FIU CHM 4130 - Chapter 22 Xiao_Electrochemistry_2018 (93 pages)

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Chapter 22 Xiao_Electrochemistry_2018



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Electrochemistry Theory The Scope of Electrochemistry Electrochemistry provide information about activities rather than concentrations of chemical species Battery Corrosion Electrolysis Electroanalysis Electrochemical Cells Consist of two electrical conductors immersed in an electrolyte solution Generally contain External wires electrons carry current Conducting electrodes metal carbon Ion solutions ions carry current Interfaces or junctions A complete electrical circuit Electrochemical Cells Galvanic cell a cell that produces electrical energy Electrolytic cell a cell that consumes electrical energy Electrochemical Galvanic Cells Copper Cu wants to ionize more than Silver Ag Cl K Open circuit no current ow measure poten3al Closed circuit current ow measure current Oxida3on occurs at the anode AnOx reduc3on occurs at the cathode RedCat Net reac3on Cu s 2Ag Cu2 2Ag s Electrochemical Electrolytic Cells Net reac3on Cu2 2Ag s Cu s 2Ag Conduction in an Electrochemical Cell Electrons serve as carriers e g moving from Cu through the conductor to the Ag In the solution electricity involves the movement of cations and anions In the salt bridge both chloride and potassium ions move At the electrode surface an oxidation or a reduction occurs Cathode the electrode at which reduction occurs Anode the electrode at which oxidation occurs Galvanic cell vs Electrolytic cell Galvanic cell 1 Requires no potential voltage input Electrolytic cell 1 Requires potential voltage input 2 The voltage becomes smaller and it ultimately reaches 0V when the system achieves equilibrium 2 The potential voltage input the cell potential must be 0 for the reactions to occur 3 The cell potential is positive produce electrical energy 3 The cell potential is negative consume electrical energy What happens at the electrode surface Electrons are transferred at electrode surface by redox reac2ons Occur at liquid solid interface solu2on electrode An electrical double layer is formed Tightly bound inner layer Stern layer Loosely bound outer layer Di use layer Types of Current in an Electrochemical Cell Two types of processes can result in current ow across the electrode solu2on interface Faradaic currents Propor2onal to ionic species concentra2on follows Faraday s Laws Due to redox reac2ons at electrodes Non faradaic currents Result from charging of the electrical double layer capacitance Not due to redox reac2ons Redox reac2ons occur close to electrode surface 10 Faradaic currents require con2nual mass transport of ions to the electrode surface This occurs by Convec2on s2rring owing Di usion concentra2on gradient Migra2on electrosta2c force Electrochemical Cell Notation Convention Anode is listed on the left Electrolyte solutions and liquid liquid interface in center Cathode is listed on the right Activities or concentrations listed in parentheses Cu CuSO4 0 02 M AgNO3 0 02 M Ag Each vertical line indicates a phase boundary Galvanic cell as written electrolytic cell if reversed Electrode Potentials An electrochemical cell as made up of two half cell reactions one at the anode and the other at the cathode each of which has its own electrode potential E When combined the two reactions will determine the overall potential of the cell Ecell and the direction of current flow The cell potential is the difference between anode and cathode potentials Ecell Ecathode Eanode By convention the half reactions are always written as reductions Ecell EAgCl Ag EH H2 Ecell Eright Eleft The Standard Hydrogen Electrode SHE Pt H2 p atm H aH x A universal reference but is really a hypothetical electrode not used in practice Uses a platinum electrode which at its surface oxidizes 2H to H2 gas Very sensitive to temperature pressure and H ion activity Because the SHE is difficult to make the saturated calomel electrode SCE is used instead Ag AgCl electrode Calomel mercury I chloride Use of the SHE to Measure Electrode Potentials An electrode potential is defined as the potential of a cell with the electrode under study as the right hand electrode and the SHE as the left hand electrode If a 1 00 M p 1 atmosphere the electrode potential E becomes the standard electrode potential E0 Some Standard Electrode Potentials Oxidizing agents E is temperature dependent E0 determined at 25 C E0 is calculated rela2ve to the SHE assigned E0 0 000 V E0 is measured at standard condi2ons 298K 1M of solutes and 1 bar of gas pressure E0 is a measure of the driving force G of the half reac2on Reducing agents Nernst Equation Compensates for non unit ac2vity i e not 1 M Rela2onship between cell poten2al and electrolyte ac2vi2es aA bB ne cC dD 2 30 RT C c D d 2 30 RT 0 0 E E log E log K a b nF A B nF At 298 K 2 3RT F 0 0592 What is the poten2al of an electrode of Zn s and 0 01 M Zn2 Zn2 2e Zn E0 0 763 V ac2vity of Zn s is 1 0 0592 1 E 0 763 log 0 822 V 2 0 01 Work done by cell G nFE RTlnK F 96 485 coulombs mole of electrons Electrochemical Cells and Analytical Methods Potentiometry Measures equilibrium E Voltammetry Control E measures I as function of time e working electrode indicator electrode detector electrode control measurement e reference electrode counter electrode Potentiometry an indicator electrode a reference electrode an a potential measuring device Indicator Electrode Reference Electrode General Principles Ecell Eind Eref Ej Electrodes and Potentiometry Reference Electrodes Standard Hydrogen Electrode Requires freshly prepared Pt surface Hg Hg2Cl2 sat d KCl xM Hg2Cl2 2e 2Hg l 2Cl SCE reference electrode AgCl s e Vycor plug Ag AgCl reference electrode Ag s Cl Indicator Electrodesmetallic indicator electrodes Electrodes of the First Kind pure metal electrode Ag Cu Zn with its cation Electrodes of the Second Kind metal electrode Hg with its precipitate and complex Electrodes of the Third kind metal electrode Hg with a different cation Inert redox Electrodes Pt Au Pd and C Liquid Junction Potentials Solution introduction of a salt bridge between the two solutions HCl 1M HCl 0 01M 30 mV or more Indicator ElectrodesIon Selective Electrodes Ion Selective Membrane Electrodes In these electrodes an ion selective potential like a junction potential develops across a membrane that separates analyte from reference solution No redox reaction occurs Properties of Ion Selective Membrane Electrodes Minimal solubility in standard and sample solutions Solids Silica glass Ionic inorganic compounds of low solubility Polymeric resins Small but non zero electrical


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