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Oxidation Numbers and Redox Reactions

Oxidation Numbers and Redox Reactions. Questions. Three questions you should be able to answer at the end of this lesson: What are oxidation numbers and how are they assigned? What is a redox reaction? What are oxidizing and reducing agents?. What is an oxidation number?.

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Oxidation Numbers and Redox Reactions

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  1. Oxidation Numbers and Redox Reactions

  2. Questions • Three questions you should be able to answer at the end of this lesson: • What are oxidation numbers and how are they assigned? • What is a redox reaction? • What are oxidizing and reducing agents?

  3. What is an oxidation number? • It is a “hypothetical” number. • shows charge an atom or ion would have if separated from its cmpd. • Doesn’t represent anything real. • Useful for tracking e- in chem. rxns. • 6 rules for assigning oxidation numbers (O.N.’s)

  4. Assigning Oxidation Numbers • 1. The O.N. of all uncombined elements is zero. • Example: The O.N. of O2, Fe, S8, etc, are all zero. • 2. The O.N. of any monatomic ion is equal to the charge of the ion. • Group 1 ions (Li+, Na+, K+, etc) = +1 • Group 2 ions (Be2+, Mg2+, Ca2+, etc) = +2 • Group 13 ions (B3+, Al3+, Ga3+, etc) = +3 • iron in FeCl3 = +3 • How do we know? • sulfur in Na2S = -2 • How do we know?

  5. Assigning Oxidation Numbers • 3. The O.N. of oxygen in most compounds is –2. • Examples: • The O.N. of oxygen in CO2, H2O, CH3OH, and HNO3 is -2 • Exceptions: • The O.N. of oxygen in peroxides is -1. • H2O2 is only important example. • The O.N. of oxygen in OF2 is +2. • Only case where oxygen’s O.N. is +.

  6. Assigning Oxidation Numbers • 4. The O.N. of H is +1 when bonded to nonmetals and –1 when bonded to metals. • Examples: • The O.N. of H in HCl, NH3, and H2O is +1. • The O.N. of H in NaH and BeH2 is –1. • When in doubt, call it +1. • The O.N. of H in NaHCO3 is +1. • The O.N. of H in Fe(OH)3 is +1.

  7. Assigning Oxidation Numbers • 5. In a neutral compound, the sum of the O.N.s of all atoms is zero. • Example: KNO3 • K = +1 (rule 2) • O = -2 (rule 3) • N: • (+1) + N + 3(-2) = 0 (Rule 5) • 1 + N – 6 = 0 • N = +5

  8. Assigning Oxidation Numbers • 6. In a polyatomic ion, the sum of the O.N.s of all atoms is equal to the charge of the ion. • Example: SO42- • O = -2 (rule 3) • S: • S + 4(-2) = -2 (rule 6) • S – 8 = -2 • S = +6

  9. -2 -2 -2 Assigning Oxidation Numbers • What are the O.N.s of the elements in NaClO3? Na+ = +1 (Rule 2) +1 O = -2 (Rule 3) Na+ Na + Cl + (3 x O) = 0 (Rule 5) O - (+1) + Cl + (3 x -2) = 0 Cl (+1) + Cl + (-6) = 0 O O +5 Cl = +5 Cl – 5 = 0

  10. Assigning Oxidation Numbers • What are the O.N.s of the elements in NaClO2? • Na = +1 (rule 2) • O = -2 (rule 3) • Cl: • (+1) + Cl + 2(-2) = 0 (rule 5) • 1 + Cl – 4 = 0 • Cl = +3

  11. Assigning Oxidation Numbers • What are the O.N.s of the elements in (NH4)2Cr2O7? • Two ions: NH4+ and Cr2O72- • H = +1 (rule 4) • N: • N + 4(+1) = +1 (rule 6) • N + 4 = 1 • N = -3

  12. Assigning Oxidation Numbers • What are the O.N.s of the elements in (NH4)2Cr2O7? (continued) • Cr2O72- • O = -2 (rule 3) • Cr: • 2Cr + 7(-2) = -2 (rule 6) • 2Cr – 14 = -2 • 2Cr = +12 • Cr = +6

  13. What is the oxidation number of Br in each of the following compounds? HBr NaBrO NaBrO2 NaBrO3 NaBrO4 -1 +1 +3 +5 +7 Assigning Oxidation Numbers • What is the oxidation number of N in each of the following compounds/ions? • N2 • NH3 • NO3- • N2O4 • NO • 0 • -3 • +5 • +4 • +2

  14. Redox Reactions • Short for oxidation-reduction reactions. • A rxn in which O.N.s change due to transfer of electrons. • Originally defined in terms of organic chemistry: • Oxidation = gain of oxygen or loss of hydrogen • Reduction = loss of oxygen or gain of hydrogen

  15. H H H Reduced H C C C H H H H H H C C C H H H H H H Oxidized Original Definition of Redox

  16. H H Oxidized H C C O H H H H O H C C O H H Reduced Original Definition of Redox Not Balanced H+ Cr2O72- H2O Cr3+

  17. Modern Definition • Has been expanded to cover reactions that don’t involve O or H. • Oxidation: Loss of electrons or increase in oxidation number. • Reduction: Gain of electrons or decrease in oxidation number.

