ES105 I 2007 February 1 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 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 2 NO 5 moles of N2 reacting with 5 moles of O2 will consume about 21 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 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 5 II 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 5 1 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 CO2 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 5 g 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 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 Fats of planktonic animals in shallow seas a Rain of dead plankton became layers of incompletely decayed organic matter b Heat and pressure of burial changed animal fat into petroleum 4 combustion products are carbon dioxide and water This is the balanced equation for octane a component of gasoline 2C8H18 18 O2 16 CO2 18 H2O a Air contains nitrogen so some is converted in combustion to nitrogen oxides b Petroleum also contains some sulfur
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