Chapter 7 Electrochemistry
description
Transcript of Chapter 7 Electrochemistry
I. N. Levine
pp. 431--443
14.7 Standard electrode potentials
14.8 Concentration cell
14.9 Liquid-junction potential
14.10 Applications of EMF measurements
14.12 ion-selective membrane electrodes
Computation of emf
For cell with single solution:
Cd(s)|CdSO4(a±) |Hg2SO4(s)|Hg(l)
2-4SO
lnRT anF
y
2CdlnRT a
nF y
Hg2SO4(s)+2e 2Hg(l) + SO4
Cd(s) Cd2++ 2e
2 24
2SO Cd
ln ln ( ) lnRT RT RTE a a anF nF nF
y y y y
Because a is a measurable quantity, E of the cell with single electrolyte can be calculated exactly.
E
For cell with two electrolytic solutions:
Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s)
2
2
Cu
Zn
lnaRTE E
nF a
y
1 ,1
2 ,2
lnmRTE E
nF m
y
we have to use mean activity coefficient () which is measurable in stead of the activity coefficient of individual ion (+ or -) which is unmeasurable.
1. Judge the strength of the oxidizing and reducing agents
⊖ (Fe3+/Fe2+) = 0.771 V
⊖ (I2/I) = 0.5362 V
Oxidative form: Fe3+, I2
Reductive form: Fe2+, I-
The oxidative form with higher (standard) electrode
potential is stronger oxidizing species, while the reductive
form with lower (standard) electrode potential is stronger
reducing agent.
(Ox)1 + (Red)2 = (Red)1+ (Ox)2
2. Determination of the reaction direction
When concentration differs far from the standard concentration, should be used in stead of ⊖.
Stronger oxidizing species oxidizes stronger reducing species to produce weaker reducing and weaker oxidizing species.
⊖ (Fe3+/Fe2+) = 0.771 V; ⊖ (I2/I) = 0.5362 V
Fe3+ + I = Fe2+ + 1/2I2
+Au 1e Au =1.7 Vy
2 2O +2H O+4e 4OH =0.401y
Example
In order to make Au in mine dissolve in alkaline solution
with the aid of oxygen, people usually add some
coordinating agent into the solution. Which coordination
agent is favorable? Please answer this question based on
simple calculation.
Divergent reaction
Cl2 + 2NaCl = NaCl + NaClO + H2O
Divergent reaction occur when R > L
HIO IO3 + I2
R 2
L 3
(HIO/I ) 1.45V
(IO /HIO) 1.13V
y
y
-1+7 +5 +1 01.7 1.13 1.45 0.534- -25 6 3H I O I O H I O I I
Can which species undergo divergent reaction?
3. Advance of reaction (equilibrium constants)
1 mol dm-3 iodine solution + Fe2+ (2 mol dm-3)
32
2
22
3
I3 2 Fe2
I Fe
IFe
Fe I
(Fe / Fe ) (I / I ) ln ln
ln ln a
a aRT RTnF a nF a
a aRT RT K EnF a a nF
y y
y y
3
2
3 2 3 2 Fe
Fe
(Fe / Fe ) (Fe / Fe ) lnaRT
nF a
y
2I2 2
I
(I / I ) (I / I ) lnaRT
nF a
y
3 22(Fe / Fe ) (I / I )
Fe3+ + I¯ Fe2+ + ½ I2
At equilibrium
Standard emf and standard equilibrium constant
lnr mG nFE RT K y y y
lnRTE KnF
y y
For any reaction that can be designed to take place in an electrochemical cell, its equilibrium constant can be measured electrochemically.
Four equilibria in solution 1) Dissolution equilibrium 2) Reaction equilibrium 3) Dissociation equilibrium 4) Coordination equilibrium
Example
Determine the solubility products of AgCl(s).
