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Electrochemistry

Electrochemistry. Chapter 20. 20.1 Oxidation states. Redox reactions always contain oxidation and reduction e.x . Sn 2+ + Fe 3+  Sn 4+ + Fe 2+ Oxidized? Reduced? Oxidizing agent/oxidant removes electrons (is reduced) Reducing agent/ reductant provides electrons (is oxidized).

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Electrochemistry

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  1. Electrochemistry Chapter 20

  2. 20.1 Oxidation states • Redox reactions always contain oxidation and reduction • e.x. Sn2+ + Fe3+  Sn4+ + Fe2+ • Oxidized? Reduced? • Oxidizing agent/oxidant removes electrons (is reduced) • Reducing agent/reductant provides electrons (is oxidized)

  3. 20.2 half reactions • e.x. Sn2+ + Fe3+  Sn4+ + Fe2+ • separate oxidation and reduction • Oxidation ½ reaction: Sn2+  Sn4+ + 2e- • Reduction ½ reaction: Fe3+ + e-  Fe2+ • Charges must balance (electrons) • Reduction rxn must be doubled (x2) • 2Fe3+ + 2e-  2Fe2+ • Add: Sn2+ + 2Fe3+ + 2e- Sn4+ + 2Fe2++ 2e- • Sn2+ + 2Fe3+  Sn4+ + 2Fe2+

  4. Spontaneous redox reactions Anode (-ve): oxidation occurs Cathode (+ve): reduction occurs Electrons flow from anode to cathode Salt bridge allows ions to migrate Anions flow towards anode Cations flow towards cathode 20.3 Voltaic Cells

  5. 20.4 Cell EMF (standard conditions) • Electrons transfer spontaneously due to difference in potential energy • Potential measured in volts. 1V = 1J/C • J = energy, C = charge on electron • Electromotive force (EMF)= potential difference • E ˚cell = E ˚red(cathode) – E ˚red(anode) • Standard conditions: 25˚C, 1M, 1atm • SHE: standard hydrogen potential • 2H+(1M) + 2e- H2 (1 atm) E ˚red=0V • E ˚ Is intensive. • [R]↑ then energy(J)↑ • ratio of energy/electron transferred does NOT change. V = J/C

  6. Free energy = G° G° = potential to be spontaneous Measured in joules (energy) Negative = spontaneous G° = -nFE° n = number of electrons transferred F = Faraday’s constant = 96485J/V•mole (C/mole) G ° = -nFE ° = -RTlnK G ° is extensive (changes with n) E° is intensive 20.5 Free Energy & Redox

  7. Nernst Equation: E = E ° – (RT/nF)LnQ E = E ° – (0.0592/n)LogQ @ 298K During reaction Q↑ so E↓ until cell discharges Concentration cell E °cell = 0 for 2 cells with the same species but E = E ° – (0.0592/n)LogQ Q depends on concentration 20.6 EMF non-standard conditions

  8. Electrolytic cell uses a battery to force a non-spontaneous reaction Quantitative electrolysis: Direct relationship: mole e-  mole product 1 mole e- = 1F = 96485C (C = J/V & C/s = Amp) Electrical work: G = wmax = -nFE Voltaic cell wmax is –ve (work done by system) Electrolysis wmax is +ve (work done on system) Work = energy/time. 1W = 1J/s 1kW•hr = 3.6 x 106J 20.9 Electrolysis

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