Transcript Slide 1
Oxidation-Reduction Reactions OXIDATION = loss of electrons Examples: Na Na+ + eAl Al3+ + 3eS2- S + 2e- LEO the lion goes GER OXIDATION = increasing the oxidation number (more positive) Example: NO NO2 Oxid. Nos: +2 -2 +4 -2 change = +2 +2 to +4 : N is oxidized Oxidation-Reduction Reactions REDUCTION = gain of electrons Examples: N + 3e- N3Fe3+ +e- Fe2+ LEO the lion goes GER REDUCTION = decreasing the oxidation number (more negative) Example: MnO4- Mn2+ Oxid. Nos: +7 +2 change = -5 +7 to +2 : Mn is reduced Voltaic Cells (a porous barrier can be used instead of salt bridge) This electromotive series presents data showing the tendency of substances to gain or lose electrons. 0.46 V Choose any two entries; the top one will act as the cathode and will be reduced; the bottom one will act as the anode and will be oxidized. The standard voltage will be the algebraic difference between the two respective potentials. Batteries Batteries are the most practical applications of voltaic cell. All batteries have self contained anode/cathode compartments. All operate using the same principles already discussed. The Classic “dry” (LeClanché) cell. zinc anode Mushy paste Of MnO2 and NH4Cl Overall reaction: Zn + 2 MnO2 + 2NH4+→ Zn2++2MnO(OH)+ 2NH3 E~1.5 v. carbon cathode Batteries Alkaline Battery (similar to dry cell but more efficient) Anode: (Zn cap) Zn(s) Zn2+ (aq) + 2eCathode: MnO2, NH4Cl and C paste: 2NH4+(aq) + 2MnO2(s) + 2e- Mn2O3(s) + 2NH3(aq) + 2H2O(l) Batteries Lead-Acid Battery Six cells in series give a total voltage of ~12 volts in an automobile battery. Batteries Fuel Cells Corrosion Corrosion is the entropy monster’s greatest weapon. It is the evil side of REDOX. It costs 100s of billions of dollars yearly to prevent and correct. Basically, it is the spontaneous process (oxidation) of iron: Fe → Fe2+ + 2enice shiny metal (steel) ugly brownish-red powder ……….RUST! Rusting cannot occur by itself. Can’t have only the OX in REDOX; So, what gets reduced? Usually H2O or O2 Corrosion Common type of “rusting” redox: (Eo ~ 0.8 V) O2(g) + 4H+(aq) + 2Fe(s) → 2Fe2+(aq) + 2H2O(l) Easy, but even more favorable in acid conditions. There are similar equations also involving water. Stopping Corrosion 1. Galvanize it (coat with Zn). Fe has higher SRP than Zn. Coupled with Zn, Fe is the cathode (cathodic protection) (look for a “matte” appearance of Zn). 2. Use “sacrificial metal” such as Mg – (this is also cathodic protection). 3. Cover it (paint). 4. Create rust-resistant alloys, e.g., stainless steel (Fe/Ni/Cr), or nickel steels (Fe/Ni). Corrosion Preventing the Corrosion of Iron Also used on ships to prevent corrosion Electrolysis – Electrolysis of Molten Salts Decomposition of molten NaCl Cathode: 2Na+(l) + 2e- 2Na(l) Anode: 2Cl-(l) Cl2(g) + 2e- Industrially, electrolysis is used to produce metals like aluminum (Hall-Héroult process, where Al2O3 is electrolyzed in molten cryolite, Na3AlF6, with a carbon electrode to give an overall reaction of 2Al2O3 + 3C 4Al + 3CO2) Electrolysis with Active Electrodes – Gold plating – protects against corrosion external power source external power source Au Au Au+(aq) cathode: Au+(aq) +e-→Au Au+(aq) anode: Au→Au+(aq) +e- Organic compounds come from organisms (original definition), but are recognized as always containing carbon atoms, generally in chains Major kinds of organic compounds in living systems: Hydrocarbons – contain only C and H Carbohydrates (saccharides; sugars) – CH2O general formula Fats and fatty acids – C-C-C-C-C-C-C-C-C-C-C-C-CO2H Proteins – contain C, H, O, and N (sometimes S) Hydrocarbons – CH Methane – CH4 (simplest hydrocarbon) Propane C3H8 Butane C4H10 Octane – C8H10 (gasoline) Kerosene – C12H26 (diesel and jet fuel) Oils - C20H42 Asphalt C100-H202 Carbohydrates – (CH2O)x Monosaccharides – C6H12O6 – fructose – corn sugar, fruit sugar (sweetist) - glucose – blood sugar, wine sugar (least sweet) Amylose Cellulose Disaccharides – C12H22O12 – sucrose – table sugar, cane sugar, beet sugar (intermediate sweetness) Lactose – milk sugar Maltose – malt sugar Polysaccharides - (C6H12O6)x where x is very large starch – digestible cellulose – digestible only by microorganisms Proteins – made up of amino acids There are 20 different amino acids -NH2-R-CO2H Globular proteins – enzymes (example, hemoglobin) Fibrous proteins – structural (muscle, hair, skin, animal tissue) Fats – made up of fatty acids and glycerol Fatty acids – CH3CH2CH2CH2CH2CO2H, or some other even number of carbon atoms. Examples: C-CO2H acetic acid (vinegar) C-C-C-CO2H butyric acid (rancid butter) C-C-C-C-C-CO2 caproic acid (goat and other barnyard animals C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-CO2 palmitic acid (palm oil) C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-CO2H stearic acid (animal fats) Sulfur organic compounds: skunk smell, cabbage, onions, garlic, rotten eggs, oil refinery areas Nitrogen organic compounds: amines (fishy odors) decaying flesh (cadaverine) Fatty acids: animal smells, spoiled beer ethylene is the simplest hormone – CH2=CH2 – ripens fruits oleic acid C-C-C-C-C-C-C-C-C=C-C-C-C-C-C-C-C-CO2H in olive oil the C=C is “unsaturation”