Chapter 7 Electrochemistry § 7.2 Conductivity and its application Out-class extensive reading: Levine: pp , 16.5 electric conductivity 16.6 Electrical conductivity of electrolyte solutions

Key problem : How to evaluate the ability of an electrolytic solution to conduct electricity?

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