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Electrochemistry විද්යුත් රසායනය Basic Physical Chemistry I CHEM 11132 2015 Dr. S. Sri Skandaraja Relationship between conductivity and the composition of the solution • Two type of ions – Cation M2+ (Z+: charge number) – anion X2- (Z-: charge number) • Molar concentrations: C+ and C• In the absent of electric field; – Disregard the random motion of ions and imagine ions as stationary point charges embedded in the solvent. • An electric field between 2 electrodes – (+)ve ions accelerated toward cathode – (-) ve ions accelerated toward anode Drift Velocity of cation = + Drift Velocity of cation = - + x s y Cathode Anode Area of plane s = a t- t+ Total volume of solution pass ‘s’ for 1 second = a + Total no. of cation in solution (s-y) = a + C+ moles Total charges carry across the ‘s’ = a + C+ Z+ F I+ = a + C+ Z+ F I- = a - C- Z- F I = I+ + I- = a F{(+ C+ Z+) + (- C- Z- )} Current density j = F{(+ C+ Z+) + (- C- Z- )} j=E = F{[(+/E) C+ Z+] + [(-/E) C- Z- ]} + = (+/E) C+ Z+ F - = (-/E) C- Z- F + = (+/E) C+ Z+ F - = (-/E) C- Z- F + = u+ C+ Z+ F - = u- C- Z- F + + - = = {(u+ C+ Z+) + (u- C- Z- )} The moving-boundary method NCl, MCl have an ion in common. The boundary, rather difference in color, refractivity, etc. is sharp. Density MCl > density NCl If uN+ > uM+; no mixing The distance boundary traveled = x The total charge pass through solution =Q The time =t Drift Velocity of M+ If applied electric field is E If current through the solution is I Q =It Multi ion solution For solution having any number of ionic species with, jB = B CB ZB F Where; jB = Current density due to B ions B = drift velocity of B ions CB = molar concentration of B ions ZB = charge number of B ions Multi ion solution cont… For solution having any number of ionic species with, B = uB CB ZB F Where; B = conductivity of B ions uB = ionic mobility of B ions Measurement of conductivity of ionic compound (KCl) Can measure the conductivity of KCl solution solution = (K+ + Cl-) + (H+ + OH-) solution = KCl + H2O KCl = solution - H2O Example • 411.82 resistance for a 0.741913 w/w % KCl solution. • 10.875 k resistance for a solution of SrCl2. • = 1.2856 Sm-1 at 25 oC for 0.741913 w/w % KCl solution. • Calculate the conductivity of SrCl2 solution. • If 368.0 k resistance is recorded for deionized water calculate the conductivity of KCl and SrCl2. Conductivity of ionic species is depend on ionic mobility charge molar concentration B = uB CB ZB F ionic mobility of ionic species is depend on total electrolyte concentration in the solution temperature solvent Concentration dependent of ionic mobility With increase in total concentration, ionic mobility will decrease. Charge Asymmetric effect + Without electric field + With electric field Electrophoretic effect • Viscous force = f B,rel • f = friction coefficient • B,rel = velocity of B ion relative to solvent Ionic mobility at infinite dilution The highest ionic mobility of a ion when the electrolyte concentration goes to zero (ionic mobility at infinite dilution) Symbol uB or uB Graph of ionic mobility vs. ionic concentration Extrapolation of graph uB Ionic mobility at infinite dilution Ion Ionic mobility 109 x (m2V-1s-1) Li+ Na+ Mg2+ OH40.1 51.9 55 205 ClBrNO3- H3O+ 79.1 81.3 74.0 363 For most of ion Ionic mobility lie in the range of 40 to 80 x 10-3 m2V-1s-1 However OH- and H3O- have unusual high value This is due to jumping mechanism (Grotthuss mechanism) Mechanism of hydrogen ions transfer Transport Number of an ionic species in the solution Transport number (tB) of an ionic species B, in a solution is defined as the fraction of current carried by that particular ion. Transport Number For one type of anion and cation (+)ve ion charge concentration in solution is equal to (-)ve charge concentration Transport Number Since solution is electrically neutral Transport Number Experimental determination of transport number • Hittorf’s Method • Moving boundary method