Chapter Nine Electrochemistry Applications

Copyright © Houghton Mifflin Company. All rights reserved.9 | 2 Batteries and Fuel Cells We’ve seen examples of batteries in our examination of electrochemical cells. Following are common examples that employ such methods.

Copyright © Houghton Mifflin Company. All rights reserved.9 | 3 A Flashlight Battery Consists of a Zinc Cup Anode and a Cathode Made of an Inert Carbon Electrode Immersed in a Paste containing MnO 2 and Mn 2 O 3 Zn(s) + 2 MnO 2 (s) + H 2 O  Zn 2 (aq) + Mn 2 O 3 (s) + 2 OH - (aq)

Copyright © Houghton Mifflin Company. All rights reserved.9 | 4 The Atomic-level View of the Reactions Involved in a Car Battery Shows Lead's Oxidation to PbSO 4 at the Anode and Reduction of PbO 2 to PbSO 4 at the Cathode Pb(s)  PbSO 4 (s) PbO 2 (s)  PbSO 4 (s) Balance this reaction

Copyright © Houghton Mifflin Company. All rights reserved.9 | 5 A Hydrogen-Oxygen Fuel Cell in an Environmentally Friendly Method for Producing Electrical Energy. The Only By-Product is Water

Copyright © Houghton Mifflin Company. All rights reserved.9 | 6 In the Fuel Cell, Electron Transfer Is Indirect O 2 (g) + 2 H 2 O(l) + 4 e -  4 OH - (aq) H 2 (g) + 2 OH - (aq)  2 H 2 O(l) + 2 e - Combine these and compute the cell voltage

Copyright © Houghton Mifflin Company. All rights reserved.9 | 7 Electrolysis As the name defines, “lysis” is to break apart, thus electro-lysis is the breaking of of materials using electricity An electrolytic cell is a batter run in reverse. This process regenerates the battery in your car while the engine is running. Question: Can one produce sodium and chlorine gas from a salt water solution?

Copyright © Houghton Mifflin Company. All rights reserved.9 | 8 Salt Water Electrolysis In a solution of NaCl, we have Na +, Cl - and H 2 O. The possible reactions are: Na + (aq) + e -  Na(s)E o = V 2 Cl - (aq)  Cl 2 (g) + 2 e - E o = V 2 H 2 O(l) + 2 e -  H 2 (g) + 2 OH - (aq)E o = V 2 H 2 O (l)  O 2 (g) + 4 H + (aq) + 4 e - E o = V Electrolysis will begin when the minimum threshold voltage is achieved. To produce Na and Cl V V = 4.07 V are required. However, when the threshold of 0.83 V V = 2.06 V is achieved, water electrolysis will begin first! So, it is not possible to produce Na and Cl 2 this way.

Copyright © Houghton Mifflin Company. All rights reserved.9 | 9 Production of Metals The production of metals usually involves electrolysis of the molten salt. Here, NaCl is kept above the melting point of 800 o C

Copyright © Houghton Mifflin Company. All rights reserved.9 | 10 Schematic of the Apparatus for the Electrolytic Production of Aluminum Showing Molten Aluminum Sinking to the Bottom of the Tank

Copyright © Houghton Mifflin Company. All rights reserved.9 | 11 Corrosion Prevention Corrosion of metals is of great concern in everything from power lines and utility poles to bridges and buildings. Active metals used in construction should be protected in some form.

Copyright © Houghton Mifflin Company. All rights reserved.9 | 12 Examining the Location of Corrosion Provides Insight into the Atomic-level Events Involved in Corrosion When placed in a solution of phenolphthalein and [Fe(CN) 6 ] -3, the solution turns pink in the presence of OH -1 ions and blue in the presence of oxidized iron, Fe +2. Oxidation occurs at defect points where exposure to the metals is greatest. Fe  Fe e - at the blue areas and O 2 (g) + 2 H 2 O + 4 e -  2 OH - in the pink areas giving an overall reaction of the production of rust. 2 Fe + O 2  2 FeO

Copyright © Houghton Mifflin Company. All rights reserved.9 | 13 The Site of Oxidation (electron source) Must Be Connected to the Site of Reduction (electron sink) by a Conductive Material

Copyright © Houghton Mifflin Company. All rights reserved.9 | 14 Sacrificial Anodes Metal such as iron is optimal for many reasons including it’s low cost and availability, conductive properties, and metallic behavior. However, it suffers from being a fairly active metal. How does one prevent the iron from decomposing? Answer: Place the iron in contact with another, more active metal. Being more active, the more active metal will oxidize preferentially to iron. Such metals are called “sacrificial anodes”

Copyright © Houghton Mifflin Company. All rights reserved.9 | 15 Underground Steel Pipes Here, Mg is more active than the pipe and will corrode first. Attack on the pipe is prevented by the transfer of electrons by the magnesium metal. Copper is very inactive and will not corrode.

Copyright © Houghton Mifflin Company. All rights reserved.9 | 16 A Steel Utility Pole Is Connected to a Magnesium Stake. The Magnesium Feeds Electrons to the Iron in the Steel Pole via the Conducting Connector, Preventing Oxidation of the Iron

Copyright © Houghton Mifflin Company. All rights reserved.9 | 17 Oxide Coating Protection Aluminum is an active metal, however, when exposed to air, it forms a tough Al 2 O 3 coating that protects the interior metal. Iron nails are often “galvanized” by coating with a layer of zinc oxide, ZnO which protects the interior metal.