CHEMISTRY The Molecular Nature of Matter SIXTH EDITION Jespersen Brady Hyslop Chapter 6 Oxidation Reduction Reactions Copyright 2012 by John Wiley & Sons, Inc.
Acids as Oxidizing Agents Metals often react with acid Form metal ions and Molecular hydrogen gas Molecular equation Zn(s) + 2HCl(aq) H 2 (g) + ZnCl 2 (aq) Net ionic equation Zn(s) + 2H + (aq) H 2 (g) + Zn 2+ (aq) Zn is oxidized H + is reduced H + is the oxidizing reagent Zn is the reducing reagent 2
Oxidation of Metals by Acids Ease of oxidation process depends on metal Metals that react with HCl or H 2 SO 4 Easily oxidized by H + More active than hydrogen (H 2 ) e.g. Mg, Zn, alkali metals Mg(s) + 2H + (aq) Mg 2+ (aq) + H 2 (g) 2Na(s) + 2H + (aq) 2Na + (aq) + H 2 (g) Metals that don t react with HCl or H 2 SO 4 Not oxidized by H + Less active than H 2 e.g. Cu, Pt 3
Anion Determines Oxidizing Power We have Nonoxidizing and Oxidazing acids: 1. Nonoxidizing Acids Anion is weaker oxidizing agent than H 3 O + Only redox reaction is 2H + + 2e H 2 or 2H 3 O + + 2e H 2 + 2H 2 O HCl(aq), HBr(aq), HI(aq) H 3 PO 4 (aq) Cold, dilute H 2 SO 4 (aq) Most organic acids (e.g., acetic acid HC 2 H 3 O 2 ) 4
2. Oxidizing Acids Anion is stronger oxidizing agent than H 3 O + Used to react metals that are less active than H 2 No H 2 gas formed HNO 3 (aq) Concentrated Dilute Very dilute, with strong reducing agent H 2 SO 4 (aq) Hot, conc d, with strong reducing agent Hot, concentrated 5
Nitrate Ion as Oxidizing Agent A. Concentrated HNO 3 NO 3 more powerful oxidizing agent than H + e.g. NO 2 is product Partial reduction of N (+5 to +4) NO 3 (aq) + 2H + (aq) + e NO 2 (g) + H 2 O oxidation reduction 0 +5 +2 +4 Cu(s) + 2NO 3 (aq) + 4H + (aq) Cu 2+ (aq) + 2NO 2 (g) + 2H 2 O Reducing agent Oxidizing agent 6
Nitrate Ion as Oxidizing Agent B. Dilute HNO 3 NO 3 is more powerful oxidizing agent than H + NO is product Partial reduction of N (+5 to +2) NO 3 (aq) + 4H + (aq) + 3e NO(g) + 2H 2 O Used to react metals that are less active than H 2 e.g. Reaction of copper with dilute nitric acid 3Cu(s) + 8HNO 3 (dil, aq) 3Cu(NO 3 ) 2 (aq) + 2NO(g) + 4H 2 O 7
Reactions of Sulfuric Acid A. Hot, Concentrated H 2 SO 4 Becomes potent oxidizing agent SO 2 is product Partial reduction of S (+6 to +4) SO 4 2 + 4H + + 2e SO 2 (g) + 2H 2 O e.g. Cu + 2H 2 SO 4 (hot, conc.) CuSO 4 + SO 2 + 2H 2 O B. Hot, concentrated with strong reducing agent H 2 S is product Complete reduction of S (+6 to 2) SO 4 2 + 10H + + 8e H 2 S(g) + 4H 2 O e.g. 4Zn + 5H 2 SO 4 (hot, conc.) 4ZnSO 4 + H 2 S + 4H 2 O 8
Redox Reactions of Metals Acids reacting with metal Special case of more general phenomena Single Replacement Reaction Reaction where one element replaces another A + BC AC + B Metal A can replace metal B If A is more active metal 9
Single Replacement Reaction Left = Zn(s) + CuSO 4 (aq) Center = Cu 2+ (aq) reduced to Cu(s) Zn(s) oxidized to Zn 2+ (aq) Right = Zn bar becomes coated with Cu(s) Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) 10
Single Replacement Reaction Zn 2+ ions replace Cu 2+ ions in solution Cu atoms replace Zn atoms in solid Cu 2+ oxidizes Zn 0 to Zn 2+ Zn 0 reduces Cu 2+ to Cu 0 More active Zn 0 replaces less active Cu 2+ Zn 0 is easier to oxidize! 11
Activity Series of Metals Cu less active, can't replace Zn 2+ Can't reduce Zn 2+ Cu(s) + Zn 2+ (aq) No reaction General phenomenon Metal that gets oxidized easily will displace one that is less easily oxidized Activity Series (Table 6.3) Metals at bottom more easily oxidized (more active) than those at top This means that any metal ion will be displaced from its compounds by another metal below in table 12
Table 6.3 Activity Series of Some Metals 13