CHAPTER 20 Electrochemistry Electrochemical reactions provide energy in all kinds of applications. Artist’s rendering of the Mars Rover Introduction to SECTION 1 Electrochemistry OBJECTIVES Identify parts of an electro- chemical cell and their functions. O xidation-reduction reactions involve energy changes. Because these Write electrode half reactions reactions involve electron transfer, the net release or net absorption of for cathodes and anodes. energy can occur in the form of electrical energy rather than as heat. This property allows for a great many practical applications of redox reactions. The branch of chemistry that deals with electricity-related applications of oxidation-reduction reactions is called electrochemistry. Electrochemical Cells Oxidation-reduction reactions involve a transfer of electrons. If the two www.scilinks.org substances are in contact with one another, a transfer of energy as heat Topic: Electrochemical Cells accompanies the electron transfer. In Figure 1 a zinc strip is in contact Code: HC60478 with a copper(II) sulfate solution. The zinc strip loses electrons to the copper(II) ions in solution. Copper(II) ions accept the electrons and fall out of solution as copper atoms. As electrons are transferred between zinc atoms and copper(II) ions, energy is released as heat, as indicated by the rise in temperature. 29 29 Before 28 After 28 27 27 26 26 25 25 24 24 23 23 22 22 21 21 Zinc strip FIGURE 1 Energy as heat given off when electrons are transferred 2+ CuSO4 Cu directly from Zn atoms to Cu ions causes the temperature of the aque- ous CuSO4 solution to rise. ELECTROCHEMISTRY 655 Conducting wire FIGURE 2 An electrochemical cell consists of two electrodes. Each electrode is in contact with an elec- Zinc electrode trolyte; the electrode and the elec- trolyte make up a half-cell. The two electrodes are connected by a wire, Copper electrode and a porous barrier separates the two electrolytes. Copper(II) sulfate Electrolyte Zinc sulfate Electrolyte Porous barrier Anode Cathode If, however, we separate the substance that is oxidized from the sub- stance that is reduced, the electron transfer is accompanied by a trans- fer of electrical energy instead of energy as heat. One means of separating oxidation and reduction half-reactions is with a porous bar- rier, or salt bridge. This barrier prevents the metal atoms of one half- reaction from mixing with the ions of the other half-reaction. Ions in the two solutions can move through the porous barrier, which keeps a charge from building up on the electrodes. Electrons can be transferred from one side to the other through an external connecting wire. Electric current moves in a closed loop path, or circuit, so this movement of elec- trons through the wire is balanced by the movement of ions in solution. Altering the system in Figure 1 as just described would simply involve separating the copper and zinc, as shown in Figure 2. The Zn strip is in an aqueous solution of ZnSO4. The Cu strip is in an aqueous solution of CuSO4. Both solutions conduct electricity, so, as you learned in Chapter 12, they are classified as electrolytes. An electrode is a conductor used to establish electrical contact with a nonmetallic part of a circuit, such as an electrolyte. In Figure 2, the Zn and Cu strips are electrodes. A sin- gle electrode immersed in a solution of its ions is a half-cell. The Half-Cells In the half-cell that contains the Zn electrode in aqueous ZnSO4 solution, the half-reaction is Zn(s) ⎯→ Zn2+(aq) + 2e−. The Zn metal loses two electrons to form Zn2+ ions in solution, and therefore oxidation is taking place in this half-cell. The electrode where oxidation occurs is called the anode. In the half-cell that contains the Cu electrode in aqueous CuSO4 solution, the half-reaction is Cu2+(aq) + 2e− ⎯→ Cu(s). In this half- reaction, the Cu2+ ions in solution gain electrons to become Cu solid; that is, reduction is taking place. The electrode where reduction occurs is called the cathode. 656 CHAPTER 20 Recall from Chapter 19 that these two half-reactions cannot occur separately. Both oxidation and reduction must occur in an electro- chemical reaction. The two half-cells taken together make an electro- www.scilinks.org chemical cell. In the Zn/Cu electrochemical cell, the electrons move Topic: Electrical Energy from the Zn electrode through the wire and down the Cu electrode to Code: HC60475 the Cu2+ ions at the electrode-solution interface. The Cu2+ ions are reduced to solid Cu, and the resulting Cu atoms attach themselves to the surface of the Cu electrode. For this reaction, charge is carried through the barrier by a combination of Zn2+(aq) ions moving from the 2− anode to the cathode and the SO4 (aq) ions moving from the