2 solvent-shared or solvent-separated Ion AssociationIon pair formationfully solvatedsolvent-shared or solvent-separatedIon paircontact ion pairIon pairs formed when opposite ions come close enough to be separated by a distance < r*
3 BjerrumAsCharge of ions increases (zi): r* increases, probability of ion pair formation increases.Temperature increases, r* decreases, probability of IP formation decreases. Kinetic energy acts against attraction.Polarity of solvent increases (er), attraction decreases, probability of IP formation decreases
4 Very difficult to bring the opposite ions so close! Electrostatic potential energy for inter- action between two univalent ions:Substitute r = r*For univalent electrolyte in water at 25oC, IP formation is negligibler* is only nm.Very difficult to bring the opposite ions so close!
15 Let 4F be passed through the cell; t+=3t- Before electrolysisOn electrolysisAfter electrolysis
16 Let 4F be passed through the cell; t+=3t- 4 Cl- -4e- 2 Cl24 H+ +4e- 2 H23 mol H+ 1 mol Cl- 3 mol H+ 1 mol Cl-
17 For anodic region:Cresidual = Cinitial – Creact + C transfer3 = – C transfert- = 1 / 4 = 0.25t+ = 3 / 4 = 0.75
18 measurement of transport numbers by Hittorf method The method of Hittorf is based on concentration changes in the anodic region and cathodic region in an electrolytic cell, caused by the passage of current through the electrolyte.
20 A solution of LiCl was electrolyzed in a Hittorf cell A solution of LiCl was electrolyzed in a Hittorf cell. After a current of 0.79 A had been passed for 2 h, the mass of LiCl in the anode compartment has decreased by g.Calculate the transport numbers of Li+ and Cl-.If Lo(LiCl) is 115 W-1cm2mol-1, what are the molar ionic conductivities and the ionic mobilities?
21 2) The moving-boundary method MA, MA’ have an ion in common. The boundary, rather difference in color, refractivity, etc. is sharp.In the steady state, the two ions move with the same velocity.
22 When Q coulomb passes, the boundary moves x, the cross-sectional area of the tube is A: No. of mole of H+ that passed from AA’ to BB’n=c.V=c+.A.xCharge carried by these moles:Q+=z+ F n+=z+ F c+ A x
23 Sample:When A = 1.05×10-5 m2, c(HCl) = 10.0 mol m-3, I = 0.01 A for 200 s, x was measured to be 0.17 m. Calculate t (H+).Solution:t+ = 0.17 m× 1.05 × 10-5 m2 × 10.0 mol m-3 ×1× C mol-1 / 0.01 A × 200 S= 0.82
24 Independent migration Kohlrausch’s Law ofIndependent migrationValid only at infinite dilution!
27 Grotthuss MechanismExplains the high conductivity of H+ and OH- in water
28 Ion Solvation Size But larger size means slower motion, the conductivity should drop?!!!!Smaller size,larger interaction with water molecule,hydrate shell larger on Li+,moving species larger,Lower conductivity
29 Viscosity of the solvent acetoneMethyl alcoholEthyl alcohol / mPas0.3160.5471.200K+0.00820.00540.0022Li+0.00750.00400.0015Walden’s Rule L.h = constant