ELECTROLYTE CONDUCTANCE
Download
Report
Transcript ELECTROLYTE CONDUCTANCE
CMT552
ELECTROCHEMISTRY AND
CORROSION SCIENCE
ELECTROLYTE
CONDUCTANCE
What is electrolyte?
Any substance that produce ions when dissolved
in a solvent (usually water) is an electrolyte.
It is the electrically conductive solution that
must be present for corrosion to occur.
Types of electrolytes
Strong electrolyte
Weak electrolyte
Non-electrolyte
Strong Electrolytes
Strong electrolytes are substances that only
exist as ions in solution.
They completely dissociate to their ions when
dissolved in solution.
Ionic compounds are typically strong
electrolytes.
Strong acids, strong bases and salts are strong
electrolytes.
They conduct electricity when molten or in
aqueous solution.
Example: Hydrochloric acid, Sodium chloride
HCl H 2O H3O Cl
NaCl H 2O Na Cl
Weak Electrolytes
A weak electrolyte only partially dissociates in
solution and produces relatively few ions (exist
in water as a mixture of individual ions and
incontact molecules).
Polar covalent compounds are typically weak
electrolytes.
Weak acids and weak bases are weak
electrolytes.
They conduct electricity weakly.
Example: Acetic acid, ammonia
CH3COOH H 2O CH3COO H
NH3 H 2O NH 4 OH
Non-electrolytes
A non-electrolyte does not dissociate at all
(present entirely as intact molecules) in
solution and therefore does not produce any
ions.
Non-electrolytes are typically polar covalent
substances that do dissolve in water as
molecules instead of ions.
They do not conduct electricity at all.
Example: Sugar
C12 H 22O11 H 2O C12 H 22O11
Acids
Are molecular compounds which ionize or turn
into ions in water.
The properties of acids were due to the
presence of hydrogen ions, H+.
All acids are soluble in water
Some acids are strong electrolytes and some
are weak electrolytes.
No acids are non-electrolytes.
Bases
Can be molecular compounds or ionic
compounds.
Some bases are soluble and some are not.
The soluble bases ionize or dissociate into ions
in water.
The properties of bases were due to the
presence of hydroxide ions, OH-.
All of the ionic bases which are soluble are also
strong electrolytes.
Salts
Are ionic compounds which are not acids or
bases.
In other words, the cation is not hydrogen and
the anion is not hydroxide.
Some salts are soluble in water and some are
not.
All of the salts which are soluble are also
strong electrolytes.
Electricals Terms
SI Term
SI Symbol
SI Unit
Electrical Current
I
Ampere (A)
Quantity of Electricity
Q
Coulomb (C)
Electric Potential
V
Volt(V)
Electric Resistance
R
Resistivity
Ohm()
m
Conductance
G
Conductivity
Siemens (S);ohm-1
Sm-1; -1m-1; -1cm-1
Molar conductivity
Sm2mol-1
Molar Conductivity of Ion
Sm2mol-1
Electric Mobility of Ion
u
m2V-1s-1
Transport Number of Ion
t
Other Symbols and Terms
Symbol
Term
C
Molar Concentration, mol dm-3 (with :mol m-3)
Degree of dissociation
l
Length
A
Area
Kcell
Cell Constant
°
Molar conductivity at infinite dilution or Limiting
Molar Conductivity
Electrolytic conductance
Electrolytic conductance occurs when a voltage is
applied to the electrode dipped into an
electrolyte solution, ions of the electrolyte move
and electric current flows through the electrolytic
solution.
This power of the electrolyte to conduct electricity
is known as conductance or conductivity.
Electrolytic solution also obey Ohm’s Law just like
metallic conductor.
Ohm’s Law: It states that the current flowing
through a conductor is directly proportional to
the potential difference across it:
V=IR
where,
V = applied potential (V)
I = current measured (A)
R = solution resistance () between the two
electrodes
Solution Resistance (R)
The increase the [ions] presence in the
solution, the lower the solution resistance, R,
will be.
A strong electrolyte like KCl is dissolve in
water, the no. of ions per unit volume increase
and the solution resistance, R, is lowered,
thus increasing the current measured for a
particular applied potential.
Thus, current can be related to the [ions] in a
particular solution.
However,
the distance between the electrodes,
the surface area of the electrodes and
the identity of the ions
also affect the solution resistance, R.
Solution Conductance (G)
The reciprocal of solution resistance (1/R) is
called Conductance, G.
Conductance is expressed as Siemens (S) or
ohm-1 (-1) or mho.
1
A
G
R
l
Where, A = surface area of each electrode
l = distance btwn electrode
= conductivity
Values of conductivity,, increased with T and
concentration.
