copyright P.A.Mabrouk,2001,All rights
reserved.
1
Electroanalytical Chemistry
Spring 2001
Patricia Ann Mabrouk
111 HT
x2845
pmabrouk@lynx.neu.edu
copyright P.A.Mabrouk,2001,All rights
reserved.
2
The Economic Case for
Electrochemical Innovation
New Horizons in Electrochemical Science and
Technology,National Academy Press,
Washington,D.C.,1986.
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
3
The Market for Electrochemistry
?Existing $30B
?Projected $20B
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reserved.? Patricia Ann Mabrouk,2001
4
Federal Support for Electrochemistry
?Basic R&D $30M
?Applications $60M
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reserved.? Patricia Ann Mabrouk,2001
5
Applications of Electroanalysis
? Batteries/fuel cells $4B
? Transportation
? Communications
? Biomedical
– H.A.O.Hill - MediSense
glucose electrode
– Defibrillators (Ag/AgCl)
? Weapons
? Coatings and Films
? Electroplating $10B
The MediSense glucose sensor
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reserved.? Patricia Ann Mabrouk,2001
6
Applications of Electroanalysis
? Manufacturing and Waste
Utilization
? Electrolytic Processes $13B
– Al - Hall Cell
– Chlorine
– Copper
– Soda Ash
? Electroorganic synthesis
Photograph of Statue of Charles Hall at Oberlin
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reserved.? Patricia Ann Mabrouk,2001
7
Applications of Electroanalytical
Chemistry
? Microelectronics
$2B
? Memories
? Microcircuitry
? LCD’s (conducting
polymers)
? Corrosion
? Conducting Polymers Measuring the conductivity of a strip of PANI
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8
Corrosion - Some Statistics
?1982 - Economic Cost in U.S,$120B
? 4% of GNP
? $500/person annually
?2000 - Economic Cost in U.S,$300B
? > cost of damage due to floods/fires
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reserved.? Patricia Ann Mabrouk,2001
9
Corrosion - Some Statistics
? 1/3 Cost is potentially
recoverable
? U.S,Steel - 40%
production for
replacement of
corroded parts
? Oil - > $2M/day for
replacement of
underground structures
New Naval Intelligent Corrosivity Sensor
Expected to save Navy $200,000 per aircraft
in preventive maintenance
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reserved.? Patricia Ann Mabrouk,2001
10
Review of Fundamental Terminology
?Electrochemistry - study of redox processes
at interfaces
? Heterogeneous
?So two reactions occurring:
? oxidation
? reduction
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reserved.? Patricia Ann Mabrouk,2001
11
?For reaction,
O + ne- R
?Oxidation,R O + ne-
? loss of electrons by R
? Reduction,O + ne- R
? gain of electrons by O
Review of Fundamental Terminology
??
??
??
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reserved.? Patricia Ann Mabrouk,2001
12
Oxidants and Reductants
?oxidant = oxidizing agent
? reactant which oxidizes another reactant and
which is itself reduced
?reductant = reducing agent
? reactant which reduces another reactant and
which is itself oxidized
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
13
Review of Fundamental Terminology
?So,reactions occur pairwise
? Cannot have oxidation without reduction
?Charge must be conserved
? Number of electrons lost in oxidation
must equal
number of electrons gained in reduction
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
14
Problem
?Identify the oxidant and reductant in each
of the following reactions:
a) Karl Fischer reaction - for quantitation
of moisture
I2 + SO2 + H2O = 2HI + SO3
b) Hall-Heroult process - production of Al
2Al2O3 + 3C = 4Al + 3CO2
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
15
What Makes Electrochemistry
Interesting?
?It is the relative tendencies of oxidants and
reductants to gain/lose electrons that
determines the extent of a redox reaction
?strong oxidant + strong reductant ?
completion
Copper Plating
? Suppose we add a strip
of Zinc metal to a
solution of CuSO4
? Zn - 2e- = Zn2+
? Cu2+ + 2e- = Cu
Zn strip
CuSO4
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Boring!
Well,what if we could separate the
oxidant from the reductant?
We would have set up a constant flow
of electrons = current = electricity!
