Question

A voltaic cell similar to that shown in Figure 20.5 is constructed. One electrode half-cell 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 $$\mathrm{Fe}(s)+2{\mathrm{Ag}}^{+}(aq)⟶{\mathrm{Fe}}^{2+}(aq)+2\mathrm{Ag}(s)$$ (a) What is being oxidized, and what is being reduced? (b) Write the half-reactions that occur in the two half-cells. (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

Iron is being oxidized and silver ions are being reduced. The half-reactions are: Oxidation (anode): $\mathrm{Fe}\to {\mathrm{Fe}}^{2+}+2e^{-}$ Reduction (cathode): $2{\mathrm{Ag}}^{+}+2e^{-}\to 2\mathrm{Ag}$ The iron electrode is the anode (negative), and the silver electrode is the cathode (positive). Electrons flow from the iron electrode to the silver electrode. Ag⁺ cations migrate towards the silver cathode, Fe²⁺ cations migrate towards the iron anode, and the anions (NO₃⁻ and Cl⁻) migrate towards the iron anode.

Step-by-step solution

(a) Identifying Oxidation and Reduction Processes

Oxidation is the process of losing electrons, while reduction is the process of gaining electrons. In the overall cell reaction given: $$\mathrm{Fe}(s)+2{\mathrm{Ag}}^{+}(aq)⟶{\mathrm{Fe}}^{2+}(aq)+2\mathrm{Ag}(s)$$ Iron (Fe) loses two electrons and turns into Fe²⁺, and the silver ion (Ag⁺) gains one electron each to turn into solid silver (Ag). So, iron is being oxidized, and silver ions are being reduced.

(b) Writing Half-reactions

From the overall cell reaction, we can derive the oxidation and reduction half-reactions. The oxidation half-reaction is for iron, and the reduction half-reaction is for silver ions: Oxidation half-reaction (anode): $$\mathrm{Fe}(s)⟶{\mathrm{Fe}}^{2+}(aq)+2e^{-}$$ Reduction half-reaction (cathode): $$2{\mathrm{Ag}}^{+}(aq)+2e^{-}⟶2\mathrm{Ag}(s)$$

(c) Identifying Anode and Cathode

The electrode where oxidation occurs is called the anode, and the electrode where reduction takes place is called the cathode. In this voltaic cell, since the iron strip is oxidized, it is the anode. The silver strip is reduced, making it the cathode.

(d) Indicating the Signs of Electrodes

In a voltaic cell, the anode (where oxidation occurs) is the negative electrode, while the cathode (where reduction occurs) is the positive electrode. Thus, the iron electrode (anode) is negative, and the silver electrode (cathode) is positive.

(e) Direction of Electron Flow

Electrons flow from the anode (negative electrode) to the cathode (positive electrode). In this case, electrons flow from the iron electrode to the silver electrode.

(f) Migration of Cations and Anions

Cations (positively charged ions) migrate towards the cathode (the positive electrode), and anions (negatively charged ions) migrate towards the anode (the negative electrode). In this voltaic cell, Ag⁺ cations migrate towards the silver electrode (cathode), and Fe²⁺ cations migrate towards the iron electrode (anode). The anions, NO₃⁻ from AgNO₃ and Cl⁻ from FeCl₂, migrate towards the iron electrode (anode).

Key concepts

Oxidation and Reduction Reactions

Oxidation and reduction are two key concepts in electrochemistry, often referred to as 'redox' reactions. These are chemical processes involving the transfer of electrons between substances. Oxidation is defined as the loss of electrons, often resulting in an increase in oxidation state. Conversely, reduction is the gain of electrons, leading to a decrease in oxidation state.

In the context of the voltaic cell exercise, we examined the reaction between iron (Fe) and silver ions (Ag⁺). Iron undergoes oxidation because it loses electrons to become Fe²⁺ ions. In contrast, silver ions undergo reduction as they gain electrons to form metallic silver (Ag). Remembering that 'LEO goes GER' - 'Lose Electrons Oxidation, Gain Electrons Reduction' - can help you identify which species is oxidized and which is reduced in a redox reaction.

Half-Reactions in Electrochemistry

To fully comprehend a redox reaction, it is useful to break it down into half-reactions that show the oxidation or reduction process separately. A half-reaction is an equation that reflects the change in oxidation state of the species involved by explicitly showing the electrons gained or lost.

In the voltaic cell exercise, we have two half-reactions:

• The oxidation half-reaction at the anode: $$\mathrm{Fe}(s)⟶{\mathrm{Fe}}^{2+}(aq)+2e^{-}$$
• The reduction half-reaction at the cathode: $$2{\mathrm{Ag}}^{+}(aq)+2e^{-}⟶2\mathrm{Ag}(s)$$

Separating the overall process into these half-reactions makes it clear how many electrons are transferred in each step and shows the actual electrochemical process at each electrode.

Anode and Cathode Identification

Identifying the anode and cathode is crucial for understanding how a voltaic cell operates. The anode is where oxidation takes place, which means it is the source of electrons that flow through the external circuit. Because of this, the anode is typically the negative electrode in a voltaic cell. Meanwhile, the cathode is where reduction occurs, receiving electrons from the circuit; thus, it is the positive electrode.

In the example provided, the iron strip is the anode where oxidation of Fe to Fe²⁺ releases electrons. These electrons travel through the external circuit to the cathode, which is the silver strip. Here, the silver ions (Ag⁺) in solution gain those electrons and are reduced to solid silver (Ag). This movement of electrons from the anode to the cathode is the electrical current harnessed from the voltaic cell.