Problem 34 - Chemistry Matter and Change

Question

In a voltaic cell represented by All \(A l^{3+} \| \mathrm{Cu}^{2+} | \mathrm{Cu},\) what is oxidized and what is reduced as the cell delivers current?

Step-by-Step Solution

Answer

Aluminum is oxidized and copper ions are reduced.

Step 1: Identify Oxidation and Reduction Half-Reactions

Examine the components of the voltaic cell, which consist of \( \text{Al} \) (aluminum) and \( \text{Cu} \) (copper) ions. Aluminum transitions from \( \text{Al} \) to \( \text{Al}^{3+} \) and copper transitions from \( \text{Cu}^{2+} \) to \( \text{Cu} \). The half-reactions help us identify what gets oxidized and reduced.

Step 2: Determine the Oxidation Reaction

The oxidation process involves losing electrons. Here, aluminum is oxidized, described by the half-reaction: \( \text{Al} \rightarrow \text{Al}^{3+} + 3e^- \). Aluminum starts as a neutral atom and loses three electrons to form \( \text{Al}^{3+} \).

Step 3: Determine the Reduction Reaction

The reduction process involves gaining electrons. In this cell, copper is reduced, as depicted in the half-reaction: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \). Copper starts as an ion \( \text{Cu}^{2+} \) and gains two electrons to become a neutral \( \text{Cu} \) atom.

Step 4: Summarize Oxidation and Reduction

Aluminum is oxidized to \( \text{Al}^{3+} \), and copper ions \( \text{Cu}^{2+} \) are reduced to copper metal \( \text{Cu} \). This transfer of electrons between the oxidation of aluminum and reduction of copper permits the cell to deliver current.

Key Concepts

Oxidation

In a voltaic cell, oxidation is a fundamental concept where a chemical species loses electrons. This loss increases the oxidation state of the element that undergoes the process. An easy way to remember oxidation is with the phrase "OIL RIG" which stands for "Oxidation Is Loss" (of electrons).
In the context of the given voltaic cell, oxidation occurs at the anode, where aluminum (\(\text{Al}\) transitions to \(\text{Al}^{3+}\)).
This half-reaction is represented as:- \(\text{Al} \rightarrow \text{Al}^{3+} + 3e^-\).
Here, aluminum starts as a neutral metal atom and loses three electrons, resulting in an increase in its oxidation state from 0 to +3.
This loss of electrons by aluminum is what triggers the flow of electrons, which is vital for generating electricity in the voltaic cell.

Reduction

Reduction is the process of gaining electrons, which lowers the oxidation state of a substance. In any redox reaction, reduction always accompanies oxidation because the electrons lost by one species are gained by another.
Reduction occurs at the cathode, and you can recall it using "OIL RIG" where "Reduction Is Gain" (of electrons).
In our voltaic cell, the copper ions (\(\text{Cu}^{2+}\)) undergo reduction as represented by the half-reaction:
- \(\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}\).
The copper ions start with an oxidation state of +2 and, by gaining two electrons, they transform into metallic copper with an oxidation state of 0.
This reduction process is essential because it captures the electrons released during oxidation, enabling the continuous flow of electric current through the cell.

Half-Reactions

Half-reactions are a way of breaking down a redox reaction into its oxidation and reduction components. Each half-reaction shows either the gain or loss of electrons, providing a clearer view of the changes in oxidation states.
In the cell scenario:

• The oxidation half-reaction is: \(\text{Al} \rightarrow \text{Al}^{3+} + 3e^-\), where aluminum loses electrons.
• The reduction half-reaction is: \(\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}\), where copper gains electrons.

Combining these half-reactions gives the overall cell reaction, ensuring all electrons are accounted for and balanced.
Half-reactions illustrate the electron transfer process essential for the electrochemical reaction, making it easier to see how the oxidation and reduction reactions complement each other to complete the circuit.

Current Delivery

The delivery of current in a voltaic cell is crucial for its operation, and it hinges upon the movement of electrons triggered by oxidation and reduction.
The flow of electric current is essentially the flow of electrons that move from the anode to the cathode.
In our cell, as aluminum is oxidized, it releases electrons that travel through an external circuit to the copper cathode, where they are accepted during the reduction of copper ions.
This continuous movement of electrons produces electrical energy that can be harnessed to perform work, such as powering a device.
Additionally, for continuous current delivery, the cell must allow ions to migrate to maintain charge balance, a process often facilitated by a salt bridge or membrane within the cell.
This movement of electrons and ions is what allows a voltaic cell to function as a source of energy and highlights the practical importance of redox reactions in everyday applications.