Chapter 2-PhyChem 2 - Electrochemical Cells Electrodes
Chapter 2-PhyChem 2 - Electrochemical Cells Electrodes
Chapter 2-PhyChem 2 - Electrochemical Cells Electrodes
ELECTROCHEMICAL
CELLS & ELECTRODES
Trần Thụy Tuyết Mai, PhD.
Email:TUYETMAI@HCMUT.EDU.VN
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1. Electrochemical cells
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ELECTROCHEMICAL CELLS
What reaction would occur if a Zn bar was immersed in a CuSO4 aqueous solution?
Zn + CuSO4 ZnSO4 + Cu H298, rxn. = -210.84 kJ, G0298= -209.9 kJ = -50710 cal
Chemical
energy
Electrochemical
cell
Electrical energy
Anodic zone Cathodic zone
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A galvanic cell is an
electrochemical cell that
produces electricity as a
result of the spontaneous
reaction occurring inside it.
An electrolytic cell is an
electrochemical cell in which
a non-spontaneous
reaction is driven by an
external source of current.
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HOW TO MEASURE
ELECTRODE POTENTIAL ( )
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Standard electrode potential table
2H+ + 2e H2 0V
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oxidation reduction
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ELECTRIC POTENTIAL JUMPS
& ELECTROMOTIVE FORCE
Electrochemical cell :
M1L1L2M2
E E = M2/M1 +M1/L1+L1/L2+L2/M2
M2
M1 M2
L1/L2 : liquid junction potential (diffusion potential) often
L1 L2
small and negligible (1 – 2 mV)
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Electromotive force
Consider cell: (A) : (-) M2 M1 L1 L2 M2 (+)
And 2 cells: (B): (-) M0 L0 L1 M1 M0 (+)
(C): (-) M0 L0 L2 M2 M0 (+)
EA= M2/M1+M1/L1+L2/M2
EB= M0/L0+L1/M1+M1/M0 EC- EB = EA
EC= M0/L0+L2/M2+M2/M0
If M0 L0 is the standard electrode (H2H+): EB= (-) và EC= (+)
Therefore: EA= (+) - (-) (Without diffusion potential)
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ELECTRIC WORK
aDd .aLl
G RT ln K a RT ln( a b ) nFE @ 298 K:
a A .aB
0.059 aLl .aDd
RT
ln K a const E0 E E0 lg a b
nF 2 a A .aB
RT a Ll a Dd
E E0 ln a b (NERNST equation) Daniel cell
nF a A aB
0.059 aZn2
E0: emf at ai = 1 E (E
0
Cu2 / Cu
E 0
Zn2 / Zn
) lg
2 aCu2
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NERNST EQUATION
( EFFECT OF CONCENTRATION ON )
Rxn. on electrode:
Ox + ne Kh
RT akh
o
T ln
nF aox 0.059 akh
@ 298 K o
298 lg
n aox
0: electrode potential at ai= 1
0298 checked in Data section
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EFFECT OF TEMPERATURE
G = H - T.S
PT Gibbs - Helmholtz :
(G ) (G )
G H T. n.E.F H T.
T P T P
H T (nFE) H E
E T.
n.F n.F T P n.F T P
H dE
E T
n.F dT
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EFFECT OF TEMPERATURE
S = ST2 – ST1
coefficient :
is proportional to the
dT nF entropy change
For electrode:
d S
emf’s temp. In the narrow temp. range d/dT = const.
coefficient dT nF
d
o
T
o
298 (T 298)
dT
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LUTHER RELATION
Considering on M metal have 2
cations of Mh+ và Mn+
Mh+ + he M (1) G1 = -hFh
ne
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TYPE 2 Metal coated by an insoluble salt immersed in a
solution containing the anion of the salt
• Notation: An- MA M
RT
Nernst equation: MA/ M , A
n
o
MA / M , An
ln a An
nF
Electrode Ag-AgCl: Cl- AgCl Ag
Half rxn. : AgCl + e = Ag + Cl-
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Calomel electrode:
Cl- Hg2Cl2 Hg, Pt
Half rxn on electrode:
Hg2Cl2 + 2e = 2Hg + 2Cl-
0.059
Cal o
Cal 2
lg aCl 0.2678 0.059 lg a
Cl
2
tuyetmai@hcmut.edu.vn 29
TYPE 3
A metal coated by two insoluble salts with common anion in a
solution containing cation of the second salt.
M’n+ M’A, MA M TMA < TM’A
Ca2+CaCO3, PbCO3Pb
Reaction: PbCO3 + Ca2+ + 2e = Pb + CaCO3
R.T
0
Pb 2 / Pb
. ln aPb 2
n.F
TPbCO3 aPb 2 .aCO 2 ; TCaCO3 aCa2 .aCO 2 R.T
3 3
'0
. ln aCa2
TPbCO3 n.F
aPb 2 aCa2
TCaCO3
tuyetmai@hcmut.edu.vn 30
GAS ELECTRODES
• An inert metal in contact with
both a gas and its ion
H2 gas electrode: H+ H2 Pt
2H+ + 2e = H2
2
RT a H
H
/ H2
Ho / H ln
2
2 F PH 2
0.059
25 C H / H
o
0.059 pH lg PH 2
2
2
http://slideplayer.com/slide/9300791/
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GAS ELECTRODE
tuyetmai@hcmut.edu.vn 32
AMALGAM ELECTRODES:
• Amalgam of a metal in a solution containing its ion.
