Chapter 7 Electrochemistry § 7.2 Conductivity and its application Key problem: How to evaluate the ability of an electrolytic solution to conduct electricity?

日期课程内容留下的问题 Sep. 10 第 1 节 知识点： 电解质、电离（离解）理论、真 实和潜在电解质、离子溶剂化 ( 水 化 ) 、离子间相互作用、离子对。 电解质导电机理、 Faraday 定律 、电流效率、应用。 淌度、迁移数。 科研方法：大胆假设、小心求证。 避免孤证，方法的可靠性。 1) 为什么测量稀溶液的依数性时， 采用渗透压法比采用凝固点降 低、沸点升高要准确？ 2) 为什么电解质水溶液会比相同 浓度的有机溶液的导电性好？ 3) 在 18- 冠 -6 的作用下， KMnO 4 可溶于苯。预测该溶液的导电 性并说明你预测的理由。 4) 可以用电解法测量单个电子的 电量吗？如何操作？ Review for the first class

1. Some concepts For metals: R: resistance, Unit: Ohm,   : resistivity Unit:  ·m Ohm’s Law For electrolyte solution: conductivity (  ) : Definition:  = 1/  Unit: S·m -1 electric conductance (G) : Definition: G = 1/R Unit:  -1, mho, Siemens, S Reciprocal of resistance ~ G A C B D R2R2 R1R1 R3R3 R4R4 I Wheatstone Bridge Circuit

conductance electrode with smooth or platinized platinum foil electrodes conductance cell Conductance cell and conductance electrode

R 3  R 2 = R 4  R 1 2. Measurement of conductance: ~ G A C B D F R2R2 R1R1 R3R3 R4R4 I R2R2 High-frequency alternative current, ca Hertz ? Conductometer a capacitor!

Calibration: The conductance cell is usually calibrated using standard aqueous KCl (potassium chloride ) solution  / S m c/ mol·dm -3 Cell constant EXAMPLE The conductance of a cell containing an aqueous mol·dm -3 KCl solution whose conductivity is  -1 ·m -1 is  -1. When the same cell is filled with an aqueous mol·dm -3 NaCl solution, its conductance is  -1. Calculate the conductivity of the NaCl solution.

3. Influential factors of conductivity (1) Concentration H 2 SO 4 KOH LiCl MgSO 4 HAc c/mol·dm  /S ·m -1 What can we learn form this figure?

wt % H 2 SO 4  / S m o C 30 o C 10 o C -10 o C -30 o C (2) Temperature 1.Why do we use 38 % H 2 SO 4 in acid-lead battery? 2.Why do we do electrolysis and electroplating using warm electrolyte? ice

4. Molar conductivity H 2 SO c/mol·dm Definition Why do we introduce the concept of molar conductivity?

(2) Concentration-dependence of molar conductivity c / mol·dm -3  m / S · mol -1 · m 2 HCl KOH NaOH KCl NaCl HAc (1) Why does molar conductivity decrease with increasing concentration? (2) Does the curve shape inspire you?

Why did Kohlrausch plot  m against c 1/2 ? Within what concentration range does the linear relation appear? Kohlrausch 5. Kohlrausch’s empirical formula  m / S·mol -1 ·m 2 HCl H 2 SO 4 KCl Na 2 SO 4 HAc

Kohlrausch empirical formula limiting molar conductivity Kohlrausch’s Square Root Law Within what concentration range is the Kohlrausch law applicable? For strong electrolyte

Salts /S mol -1 cm 2 HCl LiCl NaCl KCl LiNO KNO NaNO Molar conductivity at infinite dilution for some electrolytes in water at 298 K.

SaltsKClNaClKNO 3 NaNO 3 /S mol -1 cm ionic conductivities at infinite dilution 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. 6. Kohlrausch’s law of independent migration

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

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

7. Measuring limiting molar conductivity of ions C －, Z －, U － ; C ＋, Z ＋, U ＋ ; BAC I + = AU + Z + c + F I  = AU  Z  c  F I = I + + I  = Ac + Z + F(U + + U  )

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

ionsr / nm 10 2 ionsr / nm 10 2 H+H+  OH¯  1.98 Li F¯ Na Cl¯ K+K Br¯ Mg CO 3 2   1.66 Ca C2O42C2O42  1.48 Sr Fe(CN) 6 3   Al Fe(CN) 6 4   Fe La ) Nature of ions Charge; Radius; charge character; transfer mechanism Influential factors for

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

G   UUU ttt MacroscopicMicroscopic Dynamic Summary

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