Electrochemistry Electrochemistry Terminology 1 v Oxidation A process
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Electrochemistry
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… LEO says GER Lose Electrons = Oxidation Gain Electrons = Reduction
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 where reduction occurs Memory device: Reduction at the Cathode
Table of Reduction Potentials Measured against the Standard Hydrogen Electrode
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 negative free energy change, (- G)
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 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) || Cu 2+(aq) | Cu(s) Anode | material solution || Cathode solution | Cathode material
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 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 way:
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+ + Cu E 0 = + 1. 10 V
? ? ? 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 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 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)
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. 83 V -2. 06 V
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 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
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