The conductivity of a solution of water is highly
dependent on its concentration of dissolved
salts and sometimes other chemical species
which tend to ionize in the solution.
Electrical conductivity of water samples is
used as an indicator of how salt free or
impurity free the sample is; the purer the
water, the lower the conductivity.
Solution
Electric Conductivity
(Sm-1)
Seawater
5
Drinking water
0.0005 to 0.05
Deionized water
5.5 x 10-6
Molar Conductivity ()
Defined by:
C
Units: Sm-1
mol m-3
Example 1:
Molar conductivity of 0.005 M KCl is 144 Scm2
mol-1. Calculate its electrolytic conductivity in SI
units (Sm-1).
*(Hint: 1m2 = 104 cm2; mol/L or mol/dm3 convert to
mol m-3).
2
1
m
2
1
144Scm 2 mol1 4
0.144
Sm
mol
10 cm2
0.005mol
1 dm 3
3
5
mol
m
dm 3
(1 101 m)3
c
0.0144 5
0.072Sm -1
Measurement of Conductivity
The conductivity of a solution is measured in a
cell called conductance cell or conductivity cell.
1 l
R A
Since l and A are difficult to measure, the usual
procedure is to treat l as a cell constant, K cell
A
Therefore,
1
K cell GK cell
R
Example 2
In a certain conductivity cell, the resistance of
a 0.01 M KCl solution is 150 . The known
molar conductivity of the solution is 141.27 -1
cm2 mol-1. Calculate the cell constant (Kcell).
*(Kcell unit is cm-1)
kcell R
cR
3
141.27 0.0115010
0.2119cm
1
Exercise 1
Using the same conductance cell as in
example 2, a student measured the resistance
of a 0.10 M NaCl solution to be 19.9 .
Calculate the experimental value of the molar
conductivity of this solution.
Use the same value of kcell
kcell R
cR
0.2119 0.1 19.9
0.106485 10
3
1
106.48 Scm m ol
2
Ex
In order to determine the molar conductivity of a
0.05 M solution of AgNO3, you need to measure
the solution resistance in a conductivity cell and
found that R = 75.8 . Then, in the same cell, a
0.02 M KCl solution had a resistance of 157.9 .
Given that the accepted molar conductivity of the
KCl solution is 0.013834 -1 m2 mol-1, calculate
the molar conductivity of the AgNO3 solution.
KCl :
k cell cR
0.013834 0.02157.910-3
43.7m -1
AgNO3 : k cell ΛcR
43.7 m -1 Λ 0.05 75.8
Λ 11.5310-3 Sm 2 mol1
0.01153Sm 2 mol1
Variation of Molar Conductivity with
Concentration
Molar conductivity () of electrolytes increases
with dilution.
The variation is different for strong and weak
electrolytes.
Strong electrolytes
Fully ionized in solution
increases slowly with dilution and there is a
tendency for to approach a certain limiting value
when the concentration approaches zero(i.e. When
dilution is infinite).
The when the concentration approaches zero
(infinite dilution) is called molar conductivity at
finite dilution or limiting molar conductivity
(°).
= °
when C → 0 (at infinite dilution)
For strong electrolytes molar conductivity
increase slowly with dilution and can be
represented by:
C
DEBYE HUCKEL ONSAGER
equation
C
= Molar conductivity at a given concentration
° = Molar conductivity at infinite dilution
= constant
From the graph, it has been noted that the
variation of molar conductivity () with
concentration (C) is small so that the plot can
be extrapolated to zero concentration.
The intercept is equal to (°) and the slope is .
b) Weak electrolytes
Not fully ionized in solution
In weak electrolyte like acetic acid they have
low degree of dissociation as compared to
strong electrolyte.
However, the variation of molar conductivity ()
with concentration (C) is very large and we
can’t obtain molar conductivity at infinite
dilution (°) by extrapolation of versus C
plots.
Explanation for the variation of Molar
Conductivity with concentration
1. Conductance behaviour of strong electrolyte:
No increase in the no. of the ions with the
dilution ( completely ionized in the solution at
all concentration).
In concentrated solution:
strong inter-ionic forces
Molar conductivity is low
In dilute solution:
Inter-ionic forces low
Molar conductivity increases with dilution
When concentration very low, inter-ionic
interaction becomes almost negligible and
molar conductance approaches the limiting
value, °.
2. Conductance behaviour of weak electrolyte:
The no. of ions produced in solution depends
upon the degree of dissociation with dilution.
Higher the degree of dissociation, larger is the
molar conductance.
With increase in dilution
Degree of dissociation increases as a result
molar conductivity increases.
