Problem 14 - Chemistry: An Atoms-Focused Approach

Question

In a voltaic cell, why is the cathode labeled the positive terminal and the anode the negative terminal?

Step-by-Step Solution

Answer

Answer: In a voltaic cell, the cathode is labeled as the positive terminal because it has a more positive potential and is the site of reduction, where species gain electrons. The anode is labeled as the negative terminal because it has a more negative potential and is the site of oxidation, where species lose electrons. This convention is based on the flow of electrons and the resulting electric potential difference between the two electrodes during a redox reaction.

Step 1: Understanding a Voltaic Cell

In a voltaic cell, a spontaneous redox (reduction-oxidation) reaction occurs, and the energy released in this reaction is used to produce an electric current. A voltaic cell consists of two electrodes (cathode and anode) connected by an external circuit and a salt bridge (or porous disk).

Step 2: Oxidation and Reduction reactions

Oxidation is the process of losing electrons, while reduction is the process of gaining electrons. In a voltaic cell, the anode is the site of the oxidation half-reaction, where the species loses electrons. The cathode is the site of the reduction half-reaction, where the species gains electrons.

Step 3: Electrode potentials and Electron flow

In a redox reaction, electrons flow from the species that is being oxidized at the anode to the species that is being reduced at the cathode. The flow of electrons generates an electric potential difference between the anode and cathode. As a result, the anode has a more negative potential than the cathode.

Step 4: Convention for labeling terminals

By convention, the electrode with a higher (more positive) potential is called the positive terminal, and the electrode with a lower (more negative) potential is called the negative terminal.

Step 5: Why the cathode is labeled positive and anode is labeled negative

Since the cathode has a more positive potential and acts as the site of reduction (where species gain electrons), it is labeled as the positive terminal. On the other hand, the anode has a more negative potential and is the site of oxidation (where species lose electrons), so it is labeled as the negative terminal.

Key Concepts

Redox Reactions

Redox reactions are at the heart of how voltaic cells generate electricity. The term "redox" is a combination of two processes: reduction and oxidation. Together, they involve the transfer of electrons between species. A redox reaction in a voltaic cell consists of two half-reactions: one where electrons are lost, and another where electrons are gained. This transfer of electrons is crucial, as it creates a flow that can be harnessed to produce electrical energy.

• Oxidation: This process involves the loss of electrons. During oxidation, a species becomes more positive as it donates electrons to another species.
• Reduction: Conversely, reduction is the gain of electrons. The species accepting these electrons becomes more negative.

The beauty of a voltaic cell lies in its ability to separate these two reactions into different areas, directing electron flow through an external circuit to do work.

Cathode and Anode

In a voltaic cell, you will encounter two types of electrodes: the cathode and the anode. These electrodes are essential for maintaining the flow of electrons during a redox reaction.

• Anode: This is where oxidation occurs. The anode loses electrons, which then flow out into the external circuit. Because it loses electrons, the anode is often considered the negative electrode.
• Cathode: Here, reduction takes place. The cathode gains the electrons flowing from the anode. It is the positive electrode, as it attracts the electrons.

The electrodes are connected by a salt bridge or a porous disk, which allows ions to move between the two half-cells, maintaining electrical neutrality.

Electrode Potentials

Electrode potentials are a measure of the ability of a half-cell to gain or lose electrons. These potentials determine whether an electrode acts as a cathode or an anode. Each electrode is assigned a potential value using standard conditions and the potential difference between the electrodes drives electron flow.

• Standard Electrode Potentials: Measured under standard conditions (1M concentrations, 25°C, 1 atm pressure), these potentials are used as reference values.
• Potential Difference: Electrons naturally flow from the electrode with lower potential to the one with higher potential. This difference is what powers the external circuit.

In practice, the cell potential is always positive in a functioning voltaic cell, which is achieved by ensuring that the reduction potential of the cathode is greater than the oxidation potential of the anode.

Oxidation and Reduction

Understanding oxidation and reduction processes is key to grasping the workings of a voltaic cell. Each side of the cell performs one half of the redox reaction, with electrons traveling between them.

• Oxidation Process: Takes place at the anode. The species loses electrons, supplying them to the external circuit. This flow of electrons generates the electric current.
• Reduction Process: Occurs at the cathode. Here, the species gains electrons. This acceptance of electrons completes the circuit, allowing continuous flow.

The simultaneous occurrence of oxidation and reduction processes ensures that electrons flow in a predictable manner, driving the chemical processes that generate electricity.