What are the products and processes involved in the extraction of Aluminium from its ore?

Here are the answers to question 4:

4. (a)

(i) The formula of the main Ore X is $\boxed{\text{Al}_2\text{O}_3}$.

(ii)

(a) The main residue formed after filtration in step I is Iron(III) oxide ($\text{Fe}_2\text{O}_3$).

(b) Substance Q is Sodium hydroxide ($\text{NaOH}$).

(iii) The sodium aluminate in Solution R is separated from the impurity silicon(IV) oxide (which forms sodium silicate in solution) by cooling and diluting the solution, then seeding it with aluminium hydroxide crystals. This causes aluminium hydroxide to precipitate, leaving the sodium silicate in solution.

(v) The purpose of the addition of substance S (cryolite, $\text{Na}_3\text{AlF}_6$) in step III is to dissolve the aluminium oxide ($\text{Al}_2\text{O}_3$) and lower its melting point, which reduces the energy required for electrolysis.

4. (b)

(i) The anode in the extraction of Aluminium is replaced periodically because it is made of carbon, and the oxygen gas produced during electrolysis reacts with the carbon anode to form carbon dioxide gas, causing the anode to be consumed over time.

(ii) The equation for the formation of Aluminium at the cathode is:

$$\text{Al}^{3+}\text{(l)} + \text{3e}^- \to \text{Al(l)}$$