At infinite dilution, the electrolyte is
completely dissociated so that the degree
of dissociation become one.
= °
(at C → 0)
Thus, if
= Molar conductivity at a given concentration
° = Limiting molar conductivity or molar
conductivity at infinite dilution
Then, degree of dissociation
Ostwald Dilution Law & Dissociation
Constant of Weak Electrolyte
Consider an aqueous solution of a weak binary
electrolyte, AB, of concentration C mol dm-3 and
degree of dissociation of .
Initial/mol dm-3
Equilibrium/mol dm-3
At equilibrium:
AB (aq) ↔ A+ (aq) + B- (aq)
C
0
0
C(1-)
C
C
[ A ][B ]
K
[ AB]
(C )(C )
Kc
C (1 )
Therefore, the dissociation constant can be
expressed as:
2
C
Kc
1
Ostwald Dilution Law
However, for weak electrolyte; is very small.
Hence, (1- ) 1
Therefore,
Ka C
2
Ka
C
Since H+ = C
[H+
Ka
] = C = C
C
[ H ] K a C
KOHLRAUSCH’S LAW
At infinite dilution the ions act completely
independently, and the ° obeys a rule of
additivity:
AX AY BX BY
where AX, AY, BX and BY are strong electrolytes.
° for a weak electrolyte can be deduced from
° values obtained from strong electrolytes.
For example, consider CH3COOH denoted as
HAc’:
HAc HX MAc MX
where HX, Mac and MX are strong electrolytes.
Table 1: Limiting Molar Conductivity,
°, of some strong electrolytes
Electrolyte
HCl
° (S cm2 mol-1)
426.16
HBr
NaCl
KBr
428.10
126.45
151.80
KCl
NaNO3
KNO3
149.86
121.55
144.96
NH4Cl
KHCO3
149.70
118.00
Exercise 2
Calculate ° for a weak electrolyte NH4OH
from the ° values for these strong
electrolytes: NH4Cl: 149.7; NaCl: 126.5 and
NaOH: 248.10
( NH 4OH ) ( NH 4Cl ) ( NaOH) ( NaCl)
149.7 248.10 126.5
271.3
Kohlrausch also stated at infinite dilution when
the dissociation complete,
each ion makes a definite contribution towards
molar conductance of the electrolyte irrespective
of the nature of the other ion with which it is
associated.
It means that the molar conductivity at infinite
dilution for a given salt can be expressed as the sum of
the individual contributions from the ions of
the electrolyte.
v v
where
v+ and v-: stoichiometric coefficients for the
cation and anion in the electrolyte.
°+ and °-: ionic conductance of individual
ions (cation and anion)
Example 3
For NH4OH electrolyte: v+ = 1 and v- = 1
Since 1NH4+ ion present for each OH- ion present
in solution.
Example 4
For K4Fe(CN)6 electrolyte: v+ = 4 and v- = 1
Since there are 4K+ ions present for each
Fe(CN)4-6 ion present in solution.
Thus, the limiting ionic conductivities represent
the contributions to the total solution
conductivity made per mole of each ion present
in a dilute solution.
Exercise 3
Calculate the ° of the following electrolytes:
1) Acetic acid
2) Hydrochloric acid
3) Potassium Chloride
Ionic Conductivities at Infinite
Dilution at 25°C
Cation
°+ / Scm2mol-1
Anion
°- / Scm2mol-1
H+
349.6
OH-
197.8
Li+
38.7
Cl-
76.4
Na+
50.1
Br-
78.2
K+
73.5
I-
76.8
Fe2+
108.0
CH3COO-
40.9
Fe3+
204
CO2-3
138.6
NH4+
73.4
NO-3
71.5
Ba2+
127.3
SO2-4
160.0
(CH 3COOH ) v v
1(349.6) 1(40.9)
1
390.5 Scm m ol
2
Example 5
Molar conductivity for 0.10 M NaCl is 107 Scm2
mol-1. Calculate the degree of dissociation for the
Solution.
1) Calculate the limiting molar conductivity for
NaCl
2) Use formula
( NaCl)
1(50.1) 1(76.4)
126.5
107
126.5
0.846
Exercise 4
At 25° C, = 3.40 10-3 Sm-1 for 0.001 M
NH4OH. Values of ° are NH4Cl = 0.01497,
NaOH = 0.02481, NaCl = 0.01265 Sm2mol-1.
Calculate the dissociation constant, K, of
ammonium hydroxide.
( NH 4OH ) ( NH 4Cl ) ( NaOH ) ( NaCl)
0.01497 0.02481 0.01265
0.02713
c
3.40103
103 3.40103
0.001
3.40103
0.1253
0.02713