TRANSPORT OF IONS IN SOLUTION Conductivity of electrolyte

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 p. Ka 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 ü C is molar concentration of electrolyte and unit of Λm is S m 2 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 ü Strong electrolyte: molar conductivity depends slightly on the molar concentration ü 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: 1. Ion-ion interactions γ 1 2. 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 üΛ 0 m is the limiting molar conductivity ü 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 ü 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: 1. Konduktivitas molar larutan elektrolit pada 250 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 250 C konduktivitas larutan elektrolit kuat dalam air adalah 109, 9 S cm 2 mol-1 untuk konsentrasi 6, 2 x 10 -3 M dan 106, 1 S cm 2 mol-1 untuk konsentrasi 1, 50 x 10 -2 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)

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