Problem 72 - 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

Answer: The difference lies in the fact that a voltaic cell generates electrical energy through spontaneous redox reactions, so the cathode is the positive terminal as it receives electrons from the anode. In an electrolytic cell, the reduction reaction is forced by an external power supply, so the cathode receives electrons from the power supply and is connected to the negative terminal. The role of the cathode as the site of reduction remains the same in both cells, but the polarity is determined by whether the redox reaction is spontaneous (voltaic cell) or requires external energy (electrolytic cell).

Step 1: Understand the working of a voltaic cell

A voltaic cell, also known as a galvanic cell, is an electrochemical cell that generates electrical energy through spontaneous redox reactions. It consists of two half-cells: one half-cell with a metal electrode and a solution containing its cations (anode), and another half-cell with another metal electrode and its cations (cathode). In a voltaic cell, the anode undergoes oxidation (loses electrons) and the cathode undergoes reduction (gains electrons).

Step 2: Understand the working of an electrolytic cell

An electrolytic cell is an electrochemical cell that uses electrical energy to drive a non-spontaneous redox reaction. It also has two electrodes, the cathode and the anode, which are immersed in an electrolyte solution. In an electrolytic cell, an external power supply forces electrons from the anode to the cathode, where the reduction reaction occurs, and the anode, where the oxidation reaction occurs.

Step 3: Identify the roles of the cathode in voltaic and electrolytic cells

In both voltaic and electrolytic cells, reduction occurs at the cathode. In a voltaic cell, the cathode is the positive terminal because this is where the reduction occurs, and thus, where electrons are attracted. In an electrolytic cell, the cathode is connected to the negative terminal of the power supply, as it is the site where electrons are being pushed into the cell, causing the reduction reaction to occur.

Step 4: Explain the difference in polarity

The difference in polarity can be attributed to the fact that in a voltaic cell, the redox reaction occurs spontaneously, and the cell generates electrical energy. In this case, the cathode is the positive terminal because it receives electrons from the anode, and this is the direction of the spontaneous electron flow. In an electrolytic cell, however, an external power supply forces the redox reaction to occur, and the cell consumes electrical energy. In this case, the cathode is connected to the negative terminal of the power supply because it needs to receive electrons from the power supply to make the reduction reaction occur. Thus, the role of the cathode remains the same in both cells (reduction site), but the polarity is determined by whether the redox reaction is spontaneous (voltaic cell) or requires external energy (electrolytic cell).