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Examination of the Eyring Equation under various conditions.

Kinetics of Electron Transfer at the Solid Electrode-Solution Interface. Examination of the Eyring Equation under various conditions. Case II: At applied potentials E different than E eq such that we are applying an overpotential, h = E - E eq.

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Examination of the Eyring Equation under various conditions.

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  1. Kinetics of Electron Transfer at the Solid Electrode-Solution Interface Examination of the Eyring Equation under various conditions. Case II: At applied potentials E different than Eeq such that we are applying an overpotential, h = E- Eeq Skipping the derivation (you should be able to do it): Known as i- h Eqn NB: Not corrected for DL effects- see page 571 At E = Eeq, Ci(0,t) = Ci*, and so i = i0 High h, MT control Low h, Exponential control

  2. Kinetics of Electron Transfer at the Solid Electrode-Solution Interface This expression makes it difficult to extract key information. Must use approximations resulting from certain limiting conditions: Approximation A : 1) Well-stirred solution, and 2) i is small That means that Ci(0,t) = Ci* and E is near Eeq Butler-Volmer Eqn. This is a good approximation of the i-h Eqn. when: 0.9 ≤Ci(0,t)/Ci* ≤ 1.1 Or i is ≤ 10% of ilim

  3. Kinetics of Electron Transfer at the Solid Electrode-Solution Interface Because MT is not a component of the measured i, then i is based solely on the activation energy of the process. So, the observed potentials for oxidation are more positive and those for reduction more negative than E0’. i0 is merely a measure of a system’s ability to provide a net current w/o a significant energy loss due to activation effects. Approximation A, with Small h: For small x, ex≈ 1+x (be able to derive) Linearized Butler-Volmer Eqn. jc Use for Fast ET Region of interest +h -h ja

  4. Kinetics of Electron Transfer at the Solid Electrode-Solution Interface Approximation A, with Large ±h: For large -h Or Tafel Eqn. Holds when: 1. ib is < 0.01 if Or 2. |h| > 118/n mV Use for Slow ET (Typically for Totally Irreversible Systems) Tafel Plots to Obtain Kinetic and Other Information h large negative hlarge positive

  5. Kinetics of Electron Transfer at the Solid Electrode-Solution Interface When h is large, must recognize that MT limitations occur. When h is small, near 0, must be cognizant of back reactions. n should be there Anodic Branch Cathodic Branch Extrapolate to 0 h and obtain log i0. Obtain ah from slope. Read about exchange current plots (pp. 105-107) on your own and learn about methods to obtain a and other parameters independently (see problem 3.11).

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