Chapter-2- Conductance of electrolytes

1. Chapter-2- Conductance of electrolytes Dr/ El Hassane ANOUAR Chemistry Department, College of Sciences and Humanities, Prince Sattam bin Abdulaziz University, P.O. Box 83, Al-Kharij 11942, Saudi Arabia. College Of Science and humanities, PSAU

2. Introduction • Determination of properties of electrolyte solutions: • To study quantitatively the effects of interionic forces. • Degrees of dissociation. • Extent of ion-pairing. • Solubilities of soluble salts • Ionic products of self-ionizing solvents • Dissociation constants of weak acids • To form the basis for conductimetric titration methods It’s possible College Of Science and humanities, PSAU

3. 2.1 Conductance • The resistance (R) of a portion of an electrolyte solution: Specific resistance or Resistivity Length (m) Area (m2) Conductance (Ω-1) Conductivity (Ω-1 m-1) College Of Science and humanities, PSAU

4. 2.1 Conductance 2.1.1 Measurement of conductivity: A Wheatstone bridge circuit cell constant Serves to balance the capacity effects of the conductance cell College Of Science and humanities, PSAU

5. 2.1 Conductance 2.1.1 Measurement of conductivity: A Wheatstone bridge circuit • Maximum sensitivity in measuring high conductivities required a high cell constant • Measurement of small conductivities, l/A should be as small as possible. Small electrodes Big electrodes Large distance Small distance College Of Science and humanities, PSAU

6. 2.1 Conductance 3.1.2 Molar conductivity • Molar conductivity, , conductivity of one mole, is the conductivity, κ, multiplied by the volume which contains one mole of electrolyte. • The equivalent conductance was defined by College Of Science and humanities, PSAU

7. 2.1 Conductance 2.1.3 Variation of molar conductivity with concentration • Strong electrolytes: • Kohlrausch established an empirical relationship between and • Weak electrolytes: Molar conductivity at infinite dilution If , Arrhenius showed that ,, may be given quite well by the following: College Of Science and humanities, PSAU

8. 2.1 Conductance 2.1.3 Variation of molar conductivity with concentration Dissociation constant of acetic acid at 298 K • Weak electrolytes: College Of Science and humanities, PSAU

9. 2.1 Conductance 2.1.3 Variation of molar conductivity with concentration • Kohlrausch demonstrated that:. Infinite dilution values of the molar conductivities of some electrolytes at 291 K () Note: The equation is written for infinite dilution (i.e., ion-ion interactions are at a minimum) College Of Science and humanities, PSAU

10. 2.2 Conductance and ionic speeds • For a 1 : 1 strong electrolyte of c concentration c (c+ = c-). If the speeds of ions are u+ and u-, the amount of charge crossing unit area of solution in unit time is • For a degree of ionization of an electrolyte α, (αc = c+ = c-), we may write for the current density, Where Speeds, or absolute velocities of the ions under a potential gradient (field intensity) of 1 volt cm-1 College Of Science and humanities, PSAU

11. 2.2 Conductance and ionic speeds • The conductivity κc, for a concentration c moles per cm3is: or At infinite dilution α= 1 i.e. College Of Science and humanities, PSAU

12. 2.2 Conductance and ionic speeds 2.2.1 Strong completely dissociated electrolytes • Onsager derived an expression of ion conductivity in a very dilute solution of a strong electrolyte: where D = dielectric constant of the solvent medium N = Number of ions of either kind per unit volume in the bulk College Of Science and humanities, PSAU

13. 2.2 Conductance and ionic speeds 2.2.1 Strong completely dissociated electrolytes • For a completely dissociated electrolyte, molar conductivity by the Kohlrausch principle by the addition of two above terms, one for cationic and the other for anionic species: Or, Example: For a symmetrical electrolyte and q = 0·5, so that, for water as solvent (D = 80, η = 0·0089 poise and T = 298 K) College Of Science and humanities, PSAU

14. 2.2 Conductance and ionic speeds 2.2.1 Strong completely dissociated electrolytes • Shedlovsky observed that the value of as calculated from Equation (slide 13, red color) was not constant, but showed almost linear variation with concentration. BC: An approach to ideal behaviour in very dilute solution in which ions interact negligibly. Arrangement It holds good for a number of electrolytes up to a concentration of 0.1 M AB: Extreme departure from ideality as ions interact very significantly with one another Conductivity coefficient College Of Science and humanities, PSAU

15. 2.2 Conductance and ionic speeds 3.2.2 Weak incompletely dissociated electrolytes • The Onsager equation in the case of a weak electrolyte And we have College Of Science and humanities, PSAU

16. 2.2 Conductance and ionic speeds 2.2.3. Electrolyte systems showing ion-pairing • The slope of the versus C1/2 graph of uni-univalent electrolytes at higher concentrations is somewhat lower than the theoretical Onsager slope. • Deviations from the Onsager equation, shown by electrolytes with valency product 4 and 6, indicating ion-pairing. • The dotted lines are the Onsager slopes. College Of Science and humanities, PSAU

17. 2.3 Electrical migration and transport numbers College Of Science and humanities, PSAU

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