Chemical Changes: Electrolysis of molten aluminium oxide
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What is oxidation in terms of electron transfer? What is reduction in terms of electron transfer?
Oxidation is loss of electrons. Reduction is gain of electrons.
What metals have to be extracted by electrolysis?
Metals which are more reactive than carbon.
What is mixed with aluminium oxide before electrolysis and why?
• cryolite • this lowers the melting point • this reduces the amount of energy needed so saves money
What are the cathode and anode made of? Why
• graphite (allotrope of carbon) • good conductor of electricity • inert
What happens at the cathode during the electrolysis of aluminium oxide?
• Al³⁺ ions are attracted to the cathode • each ion gains 3 electrons • to form an aluminium atom
What is the half equation for what happens at the cathode during the electrolysis of aluminium oxide? Is this reduction or oxidation?
Al³⁺ + 3e⁻ → Al reduction
What happens at the anode during the electrolysis of aluminium oxide?
• O²⁻ ions are attracted to the anode • each oxide ion loses 2 electrons • this causes them to form an oxygen atom
What is the half equation for what happens at the anode during the electrolysis of aluminium oxide? Is this oxidation or reduction?
2O²⁻ → O₂ + 4e⁻ oxidation
Why must the anode used for the electrolysis of aluminium oxide be regularly replaced?
• the oxygen molecules produced at the anode react with the graphite • this forms carbon dioxide gas
Why is it expensive to produce metals by electrolysis?
Melting Melting the compounds requires a large amount of energy. Electricity A lot of energy is required to produce the electric current
Why can graphite conductor electricity?
• each carbon atom has 1 spare electron which becomes delocalised • this delocalised electron can now carry a charge throughout the whole structure
Why are electrodes typically inert?
to prevent them reacting with the electrolyte
What would be seen at the anode if a metal is being produced?
a solid is produced
What would be seen at an electrode where gas is being produced?
effervescence (fizzing)