Oxidation & Reduction Reactions

Transcription

1 & Reduction Reactions CHAPTER 6 Chemistry: The Molecular Nature of Matter, 6 th edition By Jesperson, Brady, & Hyslop

2 CHAPTER 6: & Reduction Learning Objectives q Define oxidation and reduction q Identify redox reactions q Recognize oxidation numbers q Balancing redox reactions q Acidic conditions q Basic conditions q Acids as oxidizing agents q Oxygen as oxidizing agents q Single replacement reactions q Navigate the activity series q Practice Redox Stoichiometry 2

3 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 3

4 Acids as Oxidizing Agents: Metals 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 4

5 Acids as Oxidizing Agents: Anions Determine Oxidizing Power Acids are divided into two classes: 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., HC 2 H 3 O 2 ) 5

6 Acids as Oxidizing Agents 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 6

7 Acids as Oxidizing Agents: Focus on Nitrate Ion Concentrated HNO 3 NO 3 more powerful oxidizing agent than H + 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 Cu(s) + 2NO 3 (aq) + 4H + (aq) Cu 2+ (aq) + 2NO 2 (g) + 2H 2 O Reducing agent Oxidizing agent 7

8 Acids as Oxidizing Agents: Focus on Nitrate Ion 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 Reaction of copper with dilute nitric acid 3Cu(s) + 8HNO 3 (dil, aq) 3Cu(NO 3 ) 2 (aq) + 2NO(g) + 4H 2 O 8

9 Acids as Oxidizing Agents: Focus on Sulfuric Acid Hot, Concentrated H 2 SO 4 Becomes potent oxidizer SO 2 is product Partial reduction of S (+6 to +4) SO H + + 2e SO 2 (g) + 2H 2 O Ex: Cu + 2H 2 SO 4 (hot, conc.) CuSO 4 + SO 2 + 2H 2 O Hot, concentrated with strong reducing agent H 2 S is product Complete reduction of S (+6 to 2) SO H + + 8e H 2 S(g) + 4H 2 O Ex: 4Zn + 5H 2 SO 4 (hot, conc.) 4ZnSO 4 + H 2 S + 4H 2 O 9

10 Single Replacement Reaction Acids reacting with metal Special case of more general phenomena: Single Replacement Reaction Reaction where one element replaces another A + BC AC + B 1. Metal A can replace metal B If A is more active metal, or 2. Nonmetal A can replace nonmetal C If A is more active than C 10

11 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 = Cu(s) plated out on Zn bar Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) 11

12 Single Replacement Reaction Zn 2+ ions take place of Cu 2+ ions in solution Cu atoms take place of 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! 12

13 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 is more easily oxidized will displace one that is less easily oxidized from its compounds Activity Series (Table 6.3) Metals at bottom more easily oxidized (more active) than those at top This means that given metal ion will be displaced from its compounds by any metal below it in table 13

14 Activity Series of Metals 2H + (aq) + Sr(s) Sr 2+ (aq) + H 2 (g) H + oxidizes Sr 0 to Sr 2+ Sr 0 reduces H + to H 2 More active Sr 0 replaces less active H + Sr 0 is easier to oxidize! H 2 (g) + Sr 2+ (aq) NO REACTION! Why? H 2 less active, can't replace Sr 2+ Can't reduce Sr 2+ 14

15 Activity Series of Metals 15

16 Activity Series of Metals Metals at very bottom of table Very strong reducing agents Very easily oxidized Na down to Cs Alkali and alkaline earth metals React with H 2 O as well as H + 2Na(s) + 2H 2 O H 2 (g) + 2NaOH(aq) 16

17 Activity Series of Metals Ag slight reaction (top of activity series) 2HCl(aq) + Ag(s) 2AgCl(aq) + H 2 (g) Zn somewhat reactive (middle of activity series) 2HCl(aq) + Zn(s) ZnCl 2 (aq) + H 2 (g) Mg very reactive (bottom of activity series) 2HCl(aq) + Mg(s) MgCl 2 (aq) + H 2 (g) 17

18 Activity Series of Metals: Predicting Reactions If M is below H Can displace H from solutions containing H + 2H + (aq) H 2 (g) If M is above H Doesn't react with nonoxidizing acids HCl, H 3 PO 4, etc. In general Metal below replaces ion above 18

19 Activity Series of Metals: Predicting Reactions Predictive tool for determining outcome of single replacement reactions Given a metal (M ) and the ion of a different metal (M ' n+ ) Look at chart and draw arrow from M to M ' n+ Arrow that points up from bottom left to top right will occur Arrow that points down from top left to bottom right will NOT occur 19

