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Redox Reactions. Oxidation = loses electrons Reduction = gains electrons. LEO the lion says GER. OIL RIG. Oxidation Is Loss Reduction Is Gain. What is an “oxidation number”?. Something to help us keep track of whether a substance is oxidized or reduced during a reaction

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redox reactions
Redox Reactions
  • Oxidation = loses electrons
  • Reduction = gains electrons
oil rig
OIL RIG
  • Oxidation Is Loss
  • Reduction Is Gain
what is an oxidation number
What is an “oxidation number”?
  • Something to help us keep track of whether a substance is oxidized or reduced during a reaction
  • “a positive or negative whole number assigned to an element on the basis of a set of rules; to some degree it reflects the positive or negative character of that atom”
    • Definition from Brown-Lemay AP Chemistry Textbook, 12th edition
rules for determining oxidation
Rules for determining oxidation #
  • For atom in elemental form, oxidation # always zero.
  • Any monoatomic ion, oxidation number = ionic charge
  • Nonmetals usually have negative oxidation numbers, can be positive
    • Oxygen is usually -2 (major exception = peroxides)
    • Hydrogen is usually +1 when bonded to non-metals, -1 when bonded to metals
    • Fluorine is always -1. other halogens usually -1, but can be positive when combined with oxygen
  • Sum of oxidation numbers in neutral compound = 0. Sum of oxidation numbers in polyatomic ion = charge of ion
peroxides
Peroxides
  • Oxidation #’s
    • Peroxides (O22-)
    • Is CO2 a peroxide? PbO2? How do you know?
    • Some common peroxides:
      • H2O2 (hydrogen peroxide)
      • Li2O2 (lithium peroxide)
      • You know it must be a peroxide because the substance the O2 is bonded to (2 Li+ ions, for example), must have a +2 charge overall (based on one of our other oxidation # rules)
slide9

ANSWERS

    • H2S
      • Hydrogen is +1 when bonded to non-metal, so +2 + x = 0, x = -2 (oxidation # of sulfur)
    • S8
      • Elemental form, oxidation # = 0
    • SCl2
      • Halogens usually -1, so S is +2
    • Na2SO3
      • SO3 must be -2 overall, oxygen usually -2 (-6 total), so S must be +4
    • SO42-
      • Oxygen usually -2 (-8 total), so S must be +6 for numbers to add up to -2
practice with reactions
Practice with Reactions
  • Many combination, decomposition, combustion, and single displacement reactions are redox reactions
  • 2Na + Cl2 2NaCl (combination reaction)
    • Which element is oxidized?
    • Which is reduced?
practice with reactions1
Practice with Reactions
  • Many combination, decomposition, combustion, and single displacement reactions are redox reactions
  • 2Na + Cl2 2NaCl (combination reaction)
    • Which element is oxidized?
    • 2Na  2Na+ + 2e- (loses electrons, becomes positive)
    • Which is reduced?
    • Cl2 + 2e-  2Cl- (gains electrons, becomes negative)
practice with reactions2
Practice with Reactions
  • Zn + FeSO4ZnSO4 + Fe (single displacement)
    • Which element is oxidized?
    • Which is reduced?
  • CH4 + 2 O2  CO2 + 2 H2O (combustion)
    • Which elementis oxidized?
    • Which is reduced?
practice with reactions3
Practice with Reactions
  • Zn + FeSO4ZnSO4 + Fe (single displacement)
    • Which element is oxidized?
    • Zn  Zn2+ + 2e- (loses electrons, becomes positive)
    • Which is reduced?
    • Fe2+ + 2e-  Fe (gains electrons, becomes LESS positive)
  • CH4 + 2 O2  CO2 + 2 H2O (combustion)
    • Which elementis oxidized?
    • C-4 C4+ + 8e- (loses electrons, becomes MORE positive)
    • Which is reduced?
    • 4 O + 8 e- 4 O 2- (gains electrons, becomes negative/less positive)
practice with reactions4
Practice with Reactions
  • 3BrO- BrO3- + 2Br - (“other” redox reactions)
    • Oxidation?
    • Reduction?
practice with reactions5
Practice with Reactions
  • 3BrO- BrO3- + 2Br- (“other” redox reactions)
    • Oxidation?
    • Br+  Br5+ + 4e- (loses e-, becomes MORE positive)
      • Or…. BrO- BrO3-
    • Reduction?
    • Br+ + 2e-  Br- (gains e-, becomes LESS positive)
      • Or… BrO- Br-