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生物电分析化学 Electroanalytical Chemsitry and Its Biological Applications 主讲人:刘宏 教授 2014 年秋
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Questions: How an electrolytic reaction is influenced by electrode potential? How is the electrode potential is established at equilibrium?
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Electrode Potential/Kinetics ( 电极电位 / 反应动力学 ) 1.Chemical thermodynamics & kinetics ( 化学热力学与动力学 ) 2.Electrochemical thermodynamics ( 基本电化学热力学 ) 3.Kinetics of electrode reactions ( 电极反应动力学 )
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Basic Thermodynamics For a chemical reaction: If the reaction is at equilibrium, K c is the equilibrium constant (K eq ) Standard Gibbs free energy: dynamic equilibrium
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Basic Thermodynamics If the reaction is not at equilibrium, the reaction quotient is The reaction will shift to the right The reaction will shift to the left ξ - extent of reaction
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Basic Thermodynamics For electrochemical reactions: For chemical reactions: So
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Kinetics of Electrode Reactions At equilibrium, Exchange velocity: Transition state theory:
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Kinetics of Electrode Reactions For an electrode reaction: The velocity of the forward reaction: The velocity of the backward reaction: Net velocity of the reaction: Faraday's law
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Butler-Volmer Model of Electrode Kinetics
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Butler-Volmer Model Because
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Butler-Volmer Model If at equilibrium and So, The rate constant is called standard rate constant k 0 So, Butler-Volmer formulation of electrode kinetics
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Standard rate constant k 0 A measure of the kinetic facility of a redox couple. A system with a large k 0 will achieve equilibrium on a short time scale, and requires small overpotential to drive the electrochemical reaction. but a system with a small k 0 will be sluggish, and requires large overpotential to drive the reaciton.
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Kinetic equilibrium At equilibrium, i =0 So, After rearrangement, (Nerst equation) Even though the net current is 0 at equilibrium, we still can evision balanced faradaic activity, which can be expressed in terms of exchange current, i 0 Becuase
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Overpotential So, Current-overpotential equation
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Overpotential
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If the solution is well stirred or current is very low, so that the surface concentration do not differ from bulk concentration, then which is historically known as Butler-Volmer equation
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Overpotential
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If the overpotential is sufficiently small, then So the charge transfer resistance is: If the overpotential is sufficiently large, then Tafel behavior at large overpotential
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Overpotential If the overpotential is sufficiently small, then So the charge transfer resistance is: If the overpotential is sufficiently large, then Tafel behavior at large overpotential
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Tafel Plot
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Assignment 1. How an electrolytic reaction is influenced by electrode potential? 2. How is the electrode potential is established at equilibrium? 3.
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