Electrochemistry Electrochemistry Terminology 1 v Oxidation A process

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Electrochemistry

Electrochemistry

Electrochemistry Terminology #1 v. Oxidation – A process in which an element attains a

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

Electrochemistry Terminology #2 An old memory device for oxidation and reduction goes like this…

Electrochemistry Terminology #2 An old memory device for oxidation and reduction goes like this… LEO says GER Lose Electrons = Oxidation Gain Electrons = Reduction

Electrochemistry Terminology #3 q Oxidizing agent The substance that is reduced is the oxidizing

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

Electrochemistry Terminology #4 Ø Anode The electrode where oxidation occurs Ø Cathode The electrode

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

Table of Reduction Potentials Measured against the Standard Hydrogen Electrode

Table of Reduction Potentials Measured against the Standard Hydrogen Electrode

Measuring Standard Electrode Potentials are measured against a hydrogen ion reduction reaction, which is

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

Galvanic (Electrochemical) Cells Spontaneous redox processes have: A positive cell potential, E 0 A

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

Zn - Cu Galvanic Cell From a table of reduction potentials: Zn 2+ +

Zn - Cu Galvanic Cell From a table of reduction potentials: Zn 2+ + 2 e- Zn Cu 2+ + 2 e- Cu E = -0. 76 V E = +0. 34 V

Zn - Cu Galvanic Cell The less positive, or more negative reduction potential becomes

Zn - Cu Galvanic Cell The less positive, or more negative reduction potential becomes the oxidation… Zn 2+ + 2 e. Cu 2+ + 2 e- Cu E = +0. 76 V E = +0. 34 V Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1. 10 V

Line Notation An abbreviated representation of an electrochemical cell Zn(s) | Zn 2+(aq) ||

Line Notation An abbreviated representation of an electrochemical cell Zn(s) | Zn 2+(aq) || Cu 2+(aq) | Cu(s) Anode | material solution || Cathode solution | Cathode material

Calculating G 0 for a Cell G 0 = -n. FE 0 n =

Calculating G 0 for a Cell G 0 = -n. FE 0 n = moles of electrons in balanced redox equation F = Faraday constant = 96, 485 coulombs/mol e- Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1. 10 V

The Nernst Equation Standard potentials assume a concentration of 1 M. The Nernst equation

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-

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

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

Equilibrium Constants and Cell Potential At equilibrium, equilibrium forward and reverse reactions occur at

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

Calculating an Equilibrium Constant from a Cell Potential Zn + Cu 2+ Zn 2+

Calculating an Equilibrium Constant from a Cell Potential Zn + Cu 2+ Zn 2+ + Cu E 0 = + 1. 10 V

? ? ? Concentration Cell Both sides have the same components but at different

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

? ? ? Anode Concentration Cell Cathode Both sides have the same components but

? ? ? Anode Concentration Cell Cathode 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. 0 M) + 2 e- Zn (reduction) Zn 2+ (0. 10 M) + 2 e. Zn 2+ (1. 0 M) Zn 2+ (0. 10 M) (oxidation)

? ? ? Concentration Cell Anode Cathode Both sides have the same components but

? ? ? Concentration Cell Anode Cathode Both sides have the same components but at different concentrations. Step 2: Calculate cell potential using the Nernst Equation (assuming 25 C). Zn 2+ (1. 0 M) Zn 2+ (0. 10 M)

Nernst Calculations Zn 2+ (1. 0 M) Zn 2+ (0. 10 M)

Nernst Calculations Zn 2+ (1. 0 M) Zn 2+ (0. 10 M)

Electrolytic Processes Electrolytic processes are NOT spontaneous. They have: A negative cell potential, (-E

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

Electrolysis of Water In acidic solution Anode rxn: Cathode rxn: -1. 23 V -0.

Electrolysis of Water In acidic solution Anode rxn: Cathode rxn: -1. 23 V -0. 83 V -2. 06 V

Electroplating of Silver Anode reaction: Ag Ag+ + e. Cathode reaction: Ag+ + e-

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

Solving an Electroplating Problem Q: How many seconds will it take to plate out

Solving an Electroplating Problem Q: How many seconds will it take to plate out 5. 0 grams of silver from a solution of Ag. NO 3 using a 20. 0 Ampere current? Ag+ + e- Ag 5. 0 g 1 mol Ag 1 mol e- 96 485 C 1 s 20. 0 C 1 mol e 107. 87 g 1 mol Ag = 2. 2 x 102 s