Question

A voltaic cell similar to that shown in Figure \(20.5\) is constructed. One electrode compartment consists of a silver strip placed in a solution of \(\mathrm{AgNO}_{3}\), and the other has an iron strip placed in a solution of \(\mathrm{FeCl}_{2}\). The overall cell reaction is $$ \mathrm{Fe}(s)+2 \mathrm{Ag}^{+}(a q) \longrightarrow \mathrm{Fe}^{2+}(a q)+2 \mathrm{Ag}(s) $$ (a) What is being oxidized, and what is being reduced? (b) Write the half-reactions that occur in the two electrode compartments. (c) Which electrode is the anode, and which is the cathode? (d) Indicate the signs of the electrodes. (e) Do electrons flow from the silver electrode to the iron electrode, or from the iron to the silver? (f) In which directions do the cations and anions migrate through the solution?

Short answer

(a) Iron (Fe) is being oxidized, and silver ions (Ag^+) are being reduced. (b) The two half-reactions are: \[ \mathrm{Fe}(s) \longrightarrow \mathrm{Fe}^{2+}(aq) + 2e^- \] \[ 2\mathrm{Ag}^{+}(aq) + 2e^- \longrightarrow 2\mathrm{Ag}(s) \] (c) The anode is the iron strip, and the cathode is the silver strip. (d) The iron electrode (anode) is negative, and the silver electrode (cathode) is positive. (e) Electrons flow from the iron electrode to the silver electrode. (f) Cations (Fe^2+ and Ag^+) migrate towards the anode (iron strip), and anions (NO3^- and Cl^-) migrate towards the cathode (silver strip).

Step-by-step solution

Identify the oxidation and reduction processes

First, let's analyze the overall cell reaction: \[ \mathrm{Fe}(s)+2\mathrm{Ag}^{+}(aq) \longrightarrow \mathrm{Fe}^{2+}(aq)+2\mathrm{Ag}(s) \] Here, the iron (Fe) is going from an elemental state (0 oxidation state) to an ion (Fe^2+), losing two electrons in the process. This change corresponds to the oxidation process. The silver ions (Ag^+) are initially in an oxidized state (+1 oxidation state) and are converted into elemental silver (Ag) through a gain of electrons, corresponding to the reduction process. (a) Iron (Fe) is being oxidized, and silver ions (Ag^+) are being reduced.

Write the half-reactions

Now let's write the half-reactions for the oxidation and reduction processes. Oxidation half-reaction: \[ \mathrm{Fe}(s) \longrightarrow \mathrm{Fe}^{2+}(aq) + 2e^- \] Reduction half-reaction: \[ 2\mathrm{Ag}^{+}(aq) + 2e^- \longrightarrow 2\mathrm{Ag}(s) \] (b) The two half-reactions are: \[ \mathrm{Fe}(s) \longrightarrow \mathrm{Fe}^{2+}(aq) + 2e^- \] \[ 2\mathrm{Ag}^{+}(aq) + 2e^- \longrightarrow 2\mathrm{Ag}(s) \]

Determine the anode and cathode

The electrode at which oxidation occurs is called the anode. In this case, the anode is the iron strip, where Fe is oxidized to Fe^2+. The electrode at which reduction occurs is called the cathode. In this case, the cathode is the silver strip, where Ag^+ is reduced to Ag. (c) The anode is the iron strip, and the cathode is the silver strip.

Assign the signs of the electrodes

The anode, where oxidation occurs, is assigned a negative sign, while the cathode, where reduction occurs, is assigned a positive sign. (d) The iron electrode (anode) is negative, and the silver electrode (cathode) is positive.

Determine the direction of electron flow

Electrons flow from the negatively charged anode (the iron strip) to the positively charged cathode (the silver strip). (e) Electrons flow from the iron electrode to the silver electrode.

