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)

Thank You