Problem 73 - Chemistry The Science in Context

Question

The positive terminal of a voltaic cell is the cathode. However, the cathode of an electrolytic cell is connected to the negative terminal of a power supply. Explain this difference in polarity.

Step-by-Step Solution

Answer

Question: Explain the difference in polarity between the cathode in a voltaic cell and an electrolytic cell. Answer: In a voltaic cell, the cathode is the positive terminal where the spontaneous reduction half-reaction takes place, attracting negatively charged electrons. On the other hand, in an electrolytic cell, the cathode is connected to the negative terminal of the power source to supply the electrons required for the non-spontaneous reduction half-reaction, making the cathode negative in this case.

Step 1: Definition of Voltaic Cell

A voltaic cell, also known as a galvanic cell, is an electrochemical cell that converts chemical energy into electrical energy. In this process, a spontaneous redox reaction occurs, which generates a flow of electrons from the anode (the negative terminal) to the cathode (the positive terminal).

Step 2: Definition of Electrolytic Cell

An electrolytic cell, on the other hand, is an electrochemical cell where electrical energy is used to drive a non-spontaneous redox reaction. In this case, the power supply provides the required energy to force the reaction to proceed in a direction that may not occur naturally.

Step 3: Cathode Function in Voltaic Cell

In a voltaic cell, the cathode is the positive terminal where the reduction half-reaction takes place. This means that electrons are gained, or added, at the cathode. The flow of electrons is from the anode (negative terminal) to the cathode (positive terminal), resulting in a spontaneous process.

Step 4: Cathode Function in Electrolytic Cell

In an electrolytic cell, the cathode is connected to the negative terminal of the power supply. The reduction half-reaction also takes place at the cathode in this case, meaning that electrons are added to the species present. However, the process is non-spontaneous, and the external power supply provides the required energy to make the reaction occur.

Step 5: Explanation of Polarity Difference

The difference in polarity between the cathode in a voltaic cell and an electrolytic cell can be explained by the nature of the redox reactions taking place in each cell. In a voltaic cell, the redox reaction is spontaneous, and the flow of electrons from the anode to the cathode drives the reaction. In this case, the cathode is positive and attracts the negatively charged electrons. In an electrolytic cell, the process is non-spontaneous, and the power source forces the reaction to take place by providing the necessary energy. As a result, the cathode is connected to the negative terminal of the power source to supply the electrons required for the reduction half-reaction to take place. Thus, the cathode in an electrolytic cell is considered negative.

Key Concepts

Voltaic Cell

Understanding the workings of a voltaic cell, also known as a galvanic cell, is fundamental in chemistry.
It's an electrochemical power source capable of generating electrical energy from spontaneous chemical reactions. Inside, two electrodes, each made of different materials, are immersed in electrolyte solutions and are connected by a conductive wire. What sets a voltaic cell apart is its ability to transfer electrons from the anode, the negative end where oxidation occurs, to the cathode, the positive side where reduction takes place.

• Anode: Loses electrons (oxidation)
• Cathode: Gains electrons (reduction)

During this process, electrons flow through the wire, creating the current that can do work, like powering a light bulb.

Electrolytic Cell

An electrolytic cell is like the voltaic cell's counterpart, but it functions in reverse.
It requires external electrical energy to provoke a chemical change, which is why it's used for processes like electroplating or electrolysis of water. In this cell, the positive terminal of a battery is connected to the anode, and the cathode is hooked to the negative terminal.

• Anode: Gains electrons from the power source (still oxidation)
• Cathode: Loses electrons to the power source (still reduction)

Despite the flow of electrons being externally driven, the fundamental chemistry at each electrode remains the same - oxidation at the anode and reduction at the cathode.

Redox Reactions

At the heart of both voltaic and electrolytic cells are redox reactions, the shorthand for reduction-oxidation reactions.
These are processes where one substance transfers electrons to another. The substance losing electrons is being oxidized, while the one gaining electrons is being reduced. Redox reactions are pivotal because they are the source of the cell's ability to generate or consume electricity.

• Oxidation: Loss of electrons, increase in oxidation state
• Reduction: Gain of electrons, decrease in oxidation state

Understanding redox is crucial, as it helps illuminate why electrons flow through a circuit, and what changes are happening at the molecular level.

Cathode Function

The role of the cathode is central to the operations of both types of cells, but its function is often misunderstood.
In an easy sense, the cathode is the electrode where reduction (gain of electrons) occurs. In a voltaic cell, the cathode is marked as positive since it attracts electrons from the external circuit. Conversely, in an electrolytic cell, the cathode is negative because it is connected to the negative terminal of the power supply, accepting the electrons forced into the cell.

• Voltaic Cell Cathode: Destination for electrons traveling through the external circuit
• Electrolytic Cell Cathode: Starting point for electrons pushed by the external power source

It's essential to note that the function of the cathode as the site of reduction remains consistent, regardless of cell type.