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Rules for Assigning Oxidation #’s

Rules for Assigning Oxidation #’s. Elements by themselves are neutral Ox # = 0 Ex: Na 0 , O 2 0 , Mg 0 Monoatomic Ions show their oxidation # as their written charge Ex: H +1 , Cl -1 , Al +3. Group #1 Elements in compounds: Ox # = +1 Ex: LiBr, NaCl Group #2 Elements in compounds:

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Rules for Assigning Oxidation #’s

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  1. Rules for Assigning Oxidation #’s

  2. Elements by themselves are neutral Ox # = 0 • Ex: Na0, O20, Mg0 • Monoatomic Ions show their oxidation # as their written charge • Ex: H+1, Cl-1, Al+3

  3. Group #1 Elements in compounds: Ox # = +1 Ex: LiBr, NaCl • Group #2 Elements in compounds: Ox # = +2 Ex: MgBr2, CaCl2

  4. Aluminum in compounds: Ox # = +3 Ex: AlCl3 • Fluorine in compounds Ox # = -1 Ex: LiF, MgF2

  5. Hydrogen: • Ox # = +1 (most common) • when combined with nonmetals • Ex: HCl, H2SO4, H2S • Ox # = -1 • When combined with metals (metal hydrides) • Ex: LiH, MgH2

  6. Oxygen: Oxidation # = -2 (usually) • Ex: CO2, H2O • Special Cases: • Peroxides: Oxygen is a diatomic ion (O2)-2 with total charge of -2, each oxygen has -1 charge • Ex: H2O2, BaO2 • When bonded with Fluorine: Ox # = +2 • Ex: OF2

  7. How to Find Charges If No “Rule”? • Sum of Individual Ox # of all the elements in a compound = 0 • All compounds are NEUTRAL • Ex: NaClO3 • What is the ox # for each element? • Draw your Lines….

  8. How to Find Charges If No “Rule”? • Sum of Ox # of all atoms in a polyatomic ion is equal to the charge of that ion. • Ex: PO4-3 • What is the charge of each element?

  9. Let’s Practice • What is the oxidation number of sulfur in Na2S2O3? (1) -1      (2) +6 (3) +2     (4) +4

  10. What is the oxidation number of chromium in the chromate ion, CrO4 2– ? (1) +6   (2) +3 (3) +2   (4) +8

  11. In which compound does chlorine have the highest oxidation number? (1) NaClO         (2) NaClO3 (3) NaClO2 (4) NaClO4

  12. What is the oxidation number assigned to manganese in KMnO4? (1) +7     (2) +3 (3) +2     (4) +4

  13. Mahjong Oxidation # Game: • http://www2.stetson.edu/mahjongchem/

  14. Oxidation and Reduction Reactions

  15. Oxidation (Read only) Original definition: When substances combined with oxygen. Ex: All combustion (burning) reactions CH4(g) + 2O2(g) CO2(g) + 2H2O(l) All “rusting” reactions 4Fe(s) + 3O2(g) 2Fe2O3(s)

  16. Reduction (Read Only) Original Definition: Reaction where a substance “gave up” oxygen. Called “reductions” because they produced products that were “reduced” in mass because gas escaped. Ex: 2Fe2O3(l) + 3C(s) 4Fe(l) + 3CO2(g)

  17. Oxidation/Reduction Reactions Oxidation: LOSS of one or more electrons. Reduction: GAIN of one or more electrons Oxidation & reduction always occur together.

  18. Deal with a movement of ELECTRONS between atoms during a reaction. • Electrons travel from what is oxidized towards what is reduced.

  19. One atom loses e-, the other gains e- This is called “electron transfer”

  20. Remember!!

  21. Or…Remember

  22. Redox Reactions: ALWAYS involve changes in charge If charges don’t change it’s NOT a Redox rxn.

  23. Conservation of “Charge” Total electrons lost = Total electrons gained Redox Reactions must balance electrons as well as atoms!

  24. Let’s Practice Which changes occur when Pt2+ is reduced? (1) Pt2+  gains electrons and its oxidation number increases. (2) Pt2+  gains electrons and its oxidation number decreases. (3) Pt2+  loses electrons and its oxidation number increases. (4) Pt2+  loses electrons and its oxidation number decreases.

  25. Answer: 1

  26. Given the balanced equation representing a redox reaction: 2Al + 3Cu2+→ 2Al3+ + 3Cu Which statement is true about this reaction? (1) Each Al loses 2e- and each Cu2+  gains 3e- (2) Each Al loses 3e- and each Cu2+  gains 2e- (3) Each Al3+  gains 2e- and each Cu loses 3e- (4) Each Al3+  gains 3e- and each Cu loses 2e-

  27. Answer: 2

  28. In an oxidation-reduction reaction, reduction is defined as the (1) loss of protons     (2) loss of electrons (3) gain of protons     (4) gain of electrons

  29. Which change in oxidation number indicates oxidation? (1) –1 to +2 (2) +2 to –3 (3) –1 to –2  (4) +3 to +2

  30. When a neutral atom undergoes oxidation, the atom’s oxidation state (1) decreases as it gains electrons (2) decreases as it loses electrons (3) increases as it gains electrons (4) increases as it loses electrons

