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): For parallel plate electrodes with area A, it follows: 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 Λm is used C is molar concentration of electrolyte and unit of Λm is S m2 mol-1 In real solutions, Λm 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, Λm depends on the concentration of the electrolyte. This could be due to: Ion-ion interactions  γ  1 Incomplete dissociation of electrolyte strong electrolyte, weak dependence of Λm on C weak electrolyte, strong dependence of Λm on C

Strong Electrolyte Fully ionized in solution Kohlrausch’s law Λ0m is the limiting molar conductivity K is a constant which typically depends on the stoichiometry of the electrolyte C1/2 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 ions migrate independently in the zero concentration limit

Weak Electrolyte Not fully ionized in solution

Weak Electrolyte The molar Conductivity (at higher concentrations) can be expressed as: 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:

Weak Electrolyte The limiting molar conductance: Hukum Pengenceran Ostwald Graph to determine the limiting value of the molar conductivity of a solution by extrapolation to zero concentration

Diskusi: Konduktivitas molar larutan elektrolit pada 250C adalah 135,5 S cm2 mol-1 dan konsentrasinya adalah 5,35 x 10-2 M. Hitunglah konduktivitas larutan! (20) 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) Pada 250C konduktivitas larutan elektrolit kuat dalam air adalah 109,9 S cm2 mol-1 untuk konsentrasi 6,2 x 10-3 M dan 106,1 S cm2 mol-1 untuk konsentrasi 1,50 x 10-2 M. Berapakah konduktivitas molar pembatas elektrolit tersebut? (30) Konduktivitas molar 0,1000 M KCl (aq) adalah 129 S cm2 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 NH4Cl (aq). Hitunglah konduktivitas molar NH4Cl (aq) pada konsentrasi ini! (25)

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