Arrange the following sequential electrochemical and physical steps occurring during the r… — JAMB UTME

Arrange the following sequential electrochemical and physical steps occurring during the reduction of alumina in the Hall-Héroult cell to extract molten aluminium metal, from initial electrolyte preparation to final anode gas emission.

1Dissolving purified alumina ( $Al_2O_3$ ) in molten cryolite ( $Na_3AlF_6$ ) at approximately $950^\circ\text{C}$ to form a conducting electrolyte solution.
2Dissociation of molten alumina into mobile aluminium ions ( $Al^{3+}$ ) and oxide ions ( $O^{2-}$ ).
3Migration of $Al^{3+}$ cations toward the carbon-lined cathode base of the cell upon applying an electric potential.
4Reduction of $Al^{3+}$ ions at the cathode by gaining electrons to form dense, molten aluminium metal.
5Oxidation of $O^{2-}$ anions at the graphite anodes to form oxygen gas, which reacts with the carbon anodes to yield carbon dioxide gas.

Answer

The correct sequence of steps in the Hall-Héroult cell is: (1) Dissolving alumina in molten cryolite, (2) Dissociation of alumina into

Al^{3+}

and

O^{2-}

ions, (3) Migration of

Al^{3+}

ions to the cathode, (4) Reduction of

Al^{3+}

to form molten aluminium, and (5) Oxidation of

O^{2-}

at the graphite anodes yielding carbon dioxide gas.

The Hall-Héroult process operates sequentially by first dissolving alumina (

Al_2O_3

) in molten cryolite (

Na_3AlF_6

) at around

950^\circ\text{C}

to create a conducting medium. Upon melting, alumina dissociates into mobile

Al^{3+}

and

O^{2-}

ions. Applying an electric current causes

Al^{3+}

cations to migrate to the carbon cathode at the bottom, where they are reduced to liquid aluminium metal (

Al^{3+} + 3e^- \rightarrow Al_{(l)}

). Concurrently,

O^{2-}

anions migrate to the top graphite anodes and undergo oxidation to oxygen gas (

2O^{2-} \rightarrow O_{2(g)} + 4e^-

), which reacts with the hot carbon anodes to form carbon dioxide gas (

C + O_2 \rightarrow CO_2

Step-by-Step Solution

Identify the electrolyte preparation step in the Hall-Héroult cell.

Alumina is dissolved in molten cryolite at about

950^\circ\text{C}

Cryolite acts as a solvent and flux to lower the high melting point of pure alumina and enhance conductivity.

Determine the ionization behavior of the dissolved alumina.

Alumina dissociates into mobile

Al^{3+}

cations and

O^{2-}

anions.

Liquid state ionic dissociation is necessary for current transport through the electrolyte.

Trace the movement of cations under the applied electric field.

Al^{3+}

cations migrate to the negatively charged carbon cathode lining at the cell floor.

Electrostatic attraction draws positive ions toward the negative electrode.

Determine the chemical reaction occurring at the cathode.

Al^{3+}

ions gain electrons to form molten aluminium metal (

Al^{3+} + 3e^- \rightarrow Al_{(l)}

Cation gain of electrons at the cathode represents the reduction process that isolates elemental aluminium.

Determine the chemical reaction occurring at the anode and the fate of the anode material.

O^{2-}

ions lose electrons to produce oxygen gas, which reacts with graphite anodes to produce

CO_2

gas.

Anode oxidation releases oxygen gas at high temperature, causing carbon anodes to burn away continuously.

Key Concept

Electrolytic reduction of alumina in the Hall-Héroult process

Estimated Time:1m 30s

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