Analyze the migration of cations and anions in the solution

Cations (positively charged ions) migrate towards the negatively charged electrode (anode), while anions (negatively charged ions) migrate towards the positively charged electrode (cathode). When the iron is oxidized, Fe^2+ ions are released into the solution, and these Fe^2+ cations migrate towards the negatively charged iron strip. Ag^+ cations from the AgNO3 solution migrate towards the positively charged silver strip (cathode) where they are reduced. The nitrate anions (NO3^-) from the AgNO3 solution migrate towards the positively charged silver strip (cathode), while the chloride anions (Cl^-) from the FeCl2 solution migrate towards the negatively charged iron strip (anode). (f) Cations (Fe^2+ and Ag^+) migrate towards the anode (iron strip), and anions (NO3^- and Cl^-) migrate towards the cathode (silver strip).

Key concepts

Oxidation and Reduction

In a voltaic cell, oxidation and reduction are key processes that drive the generation of electrical energy.

• Oxidation: This process involves the loss of electrons. For the given reaction in the cell, iron (Fe) undergoes oxidation. It starts with an oxidation state of 0 and becomes Fe²⁺ by losing two electrons. This change signifies that Fe is oxidized.


• Reduction: On the flip side, reduction is the gain of electrons. Silver ions (Ag⁺) in this voltaic cell gain electrons to form solid silver (Ag). Consequently, Ag⁺ is reduced during this process.

Remember, electrons are lost in oxidation but gained in reduction. These complementary reactions occur simultaneously in a voltaic cell.

Half-Reactions

Half-reactions break down the overall reaction into two separate parts, highlighting the specific oxidation and reduction processes.

• Oxidation Half-Reaction: This half-reaction focuses on the species that lose electrons: \[ \mathrm{Fe}(s) \rightarrow \mathrm{Fe}^{2+}(aq) + 2e^- \] Here, solid iron loses two electrons to become Fe²⁺, showing the oxidation event.


• Reduction Half-Reaction: This half-reaction represents the species that gain electrons: \[ 2\mathrm{Ag}^{+}(aq) + 2e^- \rightarrow 2\mathrm{Ag}(s) \] Silver ions gain two electrons in total (one electron per Ag⁺) to form elemental silver, marking the reduction.

Each half-reaction reveals how electrons are transferred, clarifying the electrochemical processes occurring in the cell.

Anode and Cathode

In a voltaic cell, electrodes are essential parts where oxidation and reduction occur.

• Anode: This is where oxidation takes place. In our example, the anode is the iron strip, where iron atoms lose electrons to form Fe²⁺ ions. Remember, the mnemonic "An Ox" suggests that Anode is where Oxidation happens.


• Cathode: Oppositely, the cathode is where reduction occurs. In this cell, silver ions are reduced at the cathode, which is the silver strip. Similarly, "Red Cat" helps you recall that Reduction occurs at the Cathode.

Identifying anode and cathode is crucial, as it determines the direction of electron flow and ion migration in the cell.

Electron Flow

Electrons travel through the external circuit of a voltaic cell, creating a current. - Path of Electron Flow: Electrons move from the negative electrode (anode) to the positive electrode (cathode).
- In our set-up, electrons flow from the iron electrode (anode) to the silver electrode (cathode).
- Reason for Flow: The anode, being negatively charged due to oxidation, naturally repels electrons toward the positive cathode. This movement of electrons through the circuit is what allows the voltaic cell to do work, such as lighting a bulb or powering a device.

Ion Migration

Ion migration is the movement of ions in the solution and solid structures of the cell. It's essential for maintaining the charge balance during electron flow.

• Cation Movement: Cations, which are positively charged ions, move towards the cathode. - In the discussed voltaic cell, Fe²⁺ ions remain in the solution, while Ag⁺ ions migrate to the silver electrode.


• Anion Movement: Anions, which carry a negative charge, head towards the anode. - Nitrate ions (NO₃^-) and chloride ions (Cl^-) move to neutralize the charge at their respective electrodes.

Ion migration ensures that the cell operates smoothly by preventing charge buildup and allowing continuous electron flow.