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 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 25C 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 hours. If the current efficiency is 82% how many grams of Ag is plated? AgCl (s) + e - Ag (s) + Cl - (aq)
