C6 Electrolysis
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why can solid ionic compounds not conduct electricity?
the ions are locked in place and are not free to move
why can liquid or dissolved ionic compounds conduct electricity?
the forces of attraction between molecules are broken so the ions are free to move
what is an electrolyte?
a molten or dissolved ionic compound which can conduct electricity
what are the electrodes in electrolysis made from?
a conducting material e.g. graphite or a metal
what is the negative electrode called?
the cathode it is covered with electrons from the power pack
what is the positive electrode called?
the anode it has a lack of electrons
what happens to positive ions in electrolysis?
they are attracted to the cathode. they gain electrons and become atoms this is a reduction reaction
what happens to negative ions in electrolysis?
they are attracted to the anode they lose electrons to form atoms this is an oxidation reaction
which metals are extracted with electrolysis?
ones more reactive than carbon
what is aluminium extracted from and how?
aluminium oxide electrolysis
what is aluminium oxide mixed with during electrolysis and why?
cryolite this lowers the melting point of aluminium oxide which reduced the amount of energy needed and saves money
what is the half equation for the electrolysis of aluminium oxide at the cathode?
Al3+ + 3e- –> Al this is a reduction reaction
what is the half equation for the electrolysis of aluminium oxide at the anode?
2O2- –> 2O + 4e- OR 2O2- - 4e- –> 2O this is an oxidation reaction
why does the anode have to be replaced reguarly?
the oxygen molecules produced at the anode react with the graphite (carbon), forming CO2
why is electrolysis expensive?
melting the compound requires lots of energy a lot of energy is required to produce the electric current
what do water molecules ionise to form?
hydrogen ions hydroxide ions
what is the rule for the reaction at the cathode?
hydrogen is produced at the cathode if the metal is more reactive than hydrogen
what is usually made at the anode when electrolysis is being carried out on an aqueous solution?
oxygen
what metal is often used as electrodes?
platinum it is inert (unreactive) so will not react with the products of the electrolysis
what is the half equation for the reaction at the anode in electrolysis with aqueous solutions?
4OH- –> O2 + 2H2O + 4e- OR 4OH- - 4e- –> O2 + 2H2O
what is the rule for what is produced at the anode in electrolysis of an aqueous solution?
if an aqueous solution contains halide ions, then the halogen will be produced at the anode
what is the half equation when hydrogen is produced in electrolysis?
2H+ + 2e- –> H2 at the cathode
what is the half equation when a halogen is produced? use chlorine as an example
2Cl- –> Cl2 + 2e- OR 2Cl- - 2e- –> Cl2
why must the electrodes not touch each other in electrolysis?
it would produce a short circuit