbon anodes in aluminium extraction must be continually replaced because oxygen reacts with the carbon to form carbon dioxide.
• In aqueous electrolysis, water provides hydrogen ions and hydroxide ions, so hydrogen or oxygen may be produced instead of the ions from the dissolved compound.
• At the cathode in aqueous electrolysis, hydrogen is produced if the metal ion is more reactive than hydrogen; at the anode, oxygen is produced unless halide ions are present, when chlorine, bromine or iodine is produced.

• Identify the cathode and anode in an electrolysis cell, using the rule that the cathode is negative and the anode is positive.
• Explain why a named ionic compound must be molten or dissolved before electrolysis, referring to mobile ions carrying charge.
• Classify electrode reactions as oxidation or reduction by stating whether electrons are lost or gained.
• Write balanced half equations for cathode reactions, showing positive ions gaining electrons to form neutral atoms or molecules.
• Write balanced half equations for anode reactions, showing negative ions losing electrons to form neutral atoms or molecules.
• Predict the product at the cathode during aqueous electrolysis by comparing the metal ion with hydrogen in the reactivity series.
• Predict the product at the anode during aqueous electrolysis by checking first for halide ions and otherwise selecting oxygen from hydroxide ions.
• Predict the products of electrolysis of a molten ionic compound by discharging the metal ion at the cathode and the non-metal ion at the anode.
• Describe the extraction of aluminium by naming the electrolyte, explaining the role of cryolite, and giving the products at each electrode.
• Explain why carbon anodes are consumed in aluminium extraction by linking oxygen production at the anode to carbon dioxide formation.