Mn+ M, Hg; Mn+ + ne = M(Hg)
RT aM ( Hg )
M n
/ M , Hg
o
M n / M , Hg
ln
nF a M n
Cadimi amalgam electrode: Cd2+ Cd, Hg
Ox , Red Pt
Phản ứng điện cực: Ox + ne = Red
RT aKh
Re dox o
Re dox ln
nF aOx
Simple redox electrode: Fe3+ ,Fe2+ Pt
RT aFe 2
Fe 3 / Fe 2 , Pt 3 2
o
ln
Fe / Fe , Pt F aFe 3
tuyetmai@hcmut.edu.vn 34
COMPLEX REDOX ELECTRODE
Electrode: MnO4-, H+ , Mn2+ Pt Electrode rxn.: MnO4- + 8H+ + 5e = Mn2+ + 4H2O
8RT RT aMn2
MnO , H
/ Mn2 , Pt
o
MnO4 , H / Mn2 , Pt
ln aH ln
4
5F 5F aMnO
4
o OH
C6H4O2,H+C6H4(OH)2,Pt : + -
+ 2H + 2e =
(Quinhydrone electrode)
o OH
Quinone (Q) Hydroquinone (Hq)
RT aHq
Quinh o
Quinh ln
2 F aQ aH2 RT
Quinh o
Quinh ln aH
RT RT aHq F
Quinh o
Quinh ln aH ln
F 2 F aQ (As indicator electrode for pH measurement)
tuyetmai@hcmut.edu.vn 35
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GLASS ELECTRODE
aH a H aM
tt 0.059 lg
o
tt K tt
aH
tt
aH .aH
tt
pH = 10 – 12 aH (a aH )
K tt
tt 0,059 pH
o'
tt
aH aH
tt
Alkaline medium
tt 0,059 lg aM
o ''
tt
tt 0,059 pH
o '''
tt
tuyetmai@hcmut.edu.vn 39
4. Junction (diffusion) potential
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kt: exist in loaded circuits, at interface of the two electrode solutions
(-)Ag (+)Ag
- AgNO3 diffuse from (+) to (-) (because F
of concentration difference) kt
- +
- Velocity of NO3- > velocity of Ag+
- +
t+
NO3- moves faster through the
membrane (-) charges, left of the t- - +
membrane has (+) charges kt - +
- +
-kt is often small (< 0.1V) AgNO3’(a’) AgNO3(a” )
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Eload E df
RT a ''
df ln
F a '
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Methods to avoid df
- Use salt bridge:
an agar form of concentrated salt
containing cation and anion which
have similar velocity. Ex: KCl, KNO3,
NH4Cl, NH4NO3, ...
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5. Measurement of emf
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Measurement of emf
AC
Ex .Ew
AC '
Ew: a known, highly stable emf cell
(Weston cell)
Weston cell:
(-) Cadimi amalgam electrode
(+) Hg electrode type 2
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pH measurement
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(-)Hg, Hg2Cl2 | KCl || H+ (x)| glass electrode (+)
E tt cal tt0 0.059 pH cal
tt0 cal E
pH
0.059
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Determination of activity coefficients ()
R.T R.T
E E 2 0
ln( .m) 2 . ln( )
F F
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R.T R.T
E E 2
0
ln( .m) 2 . ln( )
F F
- If Eo is known: Measure E calculate
- If Eo is unknown:
R.T R.T
E2 ln( .m) E 0 2 . ln( ) =Y
F F
If m 0 : 1 then Y Eo
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Determination of solubility product (T)
Reaction: 1
Cl 2 Ag AgCl
2
E Cl
2 / Cl
AgCl / Ag ,Cl 0
Cl2 / Cl
0.059 lg aCl AgCl
0
/ Ag , Cl
0.059 lg aCl
Besides: 0
AgCl / Ag , Cl
0
Ag / Ag
0.059 lg TAgCl
E Cl
0
2 / Cl
Ag
0
/ Ag
0.059 lg TAgCl
Cl0 2 / Cl
1.358V ; Ag
0
/ Ag
0.799V ; Emeas. = 1.136V TAgCl = 1.77x10-10
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6. Electrochemical sources
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- Primary cells/ batteries: one-time use
(non- rechargeable)
Ex: dry cell: La Clanché (Zn-C)
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Primary cells/ batteries
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(-) Pb PbSO4 H2SO4 (25-30%) PbO2 Pb (+)
Eo = 2.037 V
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Hydrogen Fuel Cell
(-) Ni H2 KOH (30-40%) O2 Ni (+)
Eo = 1.23 V
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