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Introduction to “Redox”

Introduction to “Redox”. HW. Please grab a clicker and enter your homework as the answers IN mL to questions 1-7 (Q 35 a = 1, b = 2 etc.) Molecular equations on the board, volunteers?. Oxidation Numbers. Objective sheet. Single replacement reactions: a good intro to Redox.

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Introduction to “Redox”

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  1. Introduction to “Redox”

  2. HW • Please grab a clicker and enter your homework as the answers IN mL to questions 1-7 (Q 35 a = 1, b = 2 etc.) • Molecular equations on the board, volunteers?

  3. Oxidation Numbers • Objective sheet

  4. Single replacement reactions: a good intro to Redox • Observe the following reaction • Al + CuCl2 → • What happens to the metals? • Observe the following reaction • Cu + AlCl3 → • How to know: • End S.R. intro

  5. More S.R. examples • Two reactions are occurring Zn + SnCl2 Sn + ZnCl2 Zn + 2HCl  H2 + ZnCl2

  6. A Cool Mnemonic • Leo goes ger • Lose Electrons = Oxidation • Gain Electrons = Reduction

  7. Oxidation Number • Found using these rules • Elements in their elemental state are 0 • Fluoride is -1 • Hydrogen is +1 (except in the hydride ion, H-) • Oxygen is -2 (except -1 in hydrogen peroxide) • The Halogens are -1 (except when bonded to oxygen or a halogen that is higher in the group)

  8. Practice • 3CuSO4 + 2Al  Al2(SO4)3 + 3Cu • 2SO2 + O2  2SO3 • 2H2 + O2  2H2O

  9. Redox Worksheet K =+1, +1 Mn = +7, +2 O = -2, -2 Na = +1, +1 Cl = -1, 0 H = +1, +1 S = +6, +6 Mg = 0, +2 H = +1, 0 Cl = -1, -1 Fe = 0, +3 V = +3, +2 O = -2, -2 K = +1, +1 Mn = +7, +2 N = +3, +5 H = +1, +1 S = +6, +6 O = -2, -2 K = +1, +1 Cr = +6, +3 O = -2, -2 Sn = +2, +4 Cl = -1, -1 H = +1, +1

  10. Writing Simple Half Reactions • CuCl2 + Al  AlCl3 + Cu • Cu2+ + 2 e-  Cu • Al  Al3+ + 3 e-

  11. More complex redox reactions • Check out PT • KMnO4 + KNO2 + H2SO4  MnSO4 + H2O + KNO3 + K2SO4 • Work done for you but it is really… • MnO41- + NO21- + 2H1+ Mn2+ + H2O + NO31- • And unbalanced it is… • MnO41- + NO21- Mn2+ + NO31+ • How do we get from here to there?

  12. Steps for balancing • 1st Write separate equations for oxidation and reduction ½ reactions • 2nd for each ½ reaction: • Balance all elements (except H & O) • Balance O using water • Balance H using H+ (acidic conditions) • Balance charge using e- • 3rd If needed: x each reaction by integer to = # e- gained & lost

  13. Steps for balancing • 4th add the ½ reactions and cancel identical species • 5th check to make sure everything is balanced

  14. Example • A solution of potassium permanganate is combined with a solution of Iron (II) chloride under acid conditions according to the following reaction • MnO41-(aq) + Fe2+(aq)→ Fe3+(aq) + Mn2+(aq) • Ox ½ rx’n: Fe2+(aq)→ Fe3+(aq) • Red ½ rx’n: MnO41-(aq)→ Mn2+(aq)

  15. Example • Ox ½ rx’n: Fe2+(aq)→ Fe3+(aq) • 2nd balance atoms, then water, H+ and finally e- • All good except e- so… • Fe2+(aq)→ Fe3+(aq) + e- • Now charge on each side is 2+

  16. Example • Red ½ rx’n: MnO41-(aq)→ Mn2+(aq) • 2nd balance atoms, then water, H+ and finally e- • MnO41-(aq)→ Mn2+(aq) + 4H2O(l) • Balance O atoms • 8H+(aq) + MnO41-(aq)→ Mn2+(aq) + 4H2O(l) • Balanced for H • 5e- + 8H+ + MnO41-→ Mn2+ + 4H2O

  17. Example • Mult. by integer to = electrons lost and gained • 5e- + 8H+ + MnO41-→ Mn2+ + 4H2O • 5 (Fe2+(aq)→ Fe3+(aq) + e-) • 5 Fe2+(aq)→ 5 Fe3+(aq) + 5 e- • Add reactions • 8H+ + MnO41- 5 Fe2+→ 5 Fe3++ Mn2+ + 4H2O • It’s that simple?

  18. Example • Breathalizer • Used to rely on color change • Reaction of ethanol with dichromate in acidic conditions • Cr2O72- + EtOH  ethanoic acid + chromium (III)

  19. Rules for Balancing redox reactions • Check out p.164-170 • Redox problems from book: p.177 odds only 41-51 • Webassign will be up this weekend with an assignment to help you review • Also, check out this site http://www.wfu.edu/~ylwong/redox/ (FIND LINK TO IT ON MY SITE)

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