Solutions_Manual_chapter18 - CHAPTER 18 ELECTROCHEMISTRY Review of Oxidation-Reduction Reactions 15 Oxidation increase in oxidation number loss of

Solutions_Manual_chapter18 - CHAPTER 18 ELECTROCHEMISTRY...

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684 CHAPTER 18 ELECTROCHEMISTRY Review of Oxidation - Reduction Reactions 15. Oxidation: increase in oxidation number; loss of electrons Reduction: decrease in oxidation number; gain of electrons 16. See Table 4.2 in Chapter 4 of the text for rules for assigning oxidation numbers. a. H (+1), O (!2), N (+5) b. Cl (!1), Cu (+2) c. O (0) d. H (+1), O (!1) e. H(+1), O (!2), C (0) f. Ag (0) g. Pb (+2), O (!2), S (+6) h. O (!2), Pb (+4) i. Na (+1), O ( ! 2), C (+3) j. O ( ! 2), C (+4) k. (NH 4 ) 2 Ce(SO 4 ) 3 contains NH 4 + ions and SO 4 2 ions. Thus cerium exists as the Ce 4+ ion. H (+1), N ( ! 3), Ce (+4), S (+6), O ( ! 2) l. O ( ! 2), Cr (+3) 17. The species oxidized shows an increase in oxidation numbers and is called the reducing agent. The species reduced shows a decrease in oxidation numbers and is called the oxidizing agent. The pertinent oxidation numbers are listed by the substance oxidized and the substance reduced. Substance Substance Redox?Ox. AgentRed. AgentOxidized Reduced 18. a. 4 NH 3 (g) + 5 O 2 (g) 4 NO(g) + 6 H 2 O(g) 3 +1 0 +2 2 +1 2 oxidation numbers 2 NO(g) + O 2 (g) 2 NO 2 (g) +2 2 0 +4 2
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CHAPTER 18 ELECTROCHEMISTRY 685 3 NO 2 (g) + H 2 O(l) 2 HNO 3 (aq) + NO(g) +4 2 +1 2 +1 +5 2 +2 2 All three reactions are oxidation-reduction reactions since there is a change in oxidation numbers of some of the elements in each reaction. b. 4 NH 3 + 5 O 2 4 NO + 6 H 2 O; O 2 is the oxidizing agent and NH 3 is the reducing agent. 2 NO + O 2 2 NO 2 ; O 2 is the oxidizing agent and NO is the reducing agent. 3 NO 2 + H 2 O 2 HNO 3 + NO; NO 2 is both the oxidizing and reducing agent. Questions 19. Electrochemistry is the study of the interchange of chemical and electrical energy. A redox (oxidation-reduction) reaction is a reaction in which one or more electrons are transferred. In a galvanic cell, a spontaneous redox reaction occurs that produces an electric current. In an electrolytic cell, electricity is used to force a nonspontaneous redox reaction to occur. 20. Mass balance indicates that we have the same number and type of atoms on both sides of the equation (so that mass is conserved). Similarly, net charge must also be conserved. We cannot have a buildup of charge on one side of the reaction or the other. In redox reactions, electrons are used to balance the net charge between reactants and products. 21. Magnesium is an alkaline earth metal; Mg will oxidize to Mg 2+ . The oxidation state of hydrogen in HCl is +1. To be reduced, the oxidation state of H must decrease. The obvious choice for the hydrogen product is H 2 (g), where hydrogen has a zero oxidation state. The balanced reaction is Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g). Mg goes from the 0 to the +2 oxidation state by losing two electrons. Each H atom goes from the +1 to the 0 oxidation state by gaining one electron. Since there are two H atoms in the balanced equation, then a total of two electrons are gained by the H atoms. Hence two electrons are transferred in the balanced reaction. When the electrons are transferred directly from Mg to H + , no work is obtained. In
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