This document summarizes key concepts from an electrochemistry chapter, including: - Redox reactions involve the gain or loss of electrons. Voltaic cells harness redox reactions to produce electric currents. - Standard reduction potentials indicate the tendency of half-reactions to gain or lose electrons under standard conditions. - Electrochemical cells use spontaneous redox reactions to generate voltage. Applications include batteries, electrolysis, corrosion prevention, and pH meters.

1. Chapter 18
Electrochemistry
Dr. S. M. Condren

2. Redox Reactions
Oxidation
• loss of electrons
Reduction
• gain of electrons
oxidizing agent
• substance that cause oxidation by being reduced
reducing agent
• substance that cause oxidation by being reduced
Dr. S. M. Condren

3. Electrochemistry
In the broadest sense, electrochemistry is
the study of chemical reactions that produce
electrical effects and of the chemical
phenomena that are caused by the action of
currents or voltages.
Dr. S. M. Condren

4. Dr. S. M. Condren

5. Voltaic Cells
• harnessed chemical reaction which
produces an electric current
Dr. S. M. Condren

6. Voltaic Cells
Cells and Cell Reactions
Daniel's Cell
Dr. S. M. Condren
Zn(s) + Cu+2
(aq) ---> Zn+2
(aq) + Cu(s)
oxidation half reaction
anode Zn(s) ---> Zn+2
(aq) + 2 e-reduction
half reaction
cathode Cu+2
(aq) + 2 e- ---> Cu(s)

7. Voltaic Cells
• copper electrode
dipped into a
solution of
copper(II) sulfate
• zinc electrode
dipped into a
solution of zinc
sulfate
Dr. S. M. Condren

8. Voltaic Cells
Dr. S. M. Condren

9. Hydrogen Electrode
• consists of a platinum
electrode covered with
a fine powder of
platinum around
which H2(g) is bubbled.
Its potential is defined
as zero volts.
Hydrogen Half-Cell
H2(g) = 2 H+
(aq) + 2 e-reversible
Dr. S. M. Condren
reaction

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12. Standard Reduction Potentials
• the potential under standard conditions
(25oC with all ions at 1 M concentrations
and all gases at 1 atm pressure) of a half-reaction
in which reduction is occurring
Dr. S. M. Condren

13. Some Standard Reduction Potentials
Table 18-1, pg 837
Li+ + e- ---> Li -3.045 v
Zn+2 + 2 e- ---> Zn -0.763v
Fe+2 + 2 e- ---> Fe -0.44v
2 H+
(aq) + 2 e- ---> H2(g) 0.00v
Cu+2 + 2 e- ---> Cu +0.337v
O2(g) + 4 H+
(aq) + 4 e- ---> 2 H2O(l) +1.229v
F2 + 2e- ---> 2 F- +2.87v
Dr. S. M. Condren

14. If the reduction of mercury (I) in a voltaic
cell is desired, the half reaction is:
Which of the following reactions could be
used as the anode (oxidation)?
A, B
Dr. S. M. Condren

15. Cell Potential
• the potential difference, in volts, between
the electrodes of an electrochemical cell
• Direction of Oxidation-Reduction Reactions
• positive value indicates a spontaneous
reaction
Dr. S. M. Condren

16. Standard Cell Potential
• the potential difference, in volts, between
the electrodes of an electrochemical cell
when the all concentrations of all solutes is
1 molar, all the partial pressures of any
gases are 1 atm, and the temperature at
25oC
Dr. S. M. Condren

17. Cell Diagram
• the shorthand representation of an
electrochemical cell showing the two half-cells
connected by a salt bridge or porous
Dr. S. M. Condren
barrier, such as:
Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s)
anode cathode

18. Metal Displacement Reactions
• solid of more reactive metals will displace
ions of a less reactive metal from solution
• relative reactivity based on potentials of
half reactions
• metals with very different potentials react
most vigorously
Dr. S. M. Condren

19. Ag+ + e- --->Ag E°= 0.80 V
Cu2+ + 2e- ---> Cu E°= 0.34 V
Will Ag react with Cu2+?
yes, no
Will Cu react with Ag+?
yes, no
Dr. S. M. Condren

20. Internet Report
• Due end of day, 5:00 pm, Dec. 1, 2001
Dr. S. M. Condren

21. Gibbs Free Energy
and Cell Potential
DG = - nFE
where n => number of electrons changed
F => Faraday’s constant
E => cell potential
Dr. S. M. Condren

22. Applications of
Electrochemical Cells
Batteries
– device that converts chemical energy into
electricity
Primary Cells
– non-reversible electrochemical cell
– non-rechargeable cell
Secondary Cells
– reversible electrochemical cell
– rechargeable cell
Dr. S. M. Condren

23. Applications of
Electrochemical Cells
Dr. S. M. Condren
Batteries
Primary Cells
"dry" cell & alkaline cell 1.5 v/cell
mercury cell 1.34 v/cell
fuel cell 1.23v/cell
Secondary Cells
lead-acid (automobile battery) 2 v/cell
NiCad 1.25 v/cell

24. “Dry” Cell
Dr. S. M. Condren

25. “Dry” Cell
Dr. S. M. Condren

26. “Flash Light” Batteries
"Dry" Cell
Zn(s) + 2 MnO2(s) + 2 NH4
Dr. S. M. Condren
+ ----->
Zn+2
(aq) + 2 MnO(OH)(s) + 2 NH3
Alkaline Cell
Zn(s) + 2 MnO2(s) ---> ZnO(s) + Mn2O3(s)

27. “New” Super Iron Battery
Mfe(VI)O4 + 3/2 Zn 1/2 Fe(III)2O3 + 1/2 ZnO + MZnO2
(M = K2 or Ba)
Environmentally friendlier than MnO2 containing batteries.
Dr. S. M. Condren

