Redox Reactions & Electrochemical Cells I. Balancing Redox Reactions

I. Balancing Redox Reactions STEP 1. Split Reaction into 2 Half-Reactions STEP 2. Balance Elements Other than H & O STEP 3. Balance O by Inserting H2O into eqns. as necessary STEP 4. Balance H with H+ or H2O (see 4a, 4b) STEP 5. Balance Charge by Inserting Electrons as needed STEP 6. Multiply Each 1/2 Reaction by Factor needed to make no. of Electrons in each 1/2 Reaction Equal STEP 7. Add Eqns. & Cancel Out Duplicate terms, where possible

I. Balancing Redox Reactions (continued) STEP 4a. In ACID: Balance H by Inserting H+, as needed STEP 4b. In BASE: Balance H by (i) inserting 1 H2O for each missing H & (ii) inserting same no. of OH- on OTHER SIDE OF REACTION as H2Os added in (i)

I. Balancing Redox Reactions (continued) Example Complete and Balance Following Reaction: CuS (s) + NO3 - (aq) Cu2+(aq) + SO42- (aq) + NO (g) STEP1. Split into 2 Half-Reactions a.1 CuS Cu2+ + SO42- b.1 NO3 - NO

I. Balancing Redox Reactions (continued) STEP 2. Balance Elements Other than H & O Already O.K. !

I. Balancing Redox Reactions (continued) STEP 3. Balance O by inserting H2O into equations as necessary a.3 CuS + 4H2O Cu2+ + SO42- b.3 NO3- NO + 2H2O

I. Balancing Redox Reactions (continued) STEP 4. ACIDIC, so Balance H by inserting H+ as needed a4. CuS + 4H2O Cu2+ + SO42- + 8H+ b4. NO3- + 4H+ NO + 2H2O

I. Balancing Redox Reactions (continued) STEP 5. Balance Charge by inserting Electrons, where necessary a5. CuS + 4H2O Cu2+ + SO42- + 8H+ + 8e- b5. NO3- + 4H+ + 3e- NO + 2H2O

I. Balancing Redox Reactions (continued) STEP 6. Multiply each Eqn. by factor to make No. of Electrons in Each 1/2 Reaction the Same a6. Multiply by 3x 3CuS + 12H2O 3Cu2+ + 3SO42- + 24H+ + 24e- b6.Multiply by 8x 8NO3- + 32H+ + 24e- 8NO + 16H+ + 24e-

I. Balancing Redox Reactions (continued) STEP 7. Add Eqns. and Cancel Out Duplicated Terms (a7 + b7) 8H+ 3CuS + 12H2O + 8NO3- + 32H+ + 24 e- 3Cu2+ + 3SO42- + 24H+ + 8NO +16 H2O +24e- 4H2O

I. Balancing Redox Reactions (continued) So, the final, balanced reaction is: 3CuS(s) + 8 NO3-(aq) + 8H+ (aq) 3Cu2+(aq) + 3 SO42-(aq) + 8NO(g) + + 4H2 O(l)

Checking mass balance and charge balance in Equation L.H.S 3 x Cu 3 x S 8 x N 24 x O 8 x H (8 x 1-) + (8 x H+) = 0 R.H.S. 3 x Cu 3 x S 8 x N 24 x O 8 x H (3 x 2+ )+(3 x 2- ) = 0

Redox Reactions in Electrochemistry Two Types of Electrochemical Cells: 1. Galvanic 2. Electrolytic Galvanic Cell - Converts a Chemical Potential Energy into an Electrical Potential to Perform Work Electrolytic Cell- Uses Electrical Energy to Force a Chemical Reaction to happen that would not otherwise occur

Anode and Cathode in Electrochemistry ANODE - Where OXIDATION takes place (-e-) CATHODE - Where REDUCTION takes place (+e-)

Electrochemistry and the Metals Industry Many Electrochemical Processes are used Commercially for Production of Pure Metals: e.g. Al Manufacture (by electrolysis of Al2O3) Mg Manufacture (by electrolysis of MgCl2) Na Manufacture (by electrolysis of NaCl)

Electrolylitic Production of Al using the HALL CELL (major plant in ALCOA, TN Al2O3 dissolved in molten cryolite (Na3AlF6) at 950 0C (vs. 2050 0C for pure Al2O3) Graphite Anodes (+) Steel case C lining (Cathode) (-) Al Al2O3 in molten Na3AlF6 Al Molten Al

Hall Cell for Al Manufacture

2 Al2O3 (sln) + 3C (s) 4 Al (l) + 3CO2 (g) Hall Cell Process Reaction: 2 Al2O3 (sln) + 3C (s) 4 Al (l) + 3CO2 (g) Location of Hall cell plant in E. Tennessee through availability of inexpensive Hydroelectric power. Process uses 50,000 – 100,000 A.