  18. Memory Trick! • Just remember: LEO says “GER!” • LEO: Loss of Electrons is Oxidation • GER: Gain of Electrons is Reduction

  19. Redox Reactions • Oxidation cannot happen w/o reduction, and vice versa.

  20. Redox Reactions • What is being oxidized and reduced in the following reaction? • Mg + Cl2 MgCl2

  21. Mg + Cl2 MgCl2 • First, assign oxidation numbers to all of the atoms: • Reactant side: • Mg = 0 (rule 1) • Cl = 0 (rule 1) • Product side: • Mg2+ = +2 (rule 2) • Cl- = -1 (rule 2)

  22. oxidation 0 +2 -1 0 Mg Cl2 MgCl2 reduction Mg + Cl2 MgCl2

  23. Mg Cl- Mg2+ Cl- Cl2 Mg + Cl2 MgCl2

  24. Redox Reactions • What is being oxidized and reduced in the following reaction? • Cu + 4HNO3 Cu(NO3)2 + 2NO2 + 2H2O • First assign O.N.s to all atoms. • Next, look for O.N.s that change in the course of the rxn.

  25. oxidation reduction Redox Reactions 0 +1 +5 -2 +2 +5 -2 +4 -2 +1 -2 Cu + 4HNO3 Cu(NO3)2 + 2NO2 + 2H2O

  26. Redox Reactions • See if you can determine what’s being oxidized and reduced in the following reactions: • 2NO + O2 2NO2 • 2Fe + 3Cu(NO3)2  2Fe(NO3)3 + 3Cu • HCl + NaOH  NaCl + H2O

  27. Answers • 2NO + O2 2NO2 • N is being oxidized from +2 to +4. • O is being reduced from 0 to –2. • 2Fe + 3Cu(NO3)2 2Fe(NO3)3 + 3Cu • Fe is being oxidized from 0 to +3 • Cu is being reduced from +2 to 0. • HCl + NaOH  NaCl + H2O • Nothing is being oxidized or reduced. • This is not a redox reaction.

  28. Oxidizing and Reducing Agents • Oxidizing Agent • A molecule or ion that contains an atom capable of being reduced. • Electron acceptor. • Causes oxidation in other molecules/ions. • Some examples of oxidizing agents: • MnO4-, F2, O2, ClO4-, etc.

  29. Oxidizing and Reducing Agents • Reducing Agent • A molecule or ion that contains an atom capable of being oxidized. • Electron donor. • Causes reduction in other molecules/ions. • Some examples of reducing agents: • H2, [Fe(CN)6]4-, Na, Mg, etc.

  30. Oxidizing and Reducing Agents • What is the oxidizing agent and the reducing agent in the following reaction? • Cu + 4HNO3 Cu(NO3)2 + 2NO2 + 2H2O • ANSWER: • Cu is oxidized (0 to +2), so Cu is the reducing agent. • N is reduced (+5 to +4), so HNO3 is the oxidizing agent.

  31. Oxidizing and Reducing Agents • Identify the oxidizing and reducing agents in the following redox rxns: • 3Mg + N2 Mg3N2 • SO2 + Br2 + 2H2O  2HBr + H2SO4 • 4HAuCl4 + 3N2H4  4Au + 3N2 + 16HCl

  32. Answers • 3Mg + N2 Mg3N2 • Mg is oxidized (0 to +2), so Mg is the reducing agent. • N is reduced (0 to –3), so N2 is the oxidizing agent. • SO2 + Br2 + 2H2O  2HBr + H2SO4 • S is oxidized (+4 to +6), so SO2 is the reducing agent. • Br is reduced (0 to –1), so Br2 is the oxidizing agent. • 4HAuCl4 + 3N2H4  4Au + 3N2 + 16HCl • N is oxidized (-2 to 0), so N2H4 is the reducing agent. • Au is being reduced (+5 to 0), so HAuCl4 is the oxidizing agent.

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