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For a half-reaction, the more (+) the E o red value, the

For a half-reaction, the more (+) the E o red value, the greater the tendency for that reaction to “go” in that direction (i.e., reduction). Strongest oxidizer is…. F 2. F 2 (g) + 2 e –  2 F – (aq). E o red = +2.87 V . Other strong oxidizers are… the other halogens.

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For a half-reaction, the more (+) the E o red value, the

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  1. For a half-reaction, the more (+) the Eored value, the greater the tendency for that reaction to “go” in that direction (i.e., reduction). Strongest oxidizer is… F2 F2(g) + 2 e– 2 F–(aq) Eored = +2.87 V Other strong oxidizers are… the other halogens (Cl2, Br2, I2) and oxyanions in which the central atom has a large ___ charge. (+) e.g., MnO4–, Cr2O72–, ClO3–, etc. Strong oxidizers LOVE their own e–s (and they want everybody else’s, too).

  2. (–) sign indicates poor tendency to “go” in this direction, but large magnitude (i.e., 3.05 V) shows strong tendency to “go” in other direction (i.e., oxidation). Lithium batteries take advantage of lithium’s strong tendency to BE oxidized (i.e., to REDUCE other stuff.) Poorest oxidizer is… Li. Li+(aq) + e– Li(s) Eored = –3.05 V Poor oxidizers HATE their own e–s (and have no interest in accepting anyone else’s e– s, either.)

  3. (+) V 0 V (–) V In comparing the reduction potentials of two half-reactions, consider the scale shown. The “higher-up” reaction is the reduction half-cell; the “lower-down” reaction is the oxidation half-cell. A– Red. A– Ox. C– Red. B– Red. B– Ox. C– Ox.

  4. standard conditions nonstandard conditions “e– haters” Li Rb K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H2 Sb Bi Cu Hg Ag Pt Au Spontaneity of Redox Reactions Eocell = Eored,cath – Eored,an (same equation as before) Activity Series for Metals If Eo (or E, or emf) is +…spontaneous. –…nonspontaneous. -- The Activity Series is based on standard reduction potentials. “e– lovers”

  5. ( ) C 96,500 mol e– J V . mol e– Michael Faraday Josiah Gibbs (1791–1867) (1839–1903) Relationship between E and DG… DG = –nFE DGo = –nFEo In standard states… n = # of mol of transferred e– F = Faraday’s constant =

  6. J V . mol e– ( ) = –5 mol e– (0.74 V) 96,500 For… 5 Fe2+ + MnO4– + 8 H+  5 Fe3+ + Mn2+ + 4 H2O (b) Find DGo. (a) What is n? (a) n = 5 Fe3+ + e– Fe2+Eored= 0.77 V (b) MnO4– + 8 H++ 5 e– Mn2+ + 4 H2O Eored = 1.51 V Eocell = Eored,cath – Eored,an Eocell = 1.51 V – 0.77 V = 0.74 V DGo = –nFEo (SPONTANEOUS) = –357 kJ

  7. Walther Nernst (1864–1941) Effect of Concentration on Cell EMF Cell emf drops gradually due to changing concentrations of reactants and products. When emf = 0 V, cell is “dead.” Nernst equation: At 25oC (298 K):

  8. nonspont. spont. Fe(s) + Cd2+(aq)  Cd(s) + Fe2+(aq) Find emf at 25oC when [Cd2+] = 0.030 M and [Fe2+] = 2.0 M. Eo = +0.04 V Fe2+(aq) + 2 e– Fe(s) Eored = –0.44 V Cd2+(aq) + 2 e–Cd(s) Eored = –0.40 V = –0.01 V If [Cd2+] = 2.0 M and [Fe2+] = 0.030 M… = 0.09 V

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