Ch 14: Electrode Potentials CrO 4 2- (aq) + Fe(s) + 4H 2 O(l)  Cr(OH) 3 (s) + Fe(OH) 3 (s) + 2OH - (aq) ClHC=CCl 2 (aq) H 2 C=CH 2 + HC  CH + Cl - (unbalanced) Fe catalyst Hanford, WA Fe Na 2 S 2 O 4 reduces Fe 3+ in soil to Fe 0

Redox Reaction Review 8H + + Cr 2 O H 2 S  2Cr H 2 O + 3S ox ½ rxn: red ½ rxn: reducing agent = oxidizing agent =

Current in Amp = C/s Volt (emf) = 1 Joule/Coulomb (J/C) Charge on electron (e) = 1.60 x C Faraday (F)= charge on a mole of e's = N A x e = 96,485 C/mol Some Terminology

Galvanic Cells (Sec 14-2) Be able to: draw the line diagram and cell identify cathode and anode and 1/2 rxn's determine electrode polarities & direction of electron flow give the net cell reaction Calculate E o cell, E, & K Zn | ZnCl 2 (1.0M) || CuSO 4 (1.0M) | Cu(s)

Example p. 315: Write a line diagram for the given cell. Write the half reactions and the net cell reaction.

Standard Potentials (Sec 14-3) Standard Hydrogen Electrode (SHE): a H+ = 1.00 M P H2 = 1.00 atm E o = 0.00 V (by definition)

a more complete table -

Back to the Zn/Cu galvanic cell: Zn | ZnCl 2 (1.0M) || CuSO 4 (1.0M) | Cu(s) E > 0spontaneous E = 0 equilibrium E < 0 spontaneous reverse direction

The Nernst Equation (Sec 14-4) calculating the non-standard cell potential for aA + bB = cC + dD, from Thermodynamics we know - G

Example p. 321: Find the voltage of the cell in Figure 14-4 if the right half-cell contains 0.50M KCl(aq) and the left half-cell contains 0.010M Cd(NO 3 ) 2 (aq). Write the net cell reaction and state whether it is spontaneous in the forward or reverse direction.

E o and the Equilibrium Constant (Sec 14-5) aA + bB = cC + dD at equilibrium - K

Example p. 326: Find the equilibrium constant for the reaction Fe 3+ + Ag(s) + Cl - = Fe 2+ + AgCl(s), which is the net cell reaction in Fig

Electrochemical Measurement of K w

Reference Electrodes (Sec. 14-6) The Standard Hydrogen Electrode (SHE) is inconvenient to use, so other more practical electrodes are commonly used in the lab and in the field. Their half-potentials have been measured relative to the SHE. Ag/AgCl Reference Electrode AgCl(s) + e = Ag(s) + Cl - E o = V (1.0 M KCl) = V (sat’d KCl)

Calomel Reference Electrode ½ Hg 2 Cl 2 (s) + e = Hg(l) + Cl - E o = V (1.0 M KCl) = V (sat’d KCl)

Voltage Conversions Between Different Reference Scales