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Chapter 7 Electrochemistry

0.04.  m / S ·mol -1 ·m 2. HCl. 0.03. H 2 SO 4. 0.02. KCl. Na 2 SO 4. 0.01. HAc. 0.00. 0.15. 0.20. 0.05. 0.10. Chapter 7 Electrochemistry. § 7.2 Conductivity and its application. Self reading : Ira N. Levine, Physical Chemistry, 5 th Ed., McGraw-Hill, 2002. pp. 506-521

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Chapter 7 Electrochemistry

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  1. 0.04 m / S·mol-1·m2 HCl 0.03 H2SO4 0.02 KCl Na2SO4 0.01 HAc 0.00 0.15 0.20 0.05 0.10 Chapter 7 Electrochemistry §7.2 Conductivity and its application

  2. Self reading: Ira N. Levine, Physical Chemistry, 5th Ed., McGraw-Hill, 2002. pp. 506-521 Section 16.5 electrical conductivity Section 16.6 electrical conductivity of electrolyte solutions

  3. Main contents: 7.2.1 some concepts 7.2.2 measurement of electric conductance 7.2.3 factors on conductivity 7.2.4 molar conductivity: Kohlrausch empirical formula and law of independent migration 7.2.5 measurement of limiting molar conductivity of ions 7.2.6 factors on limiting molar conductivity of ions

  4. 7.2.1 Some concepts For metals: For electrolyte solution: electric conductance (G) : Definition: G = 1/R Unit: -1, mho, Siemens, S Ohm’s Law R: resistance, Unit: Ohm,  Reciprocal of resistance conductivity () or specific conductance: Definition:  = 1/  Unit: S·m-1 resistivity/specific resistance Unit: Ohm·m, ·m

  5. conductance electrode with smooth or platinized platinum foil conductance cell

  6. R2 D F R1 R2 G R4 R3 B A C I ~ 7.2.2 Measurement of conductance: Wheatstone Bridge Circuit High-frequency alternative current, ca. 1000 Hertz R3R2 = R4R1 Conductometer

  7. Cell constant EXAMPLE The conductance of a solution is 0.689 -1. If the cell constant is 0.255 cm-1, calculate the specific conductance of the solution.

  8. The conductance cell is usually calibrated using standard aqueous KCl (potassium chloride ) solution. c/ mol·dm-3 0 0.001 0.0100 0.100 1.00 / S m-1 0 0.0147 0.1411 1.289 11.2 Relative standards are often used in scientific measurement.

  9. EXAMPLE The conductance of a cell containing an aqueous 0.0560 mol·dm-3KCl solution whose conductivity is 0.753 -1·m-1 is 0.0239 -1. When the same cell is filled with an aqueous 0.0836 mol·dm-3 NaCl solution, its conductance is 0.0285 -1. Calculate the conductivity of the NaCl solution.

  10. 80 70 H2SO4 60 50 KOH 40 LiCl 30 20 MgSO4 10 HAc 0 5 10 15 c/mol·dm-3 7.2.3. Influential factors of conductivity 1) concentration – dependence of conductivity /S·m-1

  11.  / S m-1 50 oC 30 oC 10 oC -10 oC -30 oC wt % H2SO4 2) Temperature-dependence of conductivity • Why do we usually used 38 % H2SO4 in acid-lead battery; • Why do we usually conduct electrolysis and electroplating using warm electrolyte? ice

  12. 7.2.4 Molar conductivity Why do we introduce molar conductivity? 1) Definition degree of dilution The physical meaning of m:

  13. HCl m / S·mol-1·m2 KOH NaOH KCl NaCl HAc c / mol·dm-3 2) Concentration-dependence of molar conductivity Why does m decrease with increasing concentration?

  14. 0.04 m / S·mol-1·m2 HCl 0.03 H2SO4 Kohlrausch 0.02 KCl Na2SO4 0.01 HAc 0.00 0.15 0.20 0.05 0.10 3) Kohlrausch’s empirical formula Why did Kohlrausch plot m against c1/2? Within what concentration range did the linear relation appear.

  15. limiting molar conductivity Kohlrausch empirical formula Kohlrausch’s Square Root Law Within what concentration range is the Kohlrausch law valid?

  16. 0.04 m / S·mol-1·m2 0.03 0.02 0.01 0.00 0.15 0.20 0.05 0.10 Problem: Can we obtain the limiting molar conductivity of weak electrolytes just by extrapolating the m ~ c1/2to infinite dilution?

  17. Molar conductivity at infinite dilution for some electrolytes in water at 298 K.

  18. The difference in of the two electrolytes containing the same cation or anion is the same. The same differences in led Kohlrausch to postulate that molar conductivity at infinite dilution can be broken down into two contributions by the ions. ionic conductivities at infinite dilution 4) Kohlrausch’s law of independent migration

  19. m at infinite dilution is made up of independent contributions from the cationic and anionic species. Explanation to the same difference

  20. How can we determine the limiting molar conductivity of weak electrolyte Key: How to measure the ionic conductivity at infinite dilution?

  21. 7.2.5 measurement of limiting molar conductivity of ions 1) transference number and molar conductivity I+ = AU+Z+c+F I = AUZ cF I = I++ I = Ac+Z+F(U++ U)

  22. For uni-univalent electrolyte: To measure m,+ or m,-, either t+ and t- or U+ and U- must be determined

  23. 7.2.6 Influential factors for 1) Nature of ions Charge; Radius; charge character; transfer mechanism

  24. Transport mechanism of hydrogen and hydroxyl ions Grotthus mechanism (1805) The ion can move along an extended hydrogen-bond network. Science, 2002, 297:587-590

  25. 2) Temperature Transference number of K+ in KCl solution at different concentration and temperature c /mol·dm-3 T /℃

  26. 3) Co-existing ions Table transference number on co-existing ions Problem: Why does the transference number of certain ion differ a lot in different electrolytes?

  27. G  Summary Macroscopic Microscopic  Dynamic U U t t

  28. Exercise-1 • The mobility of a chloride ion in water at 25 oC is 7.91  10-4 cm2·s-1·V-1. • Calculate the molar conductivity of the ion at infinite dilution; • How long will it take for the ion to travel between two electrodes separated by 4.0 cm if the electric field is 20 V·cm-1.

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