Question

State the reactions that take place at the cathode and anode during the process of electrolytic reduction of alumina.

Short answer

Answer: The half-reactions occurring during the electrolysis of alumina are: 1. Cathode (reduction): Al3+(aq) + 3e- → Al(s) 2. Anode (oxidation): 2O2-(aq) → O2(g) + 4e- The overall reaction for the electrolytic reduction of alumina is: 4Al3+(aq) + 6O2-(aq) → 4Al(s) + 3O2(g)

Step-by-step solution

Cathode Reaction: Reduction of Aluminum Ions

At the cathode, the reduction process involves the conversion of aluminum ions (Al3+) to aluminum metal (Al). This occurs when electrons are gained from the external circuit. The half-reaction for this process is given by: Al3+(aq) + 3e- → Al(s) Here, Al3+ ions from the electrolyte gain 3 electrons (e-) and change into aluminum metal (Al) which deposits at the cathode.

Anode Reaction: Oxidation of Oxide Ions

At the anode, the oxidation of oxide ions (O2-) into oxygen gas (O2) occurs. This involves the loss of electrons by the O2- ions, which are then transferred to the external circuit. The half-reaction for this process is given by: 2O2-(aq) → O2(g) + 4e- Here, two O2- ions from the electrolyte combine to release one oxygen gas molecule (O2) and give up a total of 4 electrons (e-).

Overall Reaction

To obtain the overall reaction for the electrolytic reduction of alumina, we balance the reduction and oxidation half-reactions. As there are 3 electrons involved in the reduction half-reaction and 4 electrons in the oxidation half-reaction, we multiply the reduction half-reaction by 4 and the oxidation half-reaction by 3 to equalize the number of electrons. The balanced overall reaction for the electrolytic reduction of alumina is: 4Al3+(aq) + 6O2-(aq) → 4Al(s) + 3O2(g) Here, 4 aluminum ions (Al3+) and 6 oxide ions (O2-) react to produce 4 atoms of aluminum metal (Al) and 3 molecules of oxygen gas (O2).

Key concepts

Cathode Reaction in Electrolysis

In the context of electrolytic reduction of alumina, the cathode plays a critical role as the site for reduction reactions. A reduction reaction is where a substance gains electrons, leading to a decrease in its oxidation state. At the cathode, aluminum ions (\( Al^{3+} \) ions) present in the alumina (\( Al_2O_3 \)) absorb electrons from the external circuit to form aluminum metal (\( Al \)), adhering to the cathode surface.

This transformation can be represented by the half-reaction:
\begin{align*} Al^{3+}(aq) + 3e^- &\rightarrow Al(s) \end{align*}
During this process, each aluminum ion gains three electrons (\( e^- \)), a classic example of an electrochemical reduction.

Anode Reaction in Electrolysis

Conversely, at the anode of the electrolysis setup for reducing alumina, an oxidation reaction occurs. Oxidation is the process where a species loses electrons, increasing its oxidation state. Here, oxide ions (\( O^{2-} \) ions) in the alumina release electrons to form oxygen gas (\( O_2 \)) that bubbles away from the anode.

The half-reaction illustrating this process is:
\begin{align*} 2O^{2-}(aq) &\rightarrow O_2(g) + 4e^- \end{align*}
This reaction showcases the oxide ions relinquishing four electrons (\( e^- \)) in total. These freed electrons then travel through the external circuit towards the cathode, facilitating the continuous flow of electricity needed for the electrolysis.

Oxidation and Reduction Half-Reactions

Understanding the interplay between oxidation and reduction half-reactions is integral to grasping the underlying principles of electrolytic processes. During electrolysis, reduction half-reactions occur at the cathode, and oxidation half-reactions occur at the anode. It's essential to remember that these half-reactions occur simultaneously, and the movement of electrons from the anode to the cathode is what powers the entire reaction.

Since these reactions are separate but connected, balancing them requires ensuring that the number of electrons lost in the oxidation half-reaction equals the number of electrons gained in the reduction half-reaction. This ensures that the electrochemical system is neutral and allows the full reaction to be accurately represented.

Balancing Chemical Equations

Balancing chemical equations is a fundamental concept in chemistry, ensuring that the Law of Conservation of Mass is satisfied; the mass of reactants must equal the mass of the products. In the context of the electrolysis of alumina, balancing involves adjusting the coefficients of reactants and products so that the number of atoms and the charge are equal on both sides of the equation.

Breaking down the steps involved in balancing the electrolytic reaction of alumina, you would first balance the aluminum and oxygen atoms individually and then ensure that the charges are balanced by adjusting the number of electrons. This might require multiplying the reduction and oxidation half-reactions by appropriate factors. By doing so, we demonstrate that the process is in accord with the physical law, reflecting the actual electron transfer that underpins the reaction.