In the metallurgy of aluminium

Options

A. `Al^(3+)` is oxidised to Al(s).
B. graphite anode is oxidised to carbon monoxide and carbon dioxide.
C. oxidation state of oxygen changes in the reaction at anode.
D. oxidation state of oxygen changes in the overall reaction involved in the process.

Explanation

### Step-by-Step Solution:

1. **Understanding the Process of Aluminium Metallurgy**:
- The metallurgy of aluminium primarily involves the extraction of aluminium from its ore, bauxite, through the Hall-Héroult process, which is an electrolytic reduction process.

2. **Identifying the Reactions**:
- In the electrolysis process, there are two main electrodes: the anode (positive electrode) and the cathode (negative electrode).
- At the anode, oxidation occurs, while at the cathode, reduction takes place.

3. **Analyzing the Anode Reaction**:
- At the anode, oxide ions (O²⁻) are oxidized to form oxygen gas (O₂). The half-reaction can be represented as:
\[ 2O^{2-} \rightarrow O_2 + 4e^- \]
- This shows that oxygen is being released, and electrons are produced.

4. **Analyzing the Cathode Reaction**:
- At the cathode, aluminium ions (Al³⁺) are reduced to form solid aluminium. The half-reaction can be represented as:
\[ Al^{3+} + 3e^- \rightarrow Al \]
- This indicates that aluminium ions gain electrons to become solid aluminium.

5. **Balancing the Reactions**:
- To balance the overall reaction, we need to ensure that the number of electrons lost at the anode equals the number of electrons gained at the cathode. In this case, we can multiply the cathode reaction by 2 to balance the 6 electrons:
\[ 2Al^{3+} + 6e^- \rightarrow 2Al \]
- Thus, we have:
- Anode: \( 4O^{2-} \rightarrow 2O_2 + 8e^- \)
- Cathode: \( 2Al^{3+} + 6e^- \rightarrow 2Al \)

6. **Identifying the Correct Option**:
- From the analysis, we see that the oxidation state of oxygen changes during the reaction at the anode. Initially, it is in the -2 oxidation state in the oxide ions and changes to 0 in molecular oxygen (O₂).
- Therefore, the correct answer to the question is that the oxidation state of oxygen changes in the reaction at the anode.

### Conclusion:
The correct answer is that the oxidation state of the oxygen changes in the reaction at the anode.

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