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29. Lead-Acid
(Automobile Battery)
Dr. S. M. Condren

30. Lead-Acid
(Automobile Battery)
Pb(s) + PbO2(s) + 2 H2SO4 = 2 PbSO4(s) + 2 H2O
2 v/cell
Dr. S. M. Condren

31. Nickel-Cadmium (Ni-Cad)
Cd(s) + 2 Ni(OH)3(s) = Cd(OH)2(s) + 2 Ni(OH)2(s)
NiCad 1.25 v/cell
Dr. S. M. Condren

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33. Automobile Oxygen Sensor
ZrO2 / CaO
Dr. S. M. Condren
Air,
constant [O 2 ]
porous Pt electrodes
migrating
O 2 - ions
exhaust gas,
unknown [O2 ]
measured potential difference

34. Automobile Oxygen Sensor
• see Oxygen Sensor Movie from Solid-State
Resources CD-ROM
Dr. S. M. Condren

35. pH Meter
pH = (Eglass electrode - constant)/0.0592
Dr. S. M. Condren

36. Effect of Concentration on Cell
Voltage: The Nernst Equation
Ecell = Eo
cell - (RT/nF)ln Q
Ecell = Eo
cell - (0.0592/n)log Q
where Q => reaction quotient
Q = [products]/[reactants]
Dr. S. M. Condren

37. EXAMPLE: What is the cell potential for the
Daniel's cell when the [Zn+2] = 10 [Cu+2] ?
Q = ([Zn+2]/[Cu+2] = (10 [Cu+2])/[Cu+2] = 10
Eo = (0.34 V)Cu couple + (-(-0.76 V)Zn couple
n = 2, 2 electron change Ecell = Eo
cell - (0.0257/n)ln Q
thus Ecell = (1.10 - (0.0257/2)ln 10) V
Ecell = (1.10 - (0.0257/2)2.303) V
Ecell = (1.10 - 0.0296) V = 1.07 V
Dr. S. M. Condren

38. Nernst Equation
nF ln Q = – 2.3 RT
F log[H+]base side
[H+]acid side
H 2 in
1 atm
e– e–
Dr. S. M. Condren
+0.83 V
salt bridge
H 2 in
1 atm
voltmeter
Pt
electrode
Pt
NaOH electrode
1 M
anode (–)
HCl
1 M
cathode (+)
F log [h+]n-type side
[h+]p-type side
n p +
+
+
+
–
–
–
–
[H+]acid side ® [H+]base side
E = Eo – RT
[h+]p-type side ® [h+]n-type side
E (in volts) = – 2.3 RT

39. Electrolysis
• non-spontaneous reaction is caused by
the passage of an electric current
through a solution
Dr. S. M. Condren

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41. Electrolysis
Electrolysis of Sodium chloride
(chlor-alkali process)
molten reactants => liquid sodium and
chlorine gas
aqueous reactants => caustic soda (sodium
hydroxide) and chlorine gas
Dr. S. M. Condren

42. Electrolysis
Preparation of Aluminum (Hall process)
Dr. S. M. Condren

43. Electrolytic Refining of Copper
Cu(s) + Cu+2
(aq) --> Cu+2
(aq) + Cu(s)
impure pure
anode cathode
impurities: anode mud; Ag, Au, Pb
Dr. S. M. Condren

44. Dr. S. M. Condren

45. Quantitative Aspects of Electrolysis
• 1 coulomb = 1 amp sec
• 1 mole e- = 96,500 coulombs
Dr. S. M. Condren

46. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
Dr. S. M. Condren

47. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
#g C r = - -(-4-5-- m---in--)-
Dr. S. M. Condren

48. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
definition of minute
#g Cr = (-4--5- -m--i-n--)-(-6-0-- -s-e-c-)--
(1 min)
Dr. S. M. Condren

49. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
(45) (60 sec) (25 amp)
#g Cr = ---------------------------
(1)
Dr. S. M. Condren

50. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
definition of a coulomb
(45)(60 sec)(25 amp)(1 C)
#g Cr = -----------------------------
(1) (1 amp sec)
Dr. S. M. Condren

51. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
Faraday’s constant
(45)(25)(60)(1 C)(1 mol e- #g Cr = --------------------------------)--
(1)(1)(96,500 C)
Dr. S. M. Condren

52. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
Dr. S. M. Condren
atomic weight
(45)(60)(25)(1)(1 mol e-)(52 g Cr)
#g Cr = -------------------------------------------
(1)(1)(96,500) (6 mol e-)

53. Electroplating
EXAMPLE: How many grams of chromium
can be plated from a Cr+6 solution in 45
minutes at a 25 amp current?
(45)(60)(25)(1)(1 mol e- #g Cr = ------------------------------)-(-5--2- -g- -C--r-)--
(1)(1)(96,500)(6 mol e-)
= 58 g Cr
Dr. S. M. Condren

54. Dr. S. M. Condren

55. Corrosion
O2(g) + 4 H+
(aq) + 4 e- -----> 2 H2O(l)
Eo = 1.23 V
Rusting
Fe(s) -----> Fe+2
(aq) + 2 e- Eo = 0.44 V
O2(g) + 4 H+
(aq) + 4 e- -----> 2 H2O(l) Eo = 1.23 V ------------------------------------------- --------------
2 Fe(s) + O2(g) + 4 H+
(aq) ----->
2 H2O(l) + Fe+2
(aq) Eo = 1.67 V
Dr. S. M. Condren

56. Preventing Corrosion
Dr. S. M. Condren
painting
galvanizing
sacrificial anode