C4 - Electrolysis of Aluminium Oxide

Why are some metals extracted using electrolysis instead of carbon?

Because metals more reactive than carbon cannot be extracted by reduction with carbon.

Give an example of a metal extracted by electrolysis.

Aluminium.

What compound is aluminium extracted from?

Aluminium oxide (Al2O3).

What ions are present in aluminium oxide?

Aluminium ions (Al3+) and oxide ions (O2-).

Why is aluminium oxide mixed with cryolite before electrolysis?

To lower its melting point, reducing energy costs.

What material are the electrodes made of in aluminium electrolysis?

Graphite (a form of carbon).

Why is graphite used for electrodes?

It conducts electricity and has a high melting point.

Which electrode are aluminium ions (Al3+) attracted to?

The cathode (negative electrode).

What happens to aluminium ions (Al3+) at the cathode?

Each ion gains 3 electrons to form an aluminium atom (reduction).

Half equation for Al3+ at the cathode (Higher only).

Al3+ + 3e- → Al.

Which electrode are oxide ions (O2-) attracted to?

The anode (positive electrode).

What happens to oxide ions (O2-) at the anode?

Each oxide ion loses 2 electrons to form an oxygen atom (oxidation).

Half equation for O2- at the anode (Higher only, version 1).

O2- → O + 2e-.

Half equation for O2- at the anode (Higher only, version 2).

2O2- → O2 + 4e-.

Why are there two versions of the anode half equation?

Both show that oxide ions lose electrons to form oxygen, either per ion or as a full O2 molecule.

What happens to oxygen atoms at the anode after being formed?

They pair up to form oxygen molecules (O2).

Why must the graphite anode be replaced regularly?

Oxygen reacts with the hot carbon anode to produce carbon dioxide gas.

Give the equation for oxygen reacting with carbon anode.

C + O2 → CO2.

Why is electrolysis of metals expensive? (Reason 1)

A lot of energy is needed to melt the ionic compound.

Why is electrolysis of metals expensive? (Reason 2)

A large amount of energy is needed to produce the electric current.

What are the products of aluminium oxide electrolysis?

Aluminium metal and oxygen gas.

Is the reaction at the cathode oxidation or reduction?

Reduction (gain of electrons).

Is the reaction at the anode oxidation or reduction?

Oxidation (loss of electrons).