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Stoichiometry Practice Related to Climate Change LESSON 6 Homework Assignment 1 Calculate the number of pounds of CO2 added to the atmosphere with the combustion of 1 gallon of gasoline. Conversions needed: 1 gallon by volume = 3785 mL density of octane = .703 g/mL 1 lb = 453.6 grams Assumptions: To make this calculation easier, we will assume the entire gasoline content is octane. The actual content is closer to 45% because there are other hydrocarbons present in gasoline but this gives a close approximation to the amount of CO2 generated. C8H18 + O2 CO2 + H2O Hint : Balance the combustion equation then start with the volume of octane burned in your stoichiometry calculation. Convert your final mass of CO2 from grams to pounds. Solution 2 C8H18 + 25 O2 16 CO2 + 18 H2O 3785 mL C8H18 1 8201 g CO2 1 .703 g C8H18 1 mL C8H18 1 lb. CO2 453.6 g CO2 1 mol C8H18 114.23 g C8H18 16 mol CO2 2 mol C8H18 44.01 g CO2 1 mol CO2 18.1 lbs CO2 So how does this make any sense? How can you burn 1 gallon of gasoline (weighing about 6 pounds depending on the temperature) and make 18 pounds of carbon dioxide? Look again at the balanced equation. You are using 25 moles of oxygen to combust 2 moles of octane. This oxygen is bonding to the carbon and hydrogen to make roughly the same amount of carbon dioxide gas and water vapor. (Water vapor is also a heat-trapping gas so this has an added affect on the temperature of the Earth.) Discussion The gasoline that fuels your car is a fossil fuel meaning it was formed millions of years ago by time, pressure, and heat applied to decayed organic matter (pre-historic plants and animals). Discussion (cont.) Most crude oil (assorted hydrogen-carbon compounds ranging from a single carbon in size to 100’s of carbons in size) was produced about 50 million years ago and has been locked below the Earth’s surface all this time. http://www.oilspillsolutions.org/oil.htm Discussion (cont.) In roughly 20 minutes of driving (if you travel 60 mph and your car gets 20 mpg) you have moved close to 20 pounds of carbon from being locked under the surface of the Earth to locking heat against our Earth. http://www.greentechnolog.com/2007/04/automobile_co2_emissions.html Burning coal: A bit more complicated. As of 2005, 59% of Alabama’s energy came from the burning of coal. The following equation is a simplified version of the combustion of coal. Coal is mostly a complex of rings of carbon bonded together but depending on the origin of the coal, other elements are attached to the carbons. Basically, when coal is burned, the result is carbon dioxide and soot which is mostly carbon dust. C2 + O2 CO2 + soot (C) http://apps1.eere.energy.gov/states/electricity.cfm/state=AL#fuel Background for Homework Assignment 2 The thermal density (how much energy you can get for a set amount of fuel) of coal is 6.67 kWh/kg. Coal fuel (as of 2000) is at 33.54% efficient. This means the other 66.46% is lost as residual heat. With only 33.54% efficiency, this means the actual useable energy for coal is 2.24 kWh/kg. Turning this value into pounds gives: 2.24 kWh 1 kg .4536 kg 1 pound 1.01 kWh 1 pound This means that 1 pound of coal produces about 1 kWh of useable energy. Homework Assignment Calculate the number of pounds of CO2 added to the atmosphere with the combustion of 1 pound of coal. This will represent the amount of CO2 added to the atmosphere for 1kWh of energy produced from coal. Conversion needed: 1 lb = 453.6 grams Assumptions: To make this calculation easier, we will use the following equation to represent the combustion of coal in a coal plant. C2+ O2 CO2 + soot (C) Hint : Start with 1 lb of coal burned in your stoichiometry calculation. Convert your final mass of CO2 from grams to pounds. Solution C2 + O2 CO2 + soot (C) 1 lb C2 1 831.1 g CO2 1 453.6 g C2 1 lb C2 1 lb. CO2 453.6 g CO2 1 mol C2 24.02 g C2 1 mol CO2 1 mol C2 44.01 g CO2 1 mol CO2 1.83 lbs CO2 So this tells us that producing electricity with 1 lb of coal (to generate about 1kWh) puts 1.83 lbs of carbon dioxide in the atmosphere. That’s about the amount of electricity it takes to keep your laptop plugged in all day. It takes about 3.4 kWh to keep your refrigerator running all day. Think about all the appliances that are plugged in to your outlets at your home. Even appliances that are not turned on but are left plugged in pull electric current and cause the dial to spin on your electric meter.