| MadSci Network: Chemistry |
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I am asking this question because currently there is great debate over
the
deleterious effects of idling one's car on a cold morning. The local
paper
ran an ad stating that one gallon of gas produces 20 pounds of CO. I took
chemistry in college awhile and go and all my intuition disagrees with
this
statement. Currently, we are allowed to idle car for 3 min. and risk a
ticket
for minutes after that. It has resulted in a few accidents from
improperly
deicing windshields before driving. I do want to protect the environment
but not at the risk of more pedestrians being run over. I would like to
raise
awareness and make sure that there is not misinformation being printed
in the paper. thanks for your time!
One gallon of gasoline (that is, about 4 liters or 4000 cc), at 0.8 g/cc, weighs in at 3.2 kg or about 7 pounds. Now, gasoline, with an average formula of about CH2, is about 85% carbon; so a gallon of gas contains about 6 pounds of carbon (oxygenated gasolines, like gasohol, will contain somewhat less). For the sake of argument we'll assume that all the carbon is converted into carbon monoxide, CO--most of it is actually converted into CO2. With these assumptions, we find that one gallon of gasoline cannot produce more than 14 pounds of CO; or if it is all converted into CO2, no more than 22 pounds of CO2. The actual CO/CO2 ratio will vary depending on conditions and how well- tuned your car is, but 20 pounds is not a bad number for the amount of carbon DIoxide--not carbon MONoxide. Automobiles are typically THE major source of municipal air pollution, particularly nitrogen oxides (which generate corrosive ground-level ozone). They are also THE primary source of CO2 in this country, simply because more fossil fuels are burned in cars than in any other type of machine, including electric power plants. That's because there are so many cars-- more than one per US citizen! It is true that cold cars can be less safe--not only because of windshield fogging, they also produce more toxic pollutants. But my experience, in rural northwestern Ohio where temperatures just came back up from 0oF with a fair bit of snow, is that a well-tuned car, even when parked outside, normally needs to be on no longer than it takes to scrape the windshield. The exception is when there's moisture trapped in the vents, in which case everything inside will fog up and stay that way for a while. It doesn't take me more than about three to five minutes to scrape the car, once the snow's cleared away; and clearing off the snow before you start the car is one of the best ways to keep moisture out of the vents and prevent inside fogging. The air intakes are often just in front of the windshield, and if you don't clear them before starting the car you may have a snowstorm inside. My guess is that the three-minute time limit is aimed at people who want to bring their passenger compartments to 70o every morning--or perhaps at people who should be using a stepladder to scrape the windshields of their monster trucks. It's silly and wasteful to use the car heater alone to clear your windshield!
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