PAGE 8-1 ANSWERS TO END-OF-CHAPTER QUESTIONS CHAPTER 8: ENERGY FROM ELECTRON TRANSFER Emphasizing Essentials 1. a. Define the terms oxidation and reduction. b. Why must these processes take place together? Answer: a. Oxidation is a process in which an atom, ion, or molecule loses one or more electrons. Reduction is a process in which an atom, ion, or molecule gains one or more electrons. b. Electrons must be transferred from the species losing electrons to the species gaining electrons. 2. Which of these half-reactions represent oxidation and which reduction? Explain your reasoning. a. Fe(s) → Fe2+(aq) + 2 e– b. Ni4+(aq) + 2 e– → Ni2+(aq) c. 2 H2O(l) + 2 e– → H2(g) + 2 OH–(aq) Answer: a. Oxidation. Iron loses two electrons to form the iron(II) ion. b. Reduction. The nickel(IV) ion gains two electrons to form the nickel(II) ion. c. Reduction. Water gains two electrons to the hydrogen ion and the hydroxide ion. 3. You have seen several examples of oxidation–reduction reactions in this chapter. Now examine these equations and decide which are oxidation–reduction reactions and which are not. Explain your decisions. a. Zn(s) + 2 MnO2(s) + H2O(l) → Zn(OH)2(s) + Mn2O3(s) b. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) c. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) Answer: Parts a and c are redox reactions. Electrons must be transferred when an element (Zn in part a, O2 in part c) reacts to form a compound. Part b is a neutralization reaction in which ions combine to form a soluble salt and water. Electron transfer does not occur in this case. 4. Comment on the statement: Every combustion reaction is an oxidation–reduction reaction. Answer: This is a true statement. Combustion is the rapid combination of a fuel and oxygen to form products. In the case of burning a hydrocarbon, these products are CO2 and H2O. The oxygen is reduced, and the hydrogen and the carbon are oxidized to form H2O and CO2, respectively.PAGE 8-2 Even when substances that do not contain carbon are burned, the reactions can still be described in terms of oxidation and reduction. 5. Two common units associated with electricity are the volt and the amp. What does each unit measure? Answer: Electric current (an amount of charge per second) is measured in amps. In contrast, the volt is a measure of electric potential, that is, the force or “pressure” behind this current. 6. Consider this galvanic cell. A coating of impure silver metal begins to appear on the surface of the silver electrode as the cell discharges. a. Identify the anode and write the oxidation half-reaction. b. Identify the cathode and write the reduction half-reaction. Answer: a. The anode is Zn(s) and the oxidation half-reaction is: Zn(s) → Zn2+(aq) + 2 e– b. The cathode is Ag(s) and the reduction half-reaction is: 2 Ag+(aq) + 2 e– → 2 Ag(s) 7. In the lithium–iodine cell, Li is oxidized to Li+; I2 is reduced to 2 I–. a. Write equations for the two half-reactions that take place in this cell, labeling one as oxidation and the other as reduction. b. Write an equation for the overall reaction in this cell. c. Identify the half-reaction that occurs at the anode and the half-reaction that occurs at the cathode. Answer: a. oxidation half-reaction: Li(s) → Li+ + e– reduction half-reaction: I2(s) + 2 e– → 2 I– b. overall reaction: 2 Li(s) + I2(s) → 2 LiI(s) c. The oxidation reaction, Li(s) → Li+(aq) + e–, occurs at the anode. The reduction reaction, I2(s) + 2 e– → 2 I–(aq), occurs at the cathode.PAGE 8-3 8. a. Is the voltage from a tiny AAA-size alkaline cell the same as that from a large D alkaline cell? Explain. b. Will both batteries sustain the flow of electrons for the same time? Why or why not? Answer: a. The voltage from both kinds of cells is the same (1.54 V) because voltage depends on the chemical reaction that is producing the electrical energy and not the size of the electrodes. The biggest difference in these two kinds of cells is the amount of current each produces. The amount of current produced by a cell does depend on the size of the cell: Larger cells generate more current than smaller cells do. The larger D cell will generate a larger current, but the same voltage as the smaller AAA cells. b. Larger cells contain more materials and can sustain the transfer of electrons over a longer period. 9. Identify the type of battery commonly used in each of these consumer electronic products. Assume none uses solar cells. a. battery-powered watch c. digital camera b. MP3 player d. handheld calculator Answer: a. battery-powered watch: silver oxide battery b. MP3 player: Lithium ion battery c. digital camera: Lithium ion battery d. calculator: AA or AAA alkaline batteries 10. The mercury battery has been used extensively in medicine and industry. Its overall reaction can be represented by this equation. HgO(l) + Zn(s) → ZnO(s) + Hg(l) a. Write the oxidation half-reaction. b. Write the reduction half-reaction. c. Why is the mercury battery no longer in common use? Answer: a. Oxidation: Zn(s) + 2 OH–(aq) → ZnO(s) + H2O(l) + 2 e– b. Reduction: HgO(s) + H2O(l) + 2 e– → Hg(l) + 2 OH–(aq) c. In both 1970 and 1980, a major use for mercury was in batteries. By 1990, awareness of the dangers of mercury in urban trash had grown. Mercury is a toxic metal and (in some forms) can accumulate in the biosphere. Safer batteries and the need to recycle batteries led to the passage of the Mercury-Containing and Rechargeable Battery Management Act (The Battery Act) in 1996. 11. a. What is the function of the electrolyte in a galvanic cell? b. What is the electrolyte in an alkaline cell? c. What is the electrolyte in a lead–acid storage battery?PAGE 8-4 Answer: a. The electrolyte completes the electrical circuit. It provides a medium for transport of ions, thus allowing charge to be transferred. b. KOH paste c. H2SO4(aq) 12. These are the incomplete equations for the half-reactions in a lead storage battery. They do not show the electrons either lost or gained. Pb(s) + SO42–(aq) → PbSO4(s) PbO2(s) + 4 H+(aq) + SO42–(aq) → PbSO4(s) + 2 H2O(l) a. Balance both equations with respect to charge by adding electrons on