cathode to the anode. The Complete Cell An electrochemical cell may be represented by the following notation: anode electrode⏐anode solution⏐⏐cathode solution⏐cathode electrode The double line represents the salt bridge, or porous barrier, between the two half-cells. For the present cell, the cell notation is Zn(s)⏐Zn2+(aq)⏐⏐Cu2+(aq)⏐Cu(s). The electrochemical reaction can be found by adding the anode half- reaction to the cathode half-reaction. This overall (or net) reaction is the following redox reaction: Zn(s) + Cu2+(aq) ⎯→ Zn2+(aq) + Cu(s) Although the two half-reactions occur at the same time, they occur at different places in the cell. Thus, for the reaction to proceed, electrons must pass through the wire that connects the two half-cells. An electrochemical cell that consists of the Zn and Cu reaction described above is called the Daniell Cell, named for the English Electrodes Porous barrier chemist John Frederick Daniell. The Daniell Cell can generate enough electricity to light up the light bulb shown in Figure 3. In electrochemi- FIGURE 3 The light bulb is cal cells, either a chemical reaction produces electrical energy or an powered by the reaction in this cell. electric current produces a chemical change. SECTION REVIEW 3. Write the half-reaction in which I−(aq) changes to 1. Why is the use of a salt bridge or porous barrier I2(s). Would this reaction occur at the anode or necessary in an electrochemical cell? cathode? + + 2. Given the Cu2 (aq)⏐Cu(s) and Mg2 (aq)⏐Mg(s) Critical Thinking half-reactions, where Cu2+(aq)⏐Cu(s) is the cath- 4. RELATING IDEAS Is the net chemical result of an ode reaction, electrochemical cell a redox reaction? Explain your a. write the overall reaction. answer. b. write the cell notation. ELECTROCHEMISTRY 657 SECTION 2 Voltaic Cells OBJECTIVES Describe the operation of voltaic cells, including dry V oltaic cells use spontaneous oxidation-reduction reactions to convert cells, lead-acid batteries, and chemical energy into electrical energy. Voltaic cells are also called galvan- fuel cells. ic cells. The most common application of voltaic cells is in batteries. Identify conditions that lead to corrosion and ways to prevent it. How Voltaic Cells Work Figure 4 shows an example of a voltaic cell: the Zn⏐⏐Cu electrochemi- Describe the relationship cal cell discussed in the previous section. between voltage and the movement of electrons. Electrons given up at the anode pass along the external connecting wire to the cathode. The movement of electrons through the wire must be balanced by the movement of ions in the solution. Thus, in Figure 4, Calculate cell voltage/poten- sulfate ions in the CuSO4 solution can move through the barrier into tials from a table of standard the ZnSO solution. electrode potentials. 4 – – Zinc atoms losing two e e Porous barrier electrons to become ions Zinc Copper Zinc atom, Zinc ion, strip strip Zn Zn2+ e– Copper(II) sulfate, CuSO4, solution Water molecule, H2O Sulfate 2– ion, SO4 Zinc sulfate, e– ZnSO4, solution Anode Cathode FIGURE 4 In a voltaic cell, electrons Copper Copper spontaneously flow from anode to cathode. + ion, Cu2 atom, Cu The copper strip gains mass as copper ions become copper atoms. The zinc strip loses Copper(II) ions gaining two mass as the zinc atoms become zinc ions. electrons to become atoms 658 CHAPTER 20 The dry cells pictured in Figure 5 are common sources of electrical energy. Like the wet cell previously described, dry cells are voltaic cells. Zinc carbon The three most common types of dry cells are the zinc-carbon battery, Alkaline batteries the alkaline battery, and the mercury battery. They differ in the sub- batteries stances being oxidized and reduced. Mercury battery Zinc-Carbon Dry Cells Batteries such as those used in flashlights are zinc-carbon dry cells. These cells consist of a zinc container, which serves as the anode, filled with a moist paste of MnO2, carbon black, NH4Cl, and ZnCl2, as illus- trated in Figure 6a. When the external circuit is closed, zinc atoms are oxidized at the negative electrode, or anode. +2 FIGURE 5 Many common batter- 0 + − Zn(s) ⎯→ Zn2 (aq) + 2e ies are simple voltaic dry cells. Electrons move across the circuit and reenter the cell through the carbon rod. The carbon rod is the cathode or positive electrode. Here MnO2 is reduced in the presence of H2O according to the following half-reaction. +4 +3 + + − ⎯→ + − 2MnO2(s) H2O(l) 2e Mn2O3(s) 2OH (aq) Alkaline Batteries www.scilinks.org The batteries found in a portable compact disc player or other small elec- Topic: Batteries tronic device are frequently alkaline dry cells. These cells do not have a Code: HC60139 carbon rod cathode, as in the zinc-carbon cell. The absence of the carbon rod allows them to be smaller.