Chapter 7 Electrochemistry m / S·mol-1·m2 §7.2 Conductivity and its application 0.04 HCl 0.03 H2SO4 0.02 KCl 0.01 0.00 0.05 HAc 0.15 0.10 Na2SO4 0.20 c / mol dm3 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 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 7.2.1 Some concepts For metals: Ohm’s Law U R I R: resistance, Unit: Ohm, l R A resistivity/specific resistance Unit: Ohm·m, ·m For electrolyte solution: electric conductance (G) : Definition: G = 1/R Unit: -1, mho, Siemens, S Reciprocal of resistance conductivity () or specific conductance: Definition: = 1/ Unit: S·m-1 A G l conductance cell conductance electrode with smooth or platinized platinum foil 7.2.2 Measurement of conductance: Wheatstone Bridge Circuit High-frequency alternative current, ca. 1000 Hertz D F R1 R2 A R2 R4 G R3 C I ~ R3 R2 = R4 R1 R2 R3 R1 R4 1 G R1 Conductometer A G l B Cell constant l G K cellG A K cell R 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. K cell R s Rs x Rx Rs x s Rx 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. EXAMPLE The conductance of a cell containing an aqueous 0.0560 mol·dm-3 KCl 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. 7.2.3. Influential factors of conductivity /S·m-1 1) concentration – dependence of conductivity 80 70 H2SO4 60 50 KOH 40 30 LiCl 20 MgSO4 10 0 HAc 5 10 15 c/mol·dm-3 2) Temperature-dependence of conductivity / S m-1 50 oC 30 oC 1.Why do we usually used 38 % 10 oC H2SO4 in acid-lead battery; -10 oC 2.Why do we usually conduct ice -30 oC electrolysis and electroplating using warm electrolyte? wt % H2SO4 7.2.4 Molar conductivity Why do we introduce molar conductivity? 1) Definition m c m 1 V V The physical meaning of m: degree of dilution m / S·mol-1·m2 2) Concentration-dependence of molar conductivity HCl KOH NaOH KCl NaCl HAc c / mol·dm-3 Why does m decrease with increasing concentration? m / S·mol-1·m2 3) Kohlrausch’s empirical formula 0.04 HCl 0.03 H2SO4 0.02 KCl Kohlrausch Why did Kohlrausch plot m against c1/2? Within what concentration range did the linear relation appear. Na2SO4 0.01 HAc 0.00 0.05 0.10 0.15 0.20 c / mol dm3 Kohlrausch empirical formula m m A c m limiting molar conductivity Kohlrausch’s Square Root Law Within what concentration range is the Kohlrausch law valid? 1·m2 m / S·mol- 0.04 0.03 0.02 0.01 0.00 0.05 0.10 0.15 0.20 c / mol dm3 Problem: Can we obtain the limiting molar conductivity of weak electrolytes just by extrapolating the m ~ c1/2 to infinite dilution? Molar conductivity at infinite dilution for some electrolytes in water at 298 K. Salts m /S mol-1 cm2 HCl 426.16 LiCl 115.03 NaCl 126.45 KCl 149.85 LiNO3 110.14 KNO3 144.96 NaNO3 121.56 4) Kohlrausch’s law of independent migration Salts m/S mol-1 cm2 Δm KCl NaCl KNO3 NaNO3 149.85 126.45 144.96 121.56 23.4 23.4 The difference in m of the two electrolytes containing the same cation or anion is the same. The same differences in m led Kohlrausch to postulate that molar conductivity at infinite dilution can be broken down into two contributions by the ions. m m, / m, m, ionic conductivities at infinite dilution m m , m , m vm, vm, m at infinite dilution is made up of independent contributions from the cationic and anionic species. Explanation to the same difference m (KCl) m (NaCl) m,K m,Cl m,Na m,Cl + - + - m,K+ m,Na + m (KNO3 ) m (NaNO3 ) m,K m,NO m,Na m,NO m,K m,Na 3 3 How can we determine the limiting molar conductivity of weak electrolyte m (HAc) m (H ) m (Ac ) m (H ) m (Cl ) m (Na ) m (Ac ) m (Na ) m (Cl ) m (HCl) m (NaAc) m (NaCl) m (HAc) (426.16 91.00 126.45)S m-1 mol-1 390.71S m-1 mol-1 Key: How to measure the ionic conductivity at infinite dilution? 7.2.5 measurement of limiting molar conductivity of ions 1) transference number and molar conductivity I+ = AU+Z+c+F I = AUZ c F I = I++ I = Ac+Z+F(U++ U) Ac Z F (U U ) G V l Ac Z F (U U ) l G A V A c Z F (U U ) c Z F (U U ) V l c Z F ( U U ) m c c Z F ( U U ) m c For uni-univalent electrolyte: m m m, m, F (U U ) m, m, m m, U F U F U F t (U U ) F m, t m m, tm To measure m,+ or m,-, either t+ and t- or U+ and U- must be determined 7.2.6 Influential factors for m 1) Nature of ions ions r / nm H+ Li+ 102 m 102 m ions r / nm 3.4982 OH¯ 1.98 0.68 0.387 F¯ 1.23 0.554 Na+ 0.98 0.501 Cl¯ 1.81 0.763 K+ 1.37 0.735 Br¯ 1.96 0.784 Mg2+ 0.74 1.061 CO32 1.66 Ca2+ 1.04 1.190 C2O42 1.48 Sr2+ 1.04 1.189 Fe(CN)63 3.030 Al3+ 0.57 1.89 Fe(CN)64 4.420 Fe3+ 0.67 2.04 La3+ 1.04 2.09 Charge; Radius; charge character; transfer mechanism 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 2) Temperature Transference number of K+ in KCl solution at different concentration and temperature c /mol·dm-3 0.000 0.005 0.01 0.02 15 0.4928 0.4926 0.4925 0.4924 25 0.4906 0.4903 0.4902 0.4901 35 0.4889 0.4887 0.4886 0.4885 T /℃ 3) Co-existing ions Table transference number on co-existing ions Electrolyte KCl KBr KI KNO3 t+ 0.4902 0.4833 0.4884 0.5084 Electrolyte LiCl NaCl KCl HCl t– 0.6711 0.6080 0.5098 0.1749 Problem: Why does the transference number of certain ion differ a lot in different electrolytes? Summary Macroscopic Microscopic G Dynamic U U t t m m m, , m, Exercise-1 The mobility of a chloride ion in water at 25 oC is 7.91 10-4 cm2·s-1·V-1. 1) Calculate the molar conductivity of the ion at infinite dilution; 2) 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.