Download Lecture 02

Electrochemistry
විද්‍යුත් රසායනය
Basic Physical Chemistry I
CHEM 11132
2015
Dr. S. Sri Skandaraja
Relationship between conductivity
and the composition of the solution
• Two type of ions
– Cation  M2+ (Z+: charge number)
– anion  X2- (Z-: charge number)
• Molar concentrations: C+ and C• In the absent of electric field;
– Disregard the random motion of ions and
imagine ions as stationary point charges
embedded in the solvent.
• An electric field between 2 electrodes
– (+)ve ions accelerated toward cathode
– (-) ve ions accelerated toward anode
Drift Velocity of cation = +
Drift Velocity of cation = -
+
x
s
y
Cathode
Anode
Area of plane s = a
t-
t+
Total volume of solution pass ‘s’ for 1 second = a +
Total no. of cation in solution (s-y) = a + C+ moles
Total charges carry across the ‘s’ = a + C+ Z+ F
I+ = a + C+ Z+ F
I- = a - C- Z- F
I = I+ + I- = a F{(+ C+ Z+) + (- C- Z- )}
Current density
j = F{(+ C+ Z+) + (- C- Z- )}
j=E
 = F{[(+/E) C+ Z+] + [(-/E) C- Z- ]}
+ = (+/E) C+ Z+ F
- = (-/E) C- Z- F
+ = (+/E) C+ Z+ F
- = (-/E) C- Z- F
+ = u+ C+ Z+ F
- = u- C- Z- F
+ + - =  = {(u+ C+ Z+) + (u- C- Z- )}
The moving-boundary
method
NCl, MCl have an ion in common.
The boundary, rather difference
in color, refractivity, etc. is sharp.
Density MCl > density NCl
If uN+ > uM+; no mixing
The distance boundary traveled = x
 The total charge pass through
solution
=Q
 The time
=t
Drift Velocity of M+
If applied electric field is E
If current through the solution is I
Q =It
Multi ion solution
For solution having any number of ionic species
with, jB = B CB ZB F
Where;
jB = Current density due to B ions
B = drift velocity of B ions
CB = molar concentration of B ions
ZB = charge number of B ions
Multi ion solution cont…
For solution having any number of ionic species
with, B = uB CB ZB F
Where;
B = conductivity of B ions
uB = ionic mobility of B ions
Measurement of conductivity of ionic
compound (KCl)
 Can measure the conductivity of KCl solution
solution = (K+ + Cl-) + (H+ + OH-)
solution = KCl + H2O
KCl = solution - H2O
Example
• 411.82  resistance for a 0.741913 w/w % KCl
solution.
• 10.875 k resistance for a solution of SrCl2.
•  = 1.2856 Sm-1 at 25 oC for 0.741913 w/w %
KCl solution.
• Calculate the conductivity of SrCl2 solution.
• If 368.0 k resistance is recorded for deionized water calculate the conductivity of KCl
and SrCl2.
Conductivity of ionic species is depend on
 ionic mobility
 charge
 molar concentration
B = uB CB ZB F
ionic mobility of ionic species is depend on
 total electrolyte concentration in the solution
 temperature
 solvent
Concentration dependent of ionic
mobility
 With increase in total concentration, ionic
mobility will decrease.
Charge Asymmetric effect
+
Without electric field
+
With electric field
Electrophoretic effect
• Viscous force = f B,rel
• f = friction coefficient
• B,rel = velocity of B ion relative to solvent
Ionic mobility at infinite dilution
The highest ionic mobility of a ion
 when the electrolyte concentration goes to zero
(ionic mobility at infinite dilution)
 Symbol uB or uB
Graph of ionic mobility vs. ionic concentration
Extrapolation of graph  uB
Ionic mobility at infinite dilution
Ion
Ionic mobility
109 x (m2V-1s-1)
Li+
Na+ Mg2+ OH40.1 51.9 55
205
ClBrNO3- H3O+
79.1 81.3 74.0 363
 For most of ion
Ionic mobility lie in the range of 40 to 80 x 10-3 m2V-1s-1
 However OH- and H3O- have unusual high value
 This is due to jumping mechanism (Grotthuss mechanism)
Mechanism of hydrogen ions transfer
Transport Number of an ionic species
in the solution
 Transport number (tB) of an ionic species B, in
a solution is defined as the fraction of current
carried by that particular ion.
Transport Number
 For one type of anion and cation
(+)ve ion charge concentration in solution is equal to (-)ve charge
concentration
Transport Number
Since solution is electrically neutral
Transport Number
Experimental determination of
transport number
• Hittorf’s Method
• Moving boundary method