Chapter 7 Electrochemistry § 7.2 Conductivity and its application Out-class extensive reading:...
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Transcript of Chapter 7 Electrochemistry § 7.2 Conductivity and its application Out-class extensive reading:...
Chapter 7 Electrochemistry
§7.2 Conductivity and its application
Out-class extensive reading:
Levine: pp. 506-515,
16.5 electric conductivity
16.6 Electrical conductivity of electrolyte solutions
Key problem:
How to evaluate the ability of an electrolytic solution to
conduct electricity?
1. Some concepts
For metals:
I
UR
R: resistance,
Unit: Ohm,
A
lR
: resistivity
Unit: ·m
Ohm’s Law
For electrolyte solution:
conductivity () : Definition: = 1/ Unit: S·m-1
electric conductance (G) :
Definition: G = 1/R
Unit: -1, mho, Siemens, S
Reciprocal of resistance
l
AG
~
G
AC
B
D
R2
R1
R3R4
I Wheatstone Bridge Circuit
conductance electrode with smooth or platinized platinum foil electrodes
conductance cell
Conductance cell and conductance electrode
R3 R2 = R4 R1
4
321 R
RRR
1
1
RG
2. Measurement of conductance:
~
G
AC
B
DF
R2
R1
R3R4
I
R2
l
AG
High-frequency alternative
current, ca. 1000 Hertz ?
Conductometer
a capacitor!
GKA
lG cell
xxss RR
Calibration: The conductance cell is usually calibrated using standard aqueous KCl (potassium chloride ) solution.
11.21.2890.14110.01470/ S m-1
1.000.1000.01000.0010c/ mol·dm-3
sx
sx R
R
Cell constant
EXAMPLE
The conductance of a cell containing an aqueous 0.0560 mol·dm-3 KCl solution whose conductivity is 0.753 -1·m-1 is 0.0239 -1. When the same cell is filled with an aqueous 0.0836 mol·dm-3 NaCl solution, its conductance is 0.0285 -1. Calculate the conductivity of the NaCl solution.
3. Influential factors of conductivity
(1) Concentration
H2SO4
KOH
LiCl
MgSO4
HAc5 10 15
c/mol·dm-3
0
10
20
30
40
50
60
70
80
/S
·m-1
What can we learn form this figure?
wt % H2SO4
/ S m-1
50 oC
30 oC
10 oC
-10 oC
-30 oC
(2) Temperature
1.Why do we use 38 % H2SO4 in
acid-lead battery?
2.Why do we do electrolysis and
electroplating using warm
electrolyte?
ice
4. Molar conductivity
H2SO4
5 10 15
c/mol·dm-3
0
10
20
30
40
50
60
70
80
Definition
Why do we introduce the concept of molar conductivity?
c
m c
mc
(2) Concentration-dependence of molar conductivity
c / mol·dm-3
m /
S·m
ol-1·m
2
HCl
KOH
NaOHKCl
NaCl
HAc
(1) Why does molar conductivity decrease with increasing concentration?
(2) Does the curve shape inspire you?
Why did Kohlrausch plot m
against c1/2?
Within what concentration range does the linear relation appear?
Kohlrausch
5. Kohlrausch’s empirical formula
0.01
0.02
0.03
0.04
0.00 0.05 0.10 0.15 0.20
m /
S·m
ol-1·m
2
3/ mol dmc
HCl
H2SO4
KCl
Na2SO4
HAc
Kohlrausch empirical formula
m m A c
limiting molar conductivitym
Kohlrausch’s Square Root Law
Within what concentration range is the Kohlrausch law applicable?
For strong electrolyte
Salts /S mol-1 cm2
HCl 426.16
LiCl 115.03
NaCl 126.45
KCl 149.85
LiNO3 110.14
KNO3 144.96
NaNO3 121.56
Molar conductivity at infinite dilution for some electrolytes in water at 298 K.
m
Salts KCl NaCl KNO3 NaNO3
/S mol-1 cm2 149.85 126.45 144.96 121.56
23.4 23.4
m m, m,
m, / ionic conductivities at infinite dilution
m
Δ m
The difference in of the two electrolytes containing the same cation or anion is the same. The same differences in led Kohlrausch to postulate that molar conductivity at infinite dilution can be broken down into two contributions by the ions.
m
m
6. Kohlrausch’s law of independent migration
m m m, ,
m at infinite dilution is made up of independent contributions from the cationic and anionic species.
m at infinite dilution is made up of independent contributions from the cationic and anionic species.
Explanation to the same difference
+ - + -
+ +
m m m,K m,Cl m,Na m,Cl
m,K m,Na
(KCl) (NaCl)
3 3m 3 m 3 m,K m,NO m,Na m,NO
m,K m,Na
(KNO ) (NaNO )
m m, m,v v
How can we determine the limiting molar conductivity of weak electrolyte
m m m(HAc) (H ) (Ac )
m m m m m m(H ) (Cl ) (Na ) (Ac ) (Na ) (Cl )
m m m(HCl) (NaAc) (NaCl)
-1 -1m
-1 -1
(HAc) (426.16 91.00 126.45)S m mol
390.71S m mol
Key:
How to measure the ionic conductivity at infinite dilution?
Key:
How to measure the ionic conductivity at infinite dilution?
m m m, ,
7. Measuring limiting molar conductivity of ions
C - , Z - , U - ; C + , Z + , U + ;
B A C
I+ = AU+Z+c+F
I = AUZ c F
I = I++ I = Ac+Z+F(U++ U)
V
UUFZAcG
)(
)()(
)(
UUFZc
lV
UUFZcA
l
V
UUFZAc
A
lG
c
UUFZcm
)(
For uni-univalent electrolyte:
)(
UUFm
,, mmm
FUm
, FUm
,
t
FUU
FU
m
m
)(,
,m mt ,m mt
mm t ,
mm t ,
To measure m,+ or m,-, either t+ and t- or U+ and U- must be determined.
c
UUFZcm
)(
UU
Ut
UU
Ut
ions r / nm 102 ions r / nm 102
H+ 3.4982 OH¯ 1.98
Li+ 0.68 0.387 F¯ 1.23 0.554
Na+ 0.98 0.501 Cl¯ 1.81 0.763
K+ 1.37 0.735 Br¯ 1.96 0.784
Mg2+ 0.74 1.061 CO32 1.66
Ca2+ 1.04 1.190 C2O42 1.48
Sr2+ 1.04 1.189 Fe(CN)63 3.030
Al3+ 0.57 1.89 Fe(CN)64 4.420
Fe3+ 0.67 2.04
La3+ 1.04 2.09
1) Nature of ions
Charge; Radius; charge character; transfer mechanism
7.2.7 Influential factors form
mm
Transport mechanism of hydrogen and hydroxyl ions
Grotthus mechanism (1805)
The ion can move along an extended hydrogen-bond network.
Science, 2002, 297:587-590
G
m
m
U U
t t
Macroscopic Microscopic
, ,, m m
Dynamic
Summary