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Electrochemistry

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Electrochemistry Terminology #1 Oxidation Oxidation – A process in which an element attains a more positive oxidation state Na(s) Na + + e - Reduction Reduction – A process in which an element attains a more negative oxidation state Cl 2 + 2e - 2Cl -

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Electrochemistry Terminology #2 G ain E lectrons = R eduction An old memory device for oxidation and reduction goes like this… LEO says GER L ose E lectrons = O xidation

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Electrochemistry Terminology #3 Oxidizing agent Oxidizing agent The substance that is reduced is the oxidizing agent Reducing agent Reducing agent The substance that is oxidized is the reducing agent

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Electrochemistry Terminology #4 Anode Anode The electrode where oxidation occurs Cathode Cathode The electrode where reduction occurs Memory device: Reduction at the Cathode

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Table of Reduction Potentials Measured against the Standard Hydrogen Electrode

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Measuring Standard Electrode Potential Potentials are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts.

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Galvanic (Electrochemical) Cells Spontaneous redox processes have: A positive cell potential, E 0 A negative free energy change, (- G)

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Zn - Cu Galvanic Cell Zn 2+ + 2e - Zn E = -0.76V Cu 2+ + 2e - Cu E = +0.34V From a table of reduction potentials:

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Zn - Cu Galvanic Cell Cu 2+ + 2e - Cu E = +0.34V The less positive, or more negative reduction potential becomes the oxidation… Zn Zn 2+ + 2e - E = +0.76V Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1.10 V

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Line Notation Zn(s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu(s) An abbreviated representation of an electrochemical cell Anodesolution Anodematerial Cathodesolution Cathodematerial ||||

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Calculating G 0 for a Cell G 0 = -nFE 0 n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e - E 0 Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1.10 V

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The Nernst Equation Standard potentials assume a concentration of 1 M. The Nernst equation allows us to calculate potential when the two cells are not 1.0 M. R = 8.31 J/(mol K) T = Temperature in K n = moles of electrons in balanced redox equation F = Faraday constant = 96,485 coulombs/mol e -

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Nernst Equation Simplified At 25 C (298 K) the Nernst Equation is simplified this way:

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Equilibrium Constants and Cell Potential equilibrium At equilibrium, forward and reverse reactions occur at equal rates, therefore: 1. 1. The battery is “dead” 2. The cell potential, E, is zero volts Modifying the Nernst Equation (at 25 C):

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E 0 Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1.10 V Calculating an Equilibrium Constant from a Cell Potential

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Concentration Cell Step 1: Determine which side undergoes oxidation, and which side undergoes reduction. Both sides have the same components but at different concentrations. ???

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Concentration Cell Both sides have the same components but at different concentrations. The 1.0 M Zn 2+ must decrease in concentration, and the 0.10 M Zn 2+ must increase in concentration Zn 2+ (1.0M) + 2e - Zn (reduction) Zn Zn 2+ (0.10M) + 2e - (oxidation) ??? Cathode Anode Zn 2+ (1.0M) Zn 2+ (0.10M)

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Concentration Cell Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C). Both sides have the same components but at different concentrations. ??? Cathode Anode Zn 2+ (1.0M) Zn 2+ (0.10M) Concentration Cell

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Nernst Calculations Zn 2+ (1.0M) Zn 2+ (0.10M)

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Electrolytic Processes A negative -E 0 cell potential, ( -E 0 ) + G A positive free energy change, ( + G ) NOT Electrolytic processes are NOT spontaneous. They have:

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Electrolysis of Water In acidic solution Anode rxn: Cathode rxn: -1.23 V -0.83 V -2.06 V

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Electroplating of Silver Anode reaction: Ag Ag + + e - Electroplating requirements: 1. Solution of the plating metal 3. Cathode with the object to be plated 2. Anode made of the plating metal 4. Source of current Cathode reaction: Ag + + e - Ag

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Solving an Electroplating Problem Q: How many seconds will it take to plate out 5.0 grams of silver from a solution of AgNO 3 using a 20.0 Ampere current? 5.0 g Ag + + e - Ag 1 mol Ag 107.87 g 1 mol e - 1 mol Ag 96 485 C 1 mol e - 1 s 20.0 C = 2.2 x 10 2 s

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