ELECTROCHEMISTRY - Wits Structural Chemistry

More documents

Recommendations

Info

Example 3 Devise a cell in which the cell reaction is: Mn(s) + Cl 2 (g) → MnCl 2 (aq) Give the half reactions at the electrodes and from the standard cell potential of 2.54 V deduce the standard potential for the Mn 2+ /Mn(s) redox couple. Given: E°(Cl 2 /Cl - ) = +1.36 V Example 4 Estimate the cell potential at 25°C for Ag(s)|AgBr(s)|KBr(aq, 0.050 mol kg –1 )||Cd(NO 3 ) 2 (aq,0.0034 mol kg –1 )|Cd(s) E°(R-H) = –0.40 V E°(L-H) = +0.07 V (assume non-ideal solutions) Write the spontaneous electrochemical reaction. Example 5 The standard potential of the cell below at 25 °C is 0.95 V. Ag(s) |AgI(s) | AgI(aq) | Ag(s) Calculate: a) its solubility constant and b) the solubility of AgI . MOLECULAR MOTION IN LIQUIDS Mass Transport Migration of ions - Conductivities of Electrolyte Solutions Mobilities of Ions Diffusion of Ions Part of Chapter 20 Mass Transport Three main ways to transport ions to the surface of an electrode → called mass transport Conductivities of Electrolyte Solutions Ions can be dragged through the solvent by applying a potential difference between two electrodes in solution. MIGRATION To measure conductivity: Incorporate conductivity cell into one arm of resistance bridge and search for balance point. R 3 Convection Movement due to mechanical intervention. e.g. stirring, pumping, shaking Affects molecules and ions in solution. Diffusion Movement due to concentration gradient. e.g. movement across a cell membrane Affects molecules and ions in solution. Migration Movement due to applied electric field. e.g. in an electrolysis cell Affects only ions in solution. 1 R = κ A 1 κ = R A C κ = R R = resistance of solution (Ω) κ = conductivity (S cm -1 ) = distance between electrodes A = area of electrodes C = cell constant κ depends on the number of ions in a solution. R 4 2 R 1 R R = 1 2 R R 3 R 4 Resistance bridge R 1R = R R3 4 2 Note: use ac current (ν~1kHz) to avoid electrolysis and polarisation of the electrodes which would change the composition of the solution at the electrode surfaces. 4
Measuring the conductivity of tap water: κ = c Λ Λ m = molar conductivity (S cm 2 mol -1 ) m c = molarity If κ ∝ c, the molar conductivity should be independent of the concentration of an electrolyte. Is this the case Parallel plates 293.3 µS / cm 1 S = 1 Siemen = 1 Ω -1 NO! Molar conductivity varies with concentration because: - no. of ions in solution may not be proportional to concentration (especially for weak electrolytes) - ions interact strongly with one another reducing effective charge NB: Don’t confuse conductivity (κ) with conductance (G) Strong electrolyte – Λ m decreases slightly as conc increases. Weak electrolyte – normal Λ m at low conc, but Λ m decreases sharply as conc increases. 1 G = R Kohlrausch’s law: At low concentrations, molar conductivities vary with the square root of conc. Λ = Λ − K c m Λ o m = ν o m + λ + + ν −λ − STRONG ELECTROLYTES Λ m o = limiting molar conductivity (ions are infinitely far apart no interaction) K → related to stoichiometry of electrolyte Kohlrausch’s law of independent migration of ions: Λ m o → expressed as sum of contributions from its individual ions. λ = limiting molar conductivity of ions ν = no. of ions per formula unit of electrolyte H + 34.96 OH - 19.91 Na + 5.01 Cl - 7.63 K + 7.35 Br - 7.81 Zn 2+ 10.56 SO 2- 4 16.00 e.g. for MgCl 2 : ν + = 1 ν − = 2 Limiting ionic conductivies in water at 298 K, λ /mS m 2 mol -1 Weak electrolytes do NOT dissociate completely, conductivity of weak electrolytes depends on the degree of ionization (α) e.g. HA + H 2 O A - + H 3 O + + − [H3O ][A ] + K [ H3O ] = αc a = [HA] − [ A ] = αc 2 α c Ka = [ HA] = ( 1− α)c 1 − α At infinite dilution the electrolyte is fully ionised : Λ o m = ν + λ + + ν −λ − At very low conc’s: Ostwald’s dilution law WEAK ELECTROLYTES Λ = α o m Λ m 1 1 Λ mc = + o Λ m Λ m K ( Λ ) o 2 1 1 1 = + Λ mc o o 2 Λ m Λ m K a ( Λ m ) y c m x a m (c = formal conc of HA) Example 6 Calculate the degree of ionization and the acid dissociation constant at 298 K for a 0.010 M acetic acid solution that has a resistance of 2220 Ω. The resistance of a 0.100 M potassium chloride solution was also found to be 28.44 Ω. Λ m (0.1 M KCl) = 129 S cm 2 mol -1 λ o (H + ) = 349.6 S cm 2 mol -1 λ o (CH 3 COO - ) = 40.9 S cm 2 mol -1 Example 7 The molar conductivity of a strong electrolyte in water at 25 °C was found to be 109.9 S cm 2 mol -1 for a concentration of 6.2 × 10 -3 mol L -1 and 106.1 S cm 2 mol -1 for a concentration of 1.5 × 10 -3 mol L -1 . Estimate the limiting molar conductivity of the electrolyte. 5
Page 1 and 2: ELECTROCHEMISTRY Mrs Billing GH840
Page 3: Standard Electrode Potentials The p
Page 7 and 8: Fick’s second law of diffusion or
Page 9 and 10: (1− α)nF ln j = ln jo + η RT Ta

ELECTROCHEMISTRY - Wits Structural Chemistry

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?