4.4.3 Electrolysis (must know) PAPER 1 Flashcards
What is electrolysis
Decomposing an ionic compound using electricity
Why must substances be liquid, melted or aq during electrolysis
So ions are free to move and carry charge
Why are electrodes graphite
Very good conductors (delocalised electrons carry charge through structure) Its inert (doesn't react with anything)
Why is cryolite used in aluminium extraction
It reduces the melting point of aluminium oxide
Less energy is needed
What is the liquid that conducts the electricity in electrolysis called
Electrolyte
What forms at the cathode (-)
Molten electrolysis
Metal eg Mg
What forms at the anode (+)
Molten
Molecules of gas
Eg Cl2
Explain the reaction occurring during electrolysis of magnesium chloride
Positive Mg ions are attracted to the cathode, they gain 2 electrons to form Mg atoms
2Cl- ions are attracted to the anode, they lose 2 electrons to form Cl2
Half equation for magnesium ions forming magnesium
Mg2+ +2e- –> Mg
Half equation for Cl- forming Cl2
2Cl- –> Cl2 + 2e-
Why would a metal be extracted using electrolysis instead of reduced with carbon
The metal is too reactive
less reactive C cannot displace more reactive metal (eg Al)
Downsides to extracting metal ores using electrolysis
Expensive as lots of energy is needed to melt the ore and produce the current
Why the positive anode must be continually replaced (extraction of Al)
As the produced oxygen reacts with the carbon in the electrode to produce carbon dioxide
This causes the anode to wear down and have to be replaced
Explain the reaction occurring during electrolysis of aluminium oxide (extraction)
The Al3+ ions are attracted to the cathode where they each gain 3 electrons to form Al ions
The O2- ions are attracted to the anode where they each lose 2 electrons to form O2 molecules
What is formed at at the cathode (negative electrode)
Aqueous solution
Copper or silver as they are less reactive than hydrogen
Hydrogen is produced if the metal is more reactive than hydrogen.
What is formed at the anode (positive electron)
aq
Oxygen is produced unless the solution contains halide ions when the halogen is produced.
Eg chloride produces chlorine Cl2
Bromide forms bromine Br2
Why are hydrogen and oxygen produced in aq electrolysis
H2O splits/breakdowns into ions
Half equation for copper II sulfate in aq electrolysis
Cu2+ + 2e- –> Cu
4OH- –> O2+ 2H2O +4e-
Half equation for sodium chloride (NaCl) in aq electrolysis
2H+ + 2e- –> H2
2Cl- –> Cl2 + 2e-
