ES105 2007 February 1 I. Energy and Chemical Reactions A. Most reactions either release heat or consume heat 1. Heat energy is commonly measured in calories a. Food calories are actually kilocalories (kcal) b. 1 Joule=0.24 Calorie 2. Exothermic reactions release heat 3. Endothermic reactions consume heat 4. The energy of the reaction can be noted in the chemical equation. The units must be included for it to be meaningful 5. Can calculate energy from the equation Problem: C3H8 + 5 O2 Æ 3 CO2 + 4 H2O + 526 kcal How much energy is released when 2 moles of propane are burned? (526 kcal/mole of propane)(2 moles propane) = 1052 kcal released Of course you could calculate heat released when a certain gram amount of propane is burned…because you can convert grams to moles. Problem: 308 g propane x (1 mole/44 grams) = 7 moles 7 moles X 526 kcal/mole = 3682 kcal released a. Energy consumed is similarly noted, and can be calculated N2 + O2 + 4.32 kcal Æ 2 NO 5 moles of N2 reacting with 5 moles of O2 will consume about 21 ½ kcal in the production of NO gas Reactions that consume heat energy are called endothermic Is photosynthesis an exothermic or endothermic reaction? B. Temperature affects rate of reactions 1. Reactions proceed more rapidly at higher temperatures a. Coal + oxygen Æ carbon dioxide + water b. faster in the presence of heat c. Molecules collide more frequently at higher temperatures d. More energy is present to break chemical bonds C. Concentration of reactants influences rate of reaction D. Presence of catalysts allows some reactions to proceed at much greater rates than without catalyst 1. Peroxide H2O2 degrades to water over time 2. Introducing platinum metal speeds the reaction to seconds instead of years 3. Heat is released in this reaction Page 1 of 5II. Thermodynamic laws A. Conservation of Energy—first law of thermodynamics 1. Energy is neither created or destroyed— 2. you have the same amount before and after any reaction or event B. Second law of thermodynamics —Heat flows from warmer substances to cooler ones C. Corollaries of these laws 1. Energy can change form, commonly to a less useful one 2. Warm places can be made cold, but with the input of energy 3. Some energy is wasted in any transfer a. Released as heat b. Released as heat of friction D. Entropy is the term to describe the increase in randomness of system 1. Systems naturally tend toward greater disorder 2. Order can be increased, with input of energy a. Easy to pollute, difficult to clean up: b. think about CFC in the atmosphere, or oil in the sea. III. Energy sources and uses A. Humans first organism to systematically exploit energy reserves 1. First used chemical energy stored in wood—prevalent until 1850 2. Kinetic energy of water harnessed about 2000 years ago—grind grain in Egypt 3. Wind energy used to grind grain, pump water to grind grain perhaps as long ago as 1000 years B. ‘Fossil fuels’ include coal, petroleum and natural gas 1. ‘Fuel’ burns readily to release energy— a. what type is released? Chemical b. what is it converted to? Mechanical (usually) 2. fuels are ‘reduced’ forms of matter a. burning oxidizes the material— b. maximum number of atoms bonded to oxygen C. Recall first law of thermodynamics—conservation of energy 1. We convert energy from high-grade to lower-grade forms a. ‘Production’ of fuel means ‘make available for exploitation’ 1) Oil isn’t available until it has been pumped 2) Coal needs to be mined 3) Turbines in dams ‘produce’ electricity from the kinetic energy of falling water b. ‘Consumption’ of energy is our utilization of the energy source, where we convert it from one form to another, and reap the rewards of the conversion Page 2 of 51) We put gasoline in our car, and convert chemical energy to forward motion 2) We turn on the light switches, and convert electrical energy to light D. Fossil fuel reserves and consumption 1. Fossil fuels are still being created today…at a very slow rate 2. For our purposes, the supply of fossil fuels is limited a. ‘non-renewable’ b. We are depleting our reserves rapidly—perhaps half of petroleum has already been burned, in about 100 years c. Will become prohibitively expensive in the next 100 years E. Coal 1. High grade coal is more than 90% carbon—anthracite C(s) +O2 (g) ÆCO2 (g) 2. Lower grade coal may have less than 70% carbon a. Bitumin—nearly 90% carbon b. Sub-bitumin—70-88% c. Lignite—almost 70% d. Peat (precursor of coal)—about 60% 3. Coal formed from incompletely decayed plants a. Plants usually revert to CO2 upon death b. Swamp conditions, with standing water, reduce decay potential c. Plants become buried, compressed 1) Cellulose breaks down, releasing small molecules containing most of the hydrogen and oxygen 2) Carbon remained as concentrated deposit d. Wide expanses of land surface were swamps in late Paleozoic 1) Climate was generally warmer 2) Great land areas existed in tropical regions e. Large reserves of coal are present in the United States 1) Must be mined, either open pit or underground 2) commonly hauled to coal-fired electrical generators to be burned to make steam, to turn turbines, to generate electricity f. the part of coal that is NOT carbon is source of pollutants 1) much coal is high in sulfur—from pyrite a) creates aerosol sulfuric acid—chief component of acid rain b) pyrite can be removed by ‘floatation’ c) sulfur bonded to carbon-rich molecules can be removed from stack fumes instead of being released 2) other components include nitrates, arsenic, mercury Page 3 of 5g. coal is a source of other materials 1) heated to make ‘coke’, a high carbon fuel used in steelmaking 2) byproducts of coke condensed to coal oil and coal tar, that are refined into various organic chemicals F. Natural Gas—mostly methane CH4 + 2 O2 Æ CO2 + 2 H2O + heat 1. Natural gas is excellent, clean-burning fuel 2. Also raw material for plastics and other chemicals 3. Natural gas also contains sulfur and nitrogen, which can be released to become acid rain. 4. from heat and pressure acting on buried organic matter 5. is trapped within geologic traps, beneath impermeable rock layers G. Petroleum 1. replaced coal as primary fuel by about 1950 2. Complex molecules of hydrocarbon probably not derived from plants 3.
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