5.2 describe and explain the extraction of aluminium from purified aluminium oxide by electrolysis including: i the use of molten cryolite as a solvent and to decrease the required operating temperature ii the need to replace the positive electrodes iii the cost of the electricity as a major factor

As aluminium is more reactive than carbon, you use electrolysis to extract aluminium from its ore. The main ore of aluminium is bauxite (which is mined, incase you were wondering). NOTE: after you purify bauxite, a white powder is left (this is aluminium oxide, Al₂O₃).

First lets start with the electrolysis process, if you are familiar with this just skip to 'problems'...

1- Aluminium oxide is melted (to form molten aluminium oxide), this contains free ions, meaning it will conduct electricity.

2- The positive Al³⁺ ions are attracted to the negative electrode (the cathode, this is lining the electrolysis cell). Here, the positive Al³⁺ ions gain electrons (3 to be precise, to balance their charge), and they turn into aluminium atoms, which sinks to the bottom as molten aluminium.

3- The negative O²⁻ ions are attractive to the positive electrodes (anodes). Are, they lose electrons. They will then react together to form O₂ (oxygen gas), which will occasionally react with the anodes forming carbon dioxide (CO₂). 

NOTE: this is a redox reaction as reduction and oxidation both take place. The equations are as follows...

at the cathode: Al³⁺ + e− ---> Al

at the anode: 2O²⁻ ---> O₂ + 4e−

The complete decomposition reaction: aluminium oxide ---> aluminium + oxygen

Problems

However, the melting point of Al₂O₃ is very high (around 2000ºC, if you were wondering). Aluminium oxide is dissolved in molten cryolite (a less common ore of aluminium), this lowers the temperature needed to melt, so less fuel is used in heating the aluminium oxide = saves energy. (NOTE: using cryolite only lowers the melting temperature to around 900ºC so, although it lowers the temperature and saves lots of energy (and also makes the extraction cheaper and easier), a very high temp is still needed.

The positive electrodes (anodes) are made of graphite (a form of carbon). During the electrolysis process, the O²⁻ ions react with the carbon anodes, forming carbon dioxide (CO₂). This means that the anodes will eventually have to be replaced, as they will start to erode where they react with oxygen.

Lots of energy is needed to heat the aluminium oxide to 900ºC and therefore electrolysis of aluminium is very expensive. Furthermore, electrolysis itself uses lots of electricity.