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CHM 2045

Electrochemistry Monday, August 05, 2013 11:00 PM 1. Redox reactions a. Oxygen usually has an oxidation state of -2 b. Reducing agents are RICH IN H, POOR IN O, metal in low oxidation state c. Oxidizing agents are RICH IN O and POOR IN H, metal in high oxidation state 2. Biological oxidation-reduction a. NADH (coenzyme)reduces carbonyls i. NAD+ is the oxidizing agent, accepting electrons to becomereduced b. FADH2 reduces alkene double bonds 3. Balancing redox reactions a. b. If after multiplying the two bridge half reactions, if the charges arent balanced, you must add either H+ or OH- to one of the sides, depending on if the reaction happens in an acid or base i. You must then rebalance the equation by adding water to the opposite side c. Oxygen isnt always -2! i. For example, in h202, 0 must be -1 to create a zero overall charge 4. Voltage and energy a. Electrochemistryinvolvesthe transfer of energy in oxidation reduction reactions reduction reactions b. The energetics of half-cells are measured relative to the reduction of hydronoium into hydrogen c. The reduction of two protons H+ to form hydrogen gas is defined as the reference standard, and assigned zero emf d. Any compound that can be reduced more favorably than a proton has a positive reduction potential e. Any compound that can be oxidized more favorably than a hydrogen gas has a negative redox potential f. The free energy change that accompanies a redox reaction is ΔG = -nFE, where n is the number of electrons transferred and F stands for a farady g. Since reduction potentials are intensive properties, we do not multiplythe half reaciton poteitnal by the nuer of times it occurs h. Each half reaction has a potential, which is the cell voltage it would have if the other electrode were a standard reference electrode i. When a species undergoes a favorable reduction half reaction (large +E ), it is a strong oxidizing agent ii. i. A species that is a strong oxidizing agent will want electrons (because it gets reduced) j. An oxidizing agent gets reduced, so it wants electrons k. A reducing agent gets oxidized, so it wants to give away electrons 5. Cell potential a. The cell potential for an oxidation-reduction reaction is a sum of the oxidation half-reaction potential and the reduction half- reaction potential i. b. WHEN YOU MULTIPLYA HALF REACTION BY AN INTEGER, YOU DO NOT MULTIPLYTHE EMF BY THAT INTERGER! 6. Conversionfrom voltage(emf) to energy a. 7. Definitions and terminology a. Electrochemicalcells convert energy produced in a chemical reaction to current b. This is accomplished by separating the xidation half reaction from the reduction half reaction and conntect the two with a waire c. Rather than generate heat energy, the reaction generates electrical flow that travels from the redcucing agent to the oxidting agent d. Oxidation
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