Problem 28 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indicate whether each statement is true or false: (a) The anode is the electrode at which oxidation takes place. (b) A voltaic cell always has a positive emf. (c) A salt bridge or permeable barrier is necessary to allow a voltaic cell to operate.

Step-by-Step Solution

Answer

(a) True: The anode is the electrode at which oxidation takes place, where chemical species lose electrons. (b) True: A voltaic cell always has a positive emf, producing electricity through a spontaneous redox reaction. (c) True: A salt bridge or permeable barrier is necessary to maintain electrical neutrality and allow a voltaic cell to operate.

Step 1: (a) The anode is the electrode at which oxidation takes place.

This statement is true. In an electrochemical cell, the anode is the electrode where oxidation occurs. Oxidation can be remembered as the process where a species loses electrons. So at the anode, the chemical species lose their electrons and generate an electric current.

Step 2: (b) A voltaic cell always has a positive emf.

This statement is true. A voltaic cell, also known as a galvanic cell, is an electrochemical cell that produces electricity through a spontaneous redox reaction. The emf is a measure of the voltage produced by the cell. Since the reaction is spontaneous, the cell generates a positive emf. If the emf were negative, it would be referred to as an electrolytic cell, which uses an external voltage source to drive a non-spontaneous redox reaction.

Step 3: (c) A salt bridge or permeable barrier is necessary to allow a voltaic cell to operate.

This statement is true. A salt bridge or permeable barrier serves a critical role in the functioning of a voltaic cell. It is responsible for maintaining electrical neutrality by allowing the migration of ions between the two half-cells. Without a salt bridge or permeable barrier, the charge buildup at the electrodes would quickly stop the redox reaction, and hence no electric current would be produced. In conclusion, all three statements (a), (b), and (c) are true.

Key Concepts

Anode and Cathode

In electrochemical cells, the anode and cathode are two types of electrodes with distinct functions. At the anode, oxidation occurs, which means this is where a species loses electrons. It's helpful to remember that oxidation can be remembered by the mnemonic "OIL" which stands for "Oxidation Is Loss" of electrons. As this happens, the chemical species at the anode become positively charged because they give away electrons.

On the other hand, the cathode is where reduction takes place. Reduction involves the gain of electrons by a chemical species, making it negatively charged, remembered by the mnemonic "RIG" - "Reduction Is Gain" of electrons.

The movement of electrons from the anode to the cathode through an external circuit generates electric current, which is essentially the purpose of electrochemical cells.

• Anode: Oxidation occurs, electrons are lost.
• Cathode: Reduction occurs, electrons are gained.

Voltaic Cell

A voltaic cell, also known as a galvanic cell, is a device that generates electrical energy from a spontaneous chemical reaction. This type of electrochemical cell utilizes redox reactions, in which oxidation and reduction processes occur simultaneously to produce an electric current.

One important aspect of a voltaic cell is that it always has a positive electromotive force (emf). This is a key characteristic because it means that the cell can work spontaneously without any external input of energy. The emf is essentially the voltage obtained from the chemical reaction inside the cell.

If the emf of a cell were negative, the cell would require external energy to function and would be termed an electrolytic cell instead.

• Voltaic Cell: Produces electricity from spontaneous reactions.
• Has a positive emf indicative of spontaneous energy production.
• Different from electrolytic cells, which require external energy sources.

Salt Bridge Function

The salt bridge is a crucial component of a voltaic cell, facilitating its long-term operation. While it might look unassuming, a salt bridge ensures that ions can flow between the two compartments of the cell. This is essential for maintaining electrical neutrality during operation.

When oxidation occurs at the anode, electrons are released, and they move towards the cathode. Hence, to balance the charge, negative ions must migrate towards the anode, while positive ions move towards the cathode. This counter-ion flow is made possible by the salt bridge or a permeable barrier, necessary to complete the internal circuit of the cell.

Without the salt bridge, ions wouldn't be able to migrate, leading to a charge buildup that would halt the chemical reaction, stopping the production of electricity.

• Salt Bridge: Enables ion flow to maintain charge balance.
• Prevents charge buildup at the electrodes.
• Vital for continuous operation of voltaic cells.