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Petroleum B Petroleum as an energy source Buried Sunshine Fossil fuels - oil, coal, natural gas, are all dead plants which grew be photosynthesizing sunlight. Therefore, fossil fuels are buried sunlight. There is no 100% conclusive evidence of where petroleum comes from. The process of making fossil fuels over time happens too slow. Potential vs. Kinetic Energy Potential Energy is stored energy. Chemical energy is a kind of potential energy = battery Kinetic Energy is energy of movement/motion During a chemical reaction the potential energy in compounds are released and the products (usually) have less potential energy than the reactants. Bond breaking and bond making In a chemical reaction, bonds must first be broken. Combustion of methane CH4 + O2 CO2 + H2O The methane and oxygen molecules must first be broken. This requires energy to be ADDED = Endothermic Next, the bonds must reform, but in different combinations. When this happens it is an exothermic reaction = energy is released Potential Energy DiagramExothermic Reaction Potential Energy DiagramEndothermic Reaction Bond breaking and bond making If the energy added (endothermic) is greater than the energy given off (exothermic), then the overall reaction is ENDOTHERMIC. If the energy added (endothermic) is less than the energy given off (exothermic), then the overall reaction is EXOTHERMIC. Bond breaking and bond making EXOTHERMIC = feels hot to the touch energy was released to the environment ENDOTHERMIC = feels cold to the touch Energy was absorbed from the environment Energy Conversion Consider the energy required to power a hair dryer. No matter how many times energy is transformed to another energy it is never “used up”. Law of Conservation of Energy - energy is never created or destroyed, it only changes forms. Energy Efficiency Devices that convert chemical energy (fossil fuels) to thermal energy (heat) then to mechanical energy (movement) are less than 50% efficient - this refers to the energy made by power plants. Solar cells and fuel cells (for cars) are cleaner and more efficient. Some energy is always “lost” when it is converted from one form to another. Energy Efficiency Low efficiency of the machine using the energy. The most efficient car engines are only 25% efficient. The rest of the energy is lost as heat. Do B.5 Energy Conversion Efficiency Problems on p 245. Combustion BURNING!! Burning a hydrocarbon will always yield Carbon Dioxide (CO2) and Water (H2O) Energy is always released - it feels hot. C25H52 + O2 CO2 + H2O combustion of candle wax. Where does the energy come from? The chemical bonds release energy when they are destroyed. Specific Heat Capacity Specific Heat Capacity - the amount of heat needed to raise the temperature of a substance by 1C. Calculating Specific Heat Capacity Suppose a 10.0 g water sample is heated from 25.0C to 30.0C. How much energy must have been added to the water? 210 J of heat Heat of Combustion/Molar Heat of Combustion Heat of Combustion - the amount of heat given off when an amount of a substance is burned The “amount” is usually 1 gram. Measured in kJ/g Molar heat of combustion - the amount of heat given off when 1 mole of any substance in burned. Measured in kJ/mol Calculating Heat of Combustion How much energy (in kilojoules) is released when 76.0 g of hectane is burned? Calculating Molar heat of combustion How much energy (in kJ) is released by completely burning 25.0 mol hexane. 104000 kJ Altering Fuels There are only 9.9 gallons of gasoline in a 55 gallon drum of crude oil after fractional distillation. Now, as much as 25.9 gallons (47%) of gasoline can be taken from a 55 gal. drum of crude oil because of altering of molecules Scientists can change existing molecules to meet the demand. Cracking Cracking - process of converting large hydrocarbon molecules to smaller ones through the application of heat (600C-700C) and catalysts. Kerosene: 16 C’s to gasoline 8 C’s Hydrocarbons up to about 14 C’s can be produced through cracking. Cracking Catalysts are used to speed up existing reactions but they themselves are not used up. Catalytic cracking is more efficient because it requires less heat Bonus: During cracking Methane, ethane, propane, and butane made and immediately burned to keep the temp. hot. Pinging/Knocking “Gasoline” is composed mainly of straight chained molecules of: hexane, heptane, and octane. Gas-Air mixture is first compressed in the cylinder (piston) just before it is ignited by the spark plug. Sometimes the compressed gas explodes before the spark plug ignites it. This causes the piston to bang back against the crankshaft = pinging/knocking These burn to rapidly - to hot causing the engine to “ping” or “knock.” Branched-chain alkanes burn more favorably in car engines This is the best “octane” - isooctane Octane Rating “Gasoline” is sold at varying grades. Isooctane is assigned an octane number of 100. Straight-chained Heptane is assigned an octane number of 0. Heptane has very poor engine performance. Finding Octane Rating Octane rating is determined under two conditions: A free-running engine And engine pulling a load (towing) The results of these two processes are averaged together. The higher the rating the more anti-knock Refer to the following for more information: http://auto.howstuffworks.com/question90. htm Octane rating Leaded Fuel: tetraethyl lead was added to gasoline to increase octane rating. However, it added lead compounds to environment. Oxygenated Fuels With lead-based fuel out, an alternative had to be found. Oxygenated fuels - a blend of additives added to fuel to increase the octane rating. The molecules of the additives contain oxygen as well as C and H. Oxygenated Fuels Oxygenated fuel provide LESS energy per gallon = less miles per gallon. BUT, they increase octane number (better performance of car) while reducing exhaust pollutants, and less emissions like CO. Oxygenated Fuels Methanol is a common oxygenated fuel (CH3OH). An additive to fuel Boosts octane rating (better engine performance). Methanol can be made from coal, natural gas, wood, and corn Conserves nonrenewable resources Gasohol - blend of 10% ethanol, 90% gasoline Can be used without engine modifications Oxygenated Fuels MTBE (methyl tertiary-butyl ether) was added to gas in late 1970’s. Octane rating of 116 - pollution reducing oxygenating chemical. However,late 1990’s: bad gas storage units allowed MTBE to seep into groundwater. Unpleasant taste to water; became a concern even though no apparent health risk. MTBE since them has been phased out. Oxygenating fuels MTHE (methyltetrahydrofuran) Octane rating = 87 same as gasoline It can increase the level of oxygenated fuel Can be obtained from renewable resources like papermill waste products. Straight-chain to branched-chain Straight-chained hydrocarbons burn too hot. Branched-chained hydrocarbons burn more favorably. Scientists can convert straight-chained to branched-chained. The straight-chained vapor is heated with a catalyst This is called isomerization.