REDOX AND ELECTROCHEMISTRY SUROVIEC SPRING 2015 Chapter 18

I. Electron Transfer Reaction Study of interconversion of electrical and chemical energy 2 types of cells

I. Electron transfer reactions Cu°(s) + 2Ag + (aq)  Cu 2+ (aq) + 2Ag ° (s)

A. Balancing in Acidic solution Balance the following in acid: Al(s) + Cu 2+ (aq)  Al 3+ (aq) + Cu(s)

B. Balancing in Base Balance the following in base I - (aq) + MnO 4 - (aq)  I 2 (aq) + MnO 2 (s)

II. Voltaic Cells Lets take: Cu°(s) + 2Ag + (aq)  Cu 2+ (aq) + 2Ag ° (s) and make a voltaic cell around it

B. Other Salt Bridge Cells

III. Standard Voltages The driving force behind a spontaneous reaction in a voltaic cell is measure by cell voltage.

A. E° RED + E° ox Any redox reaction can be split into 2 ½ reactions each with its own E° and therefore its own

A. E° cathode and E° anode Notice on the table only E° RED is listed

B. Strength of oxidizing and reducing agents Oxidizing agents = species that can electrons Reducing agents = species that supply electrons

C. Calculations of E° and spontaneity To determine if a reaction is spontaneous look at the sign of E°

IV. E°,  G° and K eq A. E°,  G° It can be shown that  G° = -nFE°

B. E° and K eq Redox equations also have a state of equilibrium which means that  G° = -RT lnK eq

Example For this reaction, determine  G° and K eq at 25°C 3Fe 2+ (aq) + NO 3 - (aq) +4H + (aq)  NO(g) + 2H 2 O(l) + 3Fe 3+ (aq)

V. Concentration and Voltage What if we are not at standard state? When the concentrations change the voltage must also change

Example Given this reaction where all the concentrations are 2.5 x M, calculate Q, n, E 0 and E Zn o (s) + 2H 2 O(l) + 2OH - (aq)  [Zn(OH) 4 ] 2- (aq) + H 2 (g)

VI. Electrolytic Cells Non-spontaneous reactions made to occur by pushing electrical energy through the system

Example Chromium can be electroplated onto other surfaces: Cr 2 O 7 2- (aq) + 14H + (aq) + 12e -  2Cr (s) + 7H 2 O (l) Using a current of 8.00A and 3.5V, how many grams of chromium can be plated is the experiment is run for a hour?

Example A baby spoon with an area of 6.25cm 2 is plated with silver from AgCl (s) using 2.00A for 2 and ¼ hours. If the current efficiency is 82% how many grams of Ag is plated? AgCl (s) + e -  Ag (s) + Cl - (aq)