20 Oxygen as an Oxidizing Agent: Combustion Oxygen reacts with many substances Combustion Rapid reaction of substance with oxygen that gives off both heat and light Hydrocarbons are important fuels Products depend on how much O 2 is available 1. Complete Combustion O 2 plentiful CO 2 and H 2 O products e.g. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O 2C 8 H 18 (g) + 25O 2 (g) 16CO 2 (g) + 18H 2 O 20

21 Oxygen as an Oxidizing Agent: Combustion 2. Incomplete Combustion Not enough O 2 a. Limited O 2 supply CO(g) is the carbon product 2CH 4 (g) + 3O 2 (g) 2CO(g) + 4H 2 O b. Very limited O 2 C(s) is the carbon product CH 4 (g) + O 2 (g) C(s) + 2H 2 O Gives tiny black particles Soot lamp black Component of air pollution 21

22 Oxygen as an Oxidizing Agent: Combustion 3. Incomplete Combustion Not enough O 2 a. Limited O 2 supply CO(g) is the carbon product 2CH 4 (g) + 3O 2 (g) 2CO(g) + 4H 2 O b. Very limited O 2 C(s) is the carbon product CH 4 (g) + O 2 (g) C(s) + 2H 2 O Gives tiny black particles Soot lamp black Component of air pollution 22

23 Oxygen as an Oxidizing Agent: Combustion 4. Combustion of Organics containing O Still produce CO 2 and H 2 O Need less added O 2 C 12 H 22 O 11 (s) + 12O 2 (g) 12CO 2 (g) + 11H 2 O 5. Combustion of Organics containing S Produce SO 2 as product 2C 4 H 9 SH + 15O 2 (g) 8CO 2 (g) + 10H 2 O + 2SO 2 (g) SO 2 turns into acid rain when mixed with water SO 2 oxidized to SO 3 SO 3 reacts with H 2 O to form H 2 SO 4 23

24 Oxygen as an Oxidizing Agent: Corrosion Corrosion Direct reaction of metals with O 2 Many metals corrode or tarnish when exposed to O 2 Ex. 2Mg(s)+ O 2 (g) 2MgO(s) 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s) 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) 4Ag(s) + O 2 (g) 2Ag 2 O(s) 24

25 Oxygen as an Oxidizing Agent: Reacting with Nonmetals Many nonmetals react directly with O 2 to form nonmetal oxides Sulfur reacts with O 2 Forms SO 2 S(s) + O 2 (g) 2SO 2 (g) Nitrogen reacts with O 2 Forms various oxides NO, NO 2, N 2 O, N 2 O 3, N 2 O 4, and N 2 O 5 Dinitrogen oxide, N 2 O Laughing gas used by dentists Propellant in canned whipped cream 25

26 Example of Redox Stoichiometry Same as other stoichiometry problems, except balancing the equation is more complicated. Ex. How many grams of Na 2 SO 3 (126.1 g/mol) are needed to completely react with 12.4 g of K 2 Cr 2 O 7 (294.2 g/mol)? First need balanced redox equation 8H + (aq) + Cr 2 O 7 2 (aq) + 3SO 3 2 (aq) 3SO 4 2 (aq) + 2Cr 3+ (aq) + 4H 2 O Then do calculations 1. g K 2 Cr 2 O 7 moles K 2 Cr 2 O 7 moles Cr 2 O 7 2 (aq) 2. moles Cr 2 O 7 2 (aq) moles 3SO 4 2 (aq) 3. moles SO 3 2 (aq) moles Na 2 SO 3 g Na 2 SO 3 26

27 Example of Redox Stoichiometry grams K 2 Cr 2 O 7 moles K 2 Cr 2 O 7 moles Cr 2 O 7 2 (aq) 12.4 g K = 2 Cr 2 O mol 7 Cr 2 O 7 1 mol moles Cr 2 O 7 2 (aq) moles 3SO 3 2 (aq) moles SO 3 2 (aq) moles Na 2 SO 3 g Na 2 SO 3 K g K 2 Cr 2 2 Cr 2 O 2 7 O mol Cr2O 1 mol K Cr O 2 3 mol SO mol Cr2O7 2 = mol SO 3 1 mol Cr O mol Na SO mol SO3 1 mol SO g Na 1 mol Na 2 2 SO SO 3 3 = 15.9 g Na 2 SO 3 27