1. Chem. 1B – 11/3 Lecture

2. Announcements I Mastering – Ch. 18 assignment due 11/12
Lab – next week
Doing Experiment 9 (thermodynamics – lnK vs. 1/T plots) and 10 (electrochemistry)
Experiment 8 due Wed./Thurs.
Quiz on Exp. 8 and 9 and on electrochemistry

3. Announcements III Today’s Lecture Electrochemistry
Standard Cell Potentials
Oxidizing and Reducing Agents
Acid Oxidation of Metals
Charge and Energy in Voltaic Cells
Relationship between DG° and E °
The Nernst Equation (for non-standard conditions)

4. Chapter 18 Electrochemistry
Example Question
An Ag/AgCl electrode is a common reference electrode. What is the standard potential of a cell made up of a Cu2+ solution being reduced to Cu(s) and AgCl(s) being reduced to Ag(s)?
E°(Cu2+ + 2e- ↔ Cu(s)) = 0.34 V
E°(AgCl(s) + e- ↔ Ag(s) + Cl- (aq)) = 0.22 V
What is the balanced reaction and what species must be present at 1 M?

5. Chapter 18 Electrochemistry
Oxidizing/Reducing Agents
Compounds with large positive or negative E° (standard reduction) values are frequently used in electrochemistry (or in redox titrations)
Example: MnO4- - E° (MnO4-(aq) + 8H+(aq) + 5e-) = 1.51 V is frequently used in redox titrations
Why? Because if E° is high, it strongly reduces, which makes it useful for oxidizing a wide variety of compounds (e.g. Cu(s))
Such a compound is called an oxidizing agent (oxidizes other compounds)

6. Chapter 18 Electrochemistry
Oxidizing/Reducing Agents – cont.
Products of reduction reactions with large negative E° values (e.g. Li(s), K(s)) are easily oxidized and can therefore reduce other compounds
Example: Al(s) - E° (Al3+(aq) + 3e-) = V is capable of reducing transition metals (reaction with iron oxide is in thermite reaction)

7. Chapter 18 Electrochemistry
Reduction Potential and Oxidation of Metals by Acids
Just as we can see which metals will oxidize or reduce when pairing two metals (Ag/Cu example), we also can see which metals will react in acid to produce H2(g)
Metals with E° (standard reduction) < 0 will react with H+
Examples: Fe, Pb, Sn, Ni, Cr, Zn, Al
Metals with E° (standard reduction) > 0 will not react with acid (except with HNO3 which is a stronger oxidizing agent) – Cu, Ag, Au, Hg

8. Chapter 18 Electrochemistry
Reducing Potential Questions
Given the table below, which of the following oxidizing agents is strong enough to oxidize Ag(s) to Ag+(aq) (under standard conditions)?
H+(aq) b) Co2+(aq) c) Cu2+(aq) d) Co3+(aq) e) Br2(l)
Reaction
Eº (V)
Ag+(aq) + e- ↔ Ag(s)
+0.799
Co2+(aq) + 2e- ↔ Co(s)
-0.277
Cu2+(aq) + 2e- ↔ Cu(s)
+0.337
Co3+(aq) + e- ↔ Co2+(aq)
+1.808
Br2(l) + 2e- ↔ 2Br- (aq)
+1.065

9. Chapter 18 Electrochemistry
Relating Standard Potential to Free Energy
Two measures of the usefulness of a battery (voltaic cell) are: potential (voltage) supplied and charge stored
The combination of these two give the energy stored for electrical work
Stored charge (allows one to calculate lifetime under given current load) = q = nF
where n = moles of e- (involved in balanced chemical equation) and F = Faraday’s constant
F = charge of a mole of electrons = NA*qelectron = (6.02 x 1023 e’s/mol e)(1.60 x C/e) = 96,485 C/mol e

10. Chapter 18 Electrochemistry
Relating Standard Potential to Free Energy – cont.
Since we know that potential is energy per charge, we can also determine that energy (in terms of maximum electrical work) = -qEcell° (negative sign reflects that work is negative if done by the system on the surroundings)
So, wmax = -nFEcell°
In terms of potential energy, wmax = DG°
Thus DG° = -nFEcell°

11. Chapter 18 Electrochemistry
Example problems:
A NiCad battery contains 12.0 g of Cd that is oxidized to Cd(OH)2. How long should the battery last if a motor is drawing A? Assume 100% efficiency. (Hint: 1 A = 1 C s-1)
Calculate the DG° (in kJ/mol) for the following reaction:
2Ag+(aq) + Cu(s) ↔ 2Ag(s) + Cu2+(aq), based on E° values given for half reactions on slide 8.

12. Chapter 18 Electrochemistry
Relating Potential to Non-Standard Conditions: The Nernst Equation
Starting from our equation for non-standard Equilibrium: DGrxn = DGºrxn + RTlnQ, we can convert this to a potential equation:
-nFEcell = -nFEºcell + RTlnQ
or if we divide both sides by –nF:
Ecell = Eºcell - (RT/nF)lnQ
Ecell = Eºcell - (0.0592/n)logQ (Nernst Equation – valid for T = 25ºC)

13. Chapter 18 Electrochemistry
Nernst Equation Application
Example: Determine the voltage for a Ag(s)/AgCl(s) electrode when [Cl-] = M if Eº = V (at T = 25°C)?
Note: this is the same as when this electrode is attached to a SHE. 13

14. Chapter 18 Electrochemistry
Nernst Equation Application – Cont.
2nd Examples:
The following cell,
Cd(s)|CdC2O4(s)|C2O42-(aq)||Cu2+(aq)1 M|Cu(s) is used to determine [C2O42-]. If Eº for the Cd reaction is V (reduction potential for oxidation reaction) and Eº for the Cu reaction is V, and the measured voltage is V, what is [C2O42-]?