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reserved.? Patricia Ann Mabrouk,2001
18
Electrochemical Cells - 2 Types
?Galvanic
? Chemical energy ? electrical energy
? Spontaneous
EXAMPLES:
– Primary (non-rechargeable)
– Le Clanche (dry cell)
– Secondary (rechargeable)
– Lead storage battery
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reserved.? Patricia Ann Mabrouk,2001
19
Electrochemical Cells - 2 Types
?Electrolytic
? Electrical energy ? chemical energy
? Non-spontaneous
EXAMPLE:
lead storage battery when recharging
1836 The Daniell Cell
ZnSO4 CuSO
4
Zn Cu
salt bridge
1.1 V
earliest electrical generator
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reserved.? Patricia Ann Mabrouk,2001
21
The Daniell Cell
?Since electrons flow from one electrode to
the other,there is a potential difference
between the electrodes
?This difference is called
? The electromotive force (EMF)
? Cell voltage
? Cell potential
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reserved.? Patricia Ann Mabrouk,2001
22
Terminology
? Electrode
? anode = electrode at which
oxidation occurs
? cathode = electrode at which
reduction occurs
? salt bridge = completes the
electrical circuit
? allows ion movement but doesn’t
allow solutions to mix
? salt in glass tube with vycor frits at
both ends
ZnSO4CuSO4
Zn Cusalt bridge
1.1 V
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
23
Problem,True or False
?In the Daniell cell,zinc metal is reduced to
zinc(II) at the cathode and copper is
oxidized to copper(II) at the anode
?In the Daniell cell,zinc is the oxidant and
copper is the reductant
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
24
The Potential Scale
?Since all redox reactions occur pairwise,
i.e.,
reduction and oxidation always occur at the
same time
we cannot measure the cell potential for just
one half cell reaction
and this means we must establish a
RELATIVE scale for cell potentials
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reserved.? Patricia Ann Mabrouk,2001
25
SHE
? Standard hydrogen electrode
(SHE)
? 1 M H+(aq)+ 2e- = H2(g) (1 atm)
? We define E0 ? 0 V for this
electrode
– where 0 stands for standard state:
– 1 M all solutes
– 1 atm all gases
– 250C (298 K)
HCl
Pt black
H2(gas)
H2SO4
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reserved.? Patricia Ann Mabrouk,2001
26
Standard Reduction Potentials
? Li+ + e- = Li -3.0 V
? 2H2O + 2e- = H2 + 2OH- -0.83 V
? Zn2+ + 2e- = Zn -0.76 V
? 2H+ + 2e- = H2 0 V (SHE)
? Cu2+ + 2e- = Cu 0.34 V
? MnO4- +8H+ +5e- = Mn2+ 1.51 V
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reserved.? Patricia Ann Mabrouk,2001
27
Calculating the Cell Potential
? Half cell reactions are reversible,I.e.,
depending on the experimental conditions any half
reaction can be either an anode or a cathode
reaction
? Changing the stoichiometry does NOT change the
reduction potential (intensive property)
? Oxidation potentials can be obtained from
reduction potentials by changing the sign
Ecell = Eanode + Ecathode
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
28
Problem:
?Calculate the
cell potential
for the Daniell
cell.
? Li+ + e- = Li -3.0 V
? 2H2O + 2e- = H2 + 2OH- -0.83 V
? Zn2+ + 2e- = Zn -0.76 V
? 2H+ + 2e- = H2 0 V
(SHE)
? Cu2+ + 2e- = Cu 0.34 V
? MnO4- +8H+ +5e- = Mn2+ 1.51 V
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
29
Line Notation,an Electrochemical
Shorthand
? Anode reaction appears leftmost while cathode
reaction appears rightmost
? All redox forms of reagents present should be
listed,Phase and concentration specified in
brackets,e.g.,ZnSO4(aq,1 M)
? A single vertical line (|) is used to indicate a
change of phase (s to l to g)
? A double vertical line (||) indicates a salt bridge
? A comma should be used to separate 2 components
in the same phase
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
30
Problem
?How do you represent the Daniell cell at
standard state using line notation?
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
31
Thermodynamic Significance of
Potentials
?We usually operate electrochemical cells at
constant P and T
?Recall,
? ?G = ?H - T ?S
? ?H = ?E + ?(PV)
?So,?GT,P=welec = -qE = -(nF)E
? since q = n F
? Recall,F is Faraday’s constant 96,485 C/mole
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
32
Thermodynamic Significance of
Potentials
?Recall sign of ?G provides information on
spontaneity:
?G negative ? spontaneous reaction
?G positive ? non-spontaneous reaction
?So,since ?G = - nFE
?E positive ? spontaneous reaction
?E negative ? non-spontaneous reaction
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
33
Thermodynamic Significance of
Potentials
?Since half-cell potentials are measured
relative to SHE,they reflect spontaneity of
redox reactions relative to SHE
?More positive potentials ? more potent
oxidants
?More negative potentials ? more potent
reductants
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
34
Predicting the Spontaneity of Redox
Reactions - The Diagonal Rule
?Using a table of standard reduction
potentials,any species on the left of a given
half reaction will react spontaneously with
any species appearing on the right of any
half reaction that appears below it when
reduction potentials are listed from highest
and most positive to lowest and most
negative.
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
35
Problem
?Is the following redox reaction
spontaneous?
Mg2+ + 2Ag = Mg + 2Ag+
given:
Ag+ + e- = Ag +0.80 V
Mg2+ + 2e- = Mg -2.37 V
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
36
How do Cell Potentials Change if We
are Not at Standard State?
?Recall,for the reaction:
aA + bB = cC + dD
??G = G0 + 2.303 RT log Q
where Q is the reaction quotient:
?where ?c is the activity for product C
??
??
ba
dc
Q ?
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
37
How do Cell Potentials Change if We
are Not at Standard State?
?Since ?G = - nFE
then
E = E0 - 2.303 (RT/nF) log Q
?At standard state,
E = E0 - (0.0591 V/n) log Q
This is called the Nernst equation
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
38
Problem
?What is the cell potential for the following
electrochemical cell? What type of cell is it?
Ni(s) | Ni2+ (aq,0.1 M) || Co2+ (aq,2.5 M) | Co(s)
copyright P.A.Mabrouk,2001,All rights
reserved.? Patricia Ann Mabrouk,2001
39
Problem:
?Devise and diagram an electrochemical cell
to measure the solubility product constant
for mercurous chloride,Evaluate Ksp for
mercurous chloride.