Transcript Document
TRANSPORT OF IONS IN SOLUTION
Conductivity of electrolyte solutions Strong and weak electrolyte
Jaslin Ikhsan, Ph.D.
Chemistry Ed. Department State University of Yogyakarta
• Mahasiswa dapat menjelaskan pengertian konduktansi dan konduktivitas • Mahasiswa dapat menghitung konduktivitas molar larutan • Mahasiswa dapat menjelaskan hukum pengenceran Ostwald • Mahasiswa dapat menentukan pKa dengan menggunakan hasil pengukuran konduktivitas
Conductivity of Electrolyte Solution
Ions in solution can be set in motion by applying a potential difference between two electrodes.
The conductance (
G) of a solution is defined as
the inverse of the resistance (
R):
1
G
,
in units of R
1 For parallel plate electrodes with area
A, it
follows:
G
A L Where
, Κ: the conductivity, L :
the distance separating the plates Units
:
G → S (siemens) R
→ Ω κ
→
S m -1
Conductivity of Electrolyte Solution
The conductivity of a solution depends on the number of ions present. Consequently, the molar conductivity Λ m
m
C
is used
C is molar concentration of electrolyte and unit of Λ m is S m 2 mol -1
In real solutions, Λ m depends on the concentration of the electrolyte. This could be due to: Ion-ion interactions γ 1 The concentration dependence of conductance indicates that there are 2 classes of electrolyte
Strong electrolyte
: molar conductivity depends slightly on the molar concentration
Weak electrolyte
: molar concentration falls sharply as the concentration increases
Conductivity of Electrolyte Solution
In real solutions, Λ m depends on the concentration of the electrolyte. This could be due to: 1. Ion-ion interactions γ 1 strong electrolyte, weak dependence of Λ m on
C
2. Incomplete dissociation of electrolyte weak electrolyte, strong dependence of Λ m on
C
Strong Electrolyte
Fully ionized in solution
Kohlrausch’s law
m
0
m
KC
Λ 0 m
is the limiting molar conductivity K is a constant which typically depends on the stoichiometry of the electrolyte
C
1/2
arises from ion-ion interactions as estimated by the Debye-Hückel theory.
Strong Electrolyte
Law of the independent migration of ions
: limiting molar conductivity can be expressed as a sum of ions contribution 0
m
ions migrate independently in the zero concentration limit
Weak Electrolyte
Not fully ionized in solution ) 2 ( 1 )
c K a
2
c
1 , 1
c K a
2
c
2
a
K a K a
K a
0
K a
K a
2
c
K a
2 4
K c a
2
c K a
2 4
K c a
2
c
c
(
aq
)
A aq
)
c
is degree of ionisation 2
K c a
K
2
c a
1 4
c K a
K
2
c a
1 4
c K a
1
Weak Electrolyte
The molar Conductivity (at higher concentrations) can be expressed as:
m
0
m
At infinite dilution, the weak acid is fully dissociated (α = 100%) It can be proven by the Ostwald dilution law which allows estimating limiting molar conductance: 1
m
1 0
m
K a c
( 0
m m
) 2
m
0
m
1
m
1
m
1 0
m
1 0
m x
1 1
m
1
m
1 0
m
1 0
m x
1
c K a
c K a
0
m x
0
m m
Weak Electrolyte
The limiting molar conductance: 1
m
1 0
m
K a c
( 0
m m
) 2
Hukum Pengenceran Ostwald
Graph to determine the limiting value of the molar conductivity of a solution by extrapolation to zero concentration
Diskusi:
1. Konduktivitas molar larutan elektrolit pada 25 0 C adalah 135,5 S cm 2 mol -1 dan konsentrasinya adalah 5,35 x 10 -2 M. Hitunglah konduktivitas larutan! (20) 2. Sel konduktivitas mempunyai elektrode bidang yang sejajar, masing-masing luasnya 2,2 cm x 2,2 cm, dan terpisah sejauh 2,75 cm. Jika sel diisi dengan larutan elektrolit, tahanannya adalah 351 ohm. Berapakah konduktivitas larutan? (25) 3. Pada 25 0 C konduktivitas larutan elektrolit kuat dalam air adalah 109,9 S cm 2 mol -1 cm 2 mol -1 untuk konsentrasi 6,2 x 10 untuk konsentrasi 1,50 x 10 -2 -3 M dan 106,1 S M. Berapakah konduktivitas molar pembatas elektrolit tersebut? (30) 4. Konduktivitas molar 0,1000 M KCl (
aq
) adalah 129 S cm 2 mol -1 dan tahanan terukur dalam sel konduktivitas adalah 28,44 ohm. Tahanan itu besarnya 28,50 ohm jika sel yang sama berisi 0,1000 M NH 4 Cl (
aq
). Hitunglah konduktivitas molar NH 4 Cl (
aq
) pada konsentrasi ini! (25)