Activity 1L (Textbook) – Chemistry Form 5 Chapter 1 (Redox Equilibrium)

Activity 1L (Discussion):
Carry out this activity in groups.
1. Scan the QR code to watch a video on aluminium extraction by electrolysis.

2. Based on the information in the video, discuss:
(a) the ionic equation for the reaction.
(b) the overall chemical equation involved.
(c) whether the extraction of aluminium is a redox reaction?

Write your discussion in your notebook.

Answer:
(2)(a)
During the electrolysis of molten aluminium oxide, Al³⁺ ions move to the cathode electrode, while O²⁻ ions move to the anode. Al³⁺ ions are discharged to form aluminium, while O²⁻ ions are discharged to form oxygen gas.
$$\begin{aligned} & \text { Half equation at the cathode }: \mathrm{Al}^{3+}+3 \mathrm{e}^{-} \rightarrow \mathrm{Al} \\ & \text { Half equation at the anode }: 2 \mathrm{O}^{2-} \rightarrow \mathrm{O}_2+4 \mathrm{e}^{-} \\ & \text {Ionic equation } \quad: 4 \mathrm{Al}^{3+}+6 \mathrm{O}^{2-} \rightarrow 4 \mathrm{Al}+3 \mathrm{O}_2 \end{aligned}$$


(2)(b)
Since the electrolysis of molten aluminium oxide is carried out at a high temperature, the oxygen gas produced at the anode reacts with the carbon electrode to form carbon dioxide gas. Therefore, the carbon anode becomes thinner and should be replaced from time to time.

Chemical equations for the reactions involved:
$$\begin{aligned} & 2 \mathrm{Al}_2 \mathrm{O}_3 \rightarrow 4 \mathrm{Al}+3 \mathrm{O}_2 \\ & \mathrm{O}_2+\mathrm{C} \rightarrow \mathrm{CO}_2 \end{aligned}$$


(2)(c)
– Yes. Oxidation reaction at the anode and reduction reaction at the cathode occur simultaneously.

– At the anode, O²⁻ ions undergo oxidation reaction because O²⁻ ions lose electrons to form oxygen molecules.

– At the cathode, Al³⁺ ions undergo reduction reaction because Al³⁺ ions gain electrons to form aluminium atoms.