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Redox Reactions

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

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  1. Redox Reactions • Oxidation = loses electrons • Reduction = gains electrons

  2. LEO the lion says GER

  3. OIL RIG • Oxidation Is Loss • Reduction Is Gain

  4. 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

  5. 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

  6. 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)

  7. Grab a whiteboard!

  8. Determine the oxidation number of sulfur in the following compounds: • H2S • S8 • SCl2 • Na2SO3 • SO42-

  9. 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

  10. 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?

  11. 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)

  12. 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?

  13. 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)

  14. Practice with Reactions • 3BrO- BrO3- + 2Br - (“other” redox reactions) • Oxidation? • Reduction?

  15. 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-

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