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24.6 The conductivities of electrolyte solutions

24.6 The conductivities of electrolyte solutions. Conductance (G, siemens) of a solution sample decreases with its length l and increases with its cross-sectional area A: k is the conductivity (Sm -1 ). Molar conductivity, Λ m , is defined as: c is the molar concentration

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24.6 The conductivities of electrolyte solutions

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  1. 24.6 The conductivities of electrolyte solutions • Conductance (G, siemens) of a solution sample decreases with its length l and increases with its cross-sectional area A: k is the conductivity (Sm-1). • Molar conductivity, Λm, is defined as: c is the molar concentration • Λm varies with the concentration due to two reasons: • Based on the concentration dependence of molar conductivities, electrolytes can be classified into two categories: 1. Strong electrolyte: its molar conductivity depends only slightly on the molar concentration. 2. Weak electrolyte: its molar conductivity is normal at diluted environment, but falls sharply as the concentration increases.

  2. Strong electrolyte • Strong electrolyte is virtually fully ionized in solution, such as ionic solid, strong acids and bases. • According to Kohlrausch’s law, the molar conductivity of strong electrolyte varies linearly with the square root of the concentration: • Λ0m can be expressed as the sum of contributions from its individual ions: where v+ and v- are the numbers of cations and anions per formula unit. (For example: HCl: v+ = 1 and v- = 1; MgCl2, v+ = 1 and v- = 2)

  3. Weak electrolyte • Weak electrolytes are not fully ionized in solution, such as weak acids and bases. • Degree of ionization (α): defined as the ratio of the amount of ions being formed in the solution and the amount of electrolyte added to the solution. • For the acid HA at a molar concentration c, [H3O+] = αc, [A-] = αc , [HA] = c –αc • Since only fraction, α, of electrolyte is actually presents as ions, the measure conductivity Λm, is given by: Λm = αΛ0m

  4. Ostwald’s dilution law

  5. 24.7 The mobility of ions • Drift speed: the terminal speed reached when the accelerating force is balanced by the viscous drag. • Accelerating force induced by a uniform electric field (E = Δø/l): F = z e E = z e Δø/l • Friction force Ffric = (6πηa)s, a is the hydrodynamic radius • Mobility of an ion: • u is called the mobility of the ion

  6. Mobility and conductivity • λ = z u F ( λ is an ion’s molar conductivity) • For the solution: Λ0m = (z+u+v+ + z-u-v-) F

  7. Transport numbers • The fraction of total current carried by the ions of a specified type. • The limiting transport number, t0±, is defined for the limit of zero concentration of the electrolyte solution.

  8. The measurement of transport numbers • Moving boundary method • Indicator solution • Leading solution

  9. Conductivities and ion-ion interactions • To explain the c1/2 dependence in the Kohlrausch law.

  10. Hückel-Onsager Theory

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