  31. Oxidizing/Reducing Agents Oxidizing Agent: substance reduced • Gains electrons Reducing Agent: substance oxidized • Loses electrons The “Agent” is the “opposite”

  32. Identify What is Changing in Charge What is oxidized and reduced? What are the oxidizing and reducing agents? Ex: 3Br2 + 2AlI3 2AlBr3 + 3I2

  33. 0 +3 -1 +3 -1 0 3Br2 + 2AlI3 2AlBr3 + 3I2 Br2 is reduced and is the oxidizing agent I-1 is oxidized and is the reducing agent

  34. What is oxidized and reduced? What are the oxidizing and reducing agents? Mg + CuSO4 MgSO4 + Cu 2K + Br2 2KBr Cu + 2AgNO3 Cu(NO3)2 + 2Ag NOTE: Atoms in a polyatomic ion DO NOT change in charge!

  35. 0 +2 +2 0 Mg + CuSO4 MgSO4 + Cu Mg oxidized (reducing agent) Cu+2 reduced (oxidizing agent) 0 0 +1 -1 2K + Br2 2KBr K oxidized (reducing agent) Br2 reduced (oxidizing agent) 0 +1 +2 0 Cu + 2AgNO3 Cu(NO3)2 + 2Ag Cu oxidized (reducing agent) Ag+1 reduced (oxidizing agent)

  36. Redox or Not Redox (that is the question…) Redox Reactions: must have atoms changing in charge. Not all reactions are redox. Easy way to spot a redox reaction!!! Look for elements entering and leaving compounds.

  37. Is it Redox? Look for Changes in Charge! Are elements entering and leaving compounds? Synthesis: Ex: 2H2 + O2 2H2O Decomposition: Ex: 2KClO3 2KCl + 3O2

  38. Is it Redox? Synthesis: YES 0 0 +1 -2 Ex: 2H2 + O2 2H2O Decomposition: YES +1 +5 -2 +1 -1 0 Ex: 2KClO3 2KCl + 3O2

  39. Is it Redox? Combustion: CH4 + 2O2 CO2 + 2H20 Single Replacement: Zn + CuCl2 ZnCl2 + Cu

  40. Is it Redox? Combustion:YES -4 +1 0 +4 -2 +1 -2 CH4 + 2O2 CO2 + 2H20 Single Replacement:YES 0 +2 -1 +2 -1 0 Zn + CuCl2 ZnCl2 + Cu

  41. Is it Redox? Double Replacement: AgNO3 + LiCl AgCl + LiNO3

  42. Is it Redox? Double Replacement:NO!!!! Ions switch partners, but don’t change in charge +1 +5 -2 +1 -1 +1 -1 +1 +5 -2 AgNO3 + LiCl AgCl + LiNO3 Remember charges of atoms inside polyatomic Ions do not change!

  43. Let’s Practice Which equation represents an oxidation reduction reaction? (1) CH4 + 2O2→ CO2 + 2H2O (2) H2SO4 + Ca(OH)2→ CaSO4 + 2H2O (3) MgCrO4 + BaCl2→ MgCl2 + BaCrO4 (4) Zn(NO3)2 + Na2CO3→ 2NaNO3 + ZnCO3

  44. Which reaction is an example of an oxidation reduction reaction? (1) AgNO3 + KI →AgI + KNO3 (2) Cu + 2 AgNO3→ Cu(NO3)2 + 2 Ag (3) 2 KOH + H2SO4→ K2SO4 + 2H2O (4) Ba(OH)2 + 2HCl → BaCl2 + 2H2O

  45. Writing Half Reactions Redox Reactions are composed of two parts or half reactions. Half Reactions Show: Element being oxidized or reduced. Change in charge # of moles of e- being lost or gained http://www.kentchemistry.com/links/Redox/flash/RedoxOxNumbers.swf

  46. Writing Half Reactions 0 0 +1 -1 2Na + F2 2NaF Oxidation: Na Na+1 + 1e- or 2Na 2Na+1 + 2e- Note: e- are “lost” (on the right of arrow) Reduction: F + 1e- F-1 or F2 + 2e- 2F-1 Note: e- are “gained” (on the left of arrow)

  47. Ox’s Have Tails!! • Oxidation Half reactions always have “tails” of electrons Na Na+1 + 1e-

  48. 0 +2 -1 +2 -1 0 Zn + CuCl2 ZnCl2 + Cu Ox: Zn Zn+2 + 2e- Red: Cu+2 + 2e- Cu https://www.youtube.com/watch?v=-wvnX1f7yRY Tutorial on Writing Half Reactions

  49. Balancing Simple Redox Rxns Must be: Balanced for Mass ATOMS balance Balanced for Charge Total e- Lost = Total e- Gained

  50. Applications of Redox Reactions Corrosion of Metals Metals gets oxidized by oxygen in the air forming metal oxides (rust) 4Fe(s) + 3O2(g) → 2Fe2O3(s) Prevention: Use paint, oil, “Plating” or attach to negative terminal of a battery. Gold doesn’t rust…Why?

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