Chemical Changes: Electrolysis of molten aluminium oxide

Question 1

What is oxidation in terms of electron transfer? What is reduction in terms of electron transfer?

Answer 1

Oxidation is loss of electrons.
Reduction is gain of electrons.

Question 2

What metals have to be extracted by electrolysis?

Answer 2

Metals which are more reactive than carbon.

Question 3

What is mixed with aluminium oxide before electrolysis and why?

Answer 3

• cryolite
• this lowers the melting point
• this reduces the amount of energy needed so saves money

Question 4

What are the cathode and anode made of? Why

Answer 4

• graphite (allotrope of carbon)
• good conductor of electricity
• inert

Question 5

What happens at the cathode during the electrolysis of aluminium oxide?

Answer 5

• Al³⁺ ions are attracted to the cathode
• each ion gains 3 electrons
• to form an aluminium atom

Question 6

What is the half equation for what happens at the cathode during the electrolysis of aluminium oxide? Is this reduction or oxidation?

Answer 6

Al³⁺ + 3e⁻ → Al

reduction

Question 7

What happens at the anode during the electrolysis of aluminium oxide?

Answer 7

• O²⁻ ions are attracted to the anode
• each oxide ion loses 2 electrons
• this causes them to form an oxygen atom

Question 8

What is the half equation for what happens at the anode during the electrolysis of aluminium oxide? Is this oxidation or reduction?

Answer 8

2O²⁻ → O₂ + 4e⁻

oxidation

Question 9

Why must the anode used for the electrolysis of aluminium oxide be regularly replaced?

Answer 9

• the oxygen molecules produced at the anode react with the graphite
• this forms carbon dioxide gas

Question 10

Why is it expensive to produce metals by electrolysis?

Answer 10

Melting
Melting the compounds requires a large amount of energy.

Electricity
A lot of energy is required to produce the electric current

Question 11

Why can graphite conductor electricity?

Answer 11

• each carbon atom has 1 spare electron which becomes delocalised
• this delocalised electron can now carry a charge throughout the whole structure

Question 12

Why are electrodes typically inert?

Answer 12

to prevent them reacting with the electrolyte

Question 13

What would be seen at the anode if a metal is being produced?

Answer 13

a solid is produced

Question 14

What would be seen at an electrode where gas is being produced?

Answer 14

effervescence (fizzing)