AgCl(s) Ag+ + Cl¯
The designed cell is
Ag(s)|AgNO3(a1)||KCl(a2)|AgCl(s)|Ag(s)
lnRTE KnF
y y
0.00 10.00 20.00 30.00 40.00 50.00
0.300
0.100
0.500
0.700
3NaOH / cmV
E / V
HAc-NaOH
HCl-NaOH
30.0020.00 40.003
NaOH / cmV
ΔΔEV 0.4
0.2
inflexion point
Differential plot
5. Determination of mean ion activity coefficients
Pt(s), H2 (g, p⊖)|HCl(m)|AgCl(s)-Ag(s) 1/2 H2 (g, p⊖) + AgCl(s) = Ag(s) + H+(m) + Cl(m)
H Cl
2 2ln ln lnRT RT RTE E a a E mnF nF nF
y y
For combined concentration cell
1,1
2,2ln2
mm
FRTE
Using one electrolytic solution with known mean activity coefficient, the mean activity coefficient of another unknown solution can be determined.
Answer: = 0.9946
Example:
Pt(s), H2 (g, p) |HBr(m) | AgBr(s)-Ag(s)
Given E = 0.0714 V, m = 1.262 10-4 mol·kg-1, E = 0.5330 V,
calculate .
lnRTE E mF
y
6. Determination of transference number
Zn|ZnSO4(a,1) |ZnSO4(a,2) |Zn
Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s)
'ln)12(
'ln)(
mm
FRTt
mm
FRTttE j
The relationship between transference number and liquid junction potential can be made use of to determine the transference number of ions.
Electromotive forces of cell with and without liquid junction potential gives liquid junction potential.
7. Measurement of pH
1909, Sorensen defined: pH = log [H+]
present definition: H
pH log a Non-operational definition
1) Hydrogen electrode
Pt(s), H2 (g, p⊖)|H +(x) |SCE
+ +2
SCE H /H H
SCE
lg
0.05916pH
RTE anF
y
poison of platinized platinum
The way to determine pH
2) Quinhydrone electrode
supramolecule : 1:1 quinone: hydroquinone
1) Equal concentrations of both species in the solution.
2) Being nonelectrolytes, activity coefficients of dilute Q and H2
Q is unity.
Q + 2H + + 2e- H2Q
Pt(s)|Q, H2Q, H+(mx) |SCE
2
2
2
SCE Q/H Q
H QSCE Q/H Q 2
Q H
SCE
ln2
0.6995 0.05916pH
E
aRTF a a
y
O
O H
H O
O
O
O
2e-2H+
OH
OH
+ +
membrane potential
2 4 6 8 10 12 14 160-2
GE
/ m
V
pH
GE = ⊖ GE - 0.05915 pH
Linear relation of GE and pH exists within pH range from 0 to 14.
GE H+(mx)(SCE)
Test cell:
4) Operational definition of pH
s x( )pH(x) pH(s)2.303E E F
RT
Buffer A B C D E
pH 3.557 4.008 6.865 7.413 9.180
pH meter with standard buffer solution
pH of standard buffer solutions at 25 oC
Es = ⊖SCE –(⊖GE - 0.05915 pHs )
Ex = ⊖SCE –(⊖GE - 0.05915 pHx
)
Calibration
Measurement
What is the concentration of hydrogen ion in this solution?
Composite electrode:
with reference electrode, usually AgCl/Ag electrode embedded on the side of glass electrode.
7. Determination of ion concentrationIon-selective electrode
Cutaway view of an ion selective electrode
For F- electrode, thin film of LaF3 single crystal is used as ion selective membrane.
For S2- electrode, compressed thin film of AgCl-Ag2S mixture is used as ion-selective membrane.
antigen antibody
electrochemical sensor of potential type
8. Electrochemical sensor
electrochemical sensor of current type
Electrochemical nose
Electroanalytical chip
PbO2
Ion-exchange membrane
amplifierannunciator
Pt electrode
Gas-permeable membrane