Problem 5 - Chemistry: Principles and Reactions

Question

Consider a salt bridge voltaic cell represented by the following reaction: $$ \mathrm{Fe}(s)+2 \mathrm{Tl}^{+}(a q) \longrightarrow \mathrm{Fe}^{2+}(a q)+2 \mathrm{Tl}(s) $$ Choose the best answer from the choices in each part below: (a) What is the path of electron flow? Through the salt bridge, or through the external circuit? (b) To which half-cell do the negative ions in the salt bridge move? The anode, or the cathode? (c) Which metal is the electrode in the anode?

Step-by-Step Solution

Answer

Question: In a voltaic cell with a salt bridge and the redox reaction \(\mathrm{Fe}(s)+2\mathrm{Tl}^{+}(a q)\longrightarrow\mathrm{Fe}^{2+}(a q)+2\mathrm{Tl}(s)\), (a) describe the path of electron flow, (b) to which half-cell do the negative ions in the salt bridge move, and (c) identify the metal in the anode. Answer: (a) The electrons flow through the external circuit, not the salt bridge. (b) The negative ions in the salt bridge move to the anode half-cell to maintain electrical neutrality as the anode releases positive ions. (c) The metal in the anode is iron (Fe) because the half-reaction at the anode involves Fe losing electrons and turning into Fe\(^{2+}\).

Step 1: (a) Path of electron flow

The electrons flow through the external circuit. The purpose of the salt bridge is to maintain the electrical neutrality of the solutions in the half-cells by allowing the exchange of ions, not electrons.

Step 2: (b) Movement of negative ions

The negative ions in the salt bridge move to the anode half-cell. In the reaction \(\mathrm{Fe}(s)+2\mathrm{Tl}^{+}(a q)\longrightarrow\mathrm{Fe}^{2+}(a q)+2\mathrm{Tl}(s)\), solid iron (Fe) loses electrons and becomes \(\mathrm{Fe}^{2+}(a q)\). This means the anode half-cell gains positively charged ions, so the negative ions from the salt bridge will move to the anode to maintain electrical neutrality.

Step 3: (c) Metal in the anode

The metal in the anode is iron (Fe). This is because the half-reaction at the anode involves Fe losing electrons and turning into Fe\(^{2+}\), as shown in the given reaction: \(\mathrm{Fe}(s) \rightarrow \mathrm{Fe}^{2+}(a q) + 2e^{-}\).

Key Concepts

Electrochemistry

Electrochemistry is the branch of chemistry that deals with the relationship between electrical energy and chemical reactions. It is a study of chemical processes that cause electrons to move. This field has vast applications, including batteries, fuel cells, and electrolysis.

A key component in the study of electrochemistry is the voltaic cell, also known as a galvanic cell. It is a device that converts chemical energy into electrical energy through spontaneous redox reactions. A salt bridge is a vital part of a voltaic cell as it maintains the charge balance by allowing ions to flow between the two half-cells, preventing the cell from quickly depleting its reactants.

Electron Flow

Electron flow is the movement of electrons from one point to another, which, in electrochemistry, occurs through conductive material. Electrons flow from the anode to the cathode in an external circuit within a voltaic cell, powering electronic devices or performing work as they go.

Understanding the direction of electron flow is crucial for determining the anode and cathode in a cell. Electrons are negatively charged, so they are repelled from the negative anode and attracted to the positive cathode. The salt bridge does not permit electron flow; instead, it enables ionic flow to maintain the electrical neutrality in the half-cells.

Half-Cell Reactions

Half-cell reactions are the individual oxidation and reduction reactions that occur in each half of a voltaic cell. In the context of the exercise, the oxidation half-reaction happens at the anode, where iron (Fe) is oxidized to \(\mathrm{Fe}^{2+}\) ions, releasing electrons in the process. These electrons travel through the external circuit to the cathode.

At the cathode, the reduction half-reaction occurs, where \(\mathrm{Tl}^{+}\) ions gain electrons and get reduced to thallium (Tl) metal. These two half-reactions are connected by the external circuit and the salt bridge, enabling the continuous flow of electrons and ions, respectively, thereby sustaining the cell's overall reaction.