Whenever we burn fossil fuels, be it gasoline, natural gas or charcoal, one of the byproducts is carbon dioxide ( CO2). That’s a big worry these days because the huge amounts of carbon dioxide we’re putting into the atmosphere is a primary cause of global warming.
Back in 2006, Slate published this article about the CO2 produced by burning gasoline in our cars. I thought it might be interesting to look at what happens when we burn gasoline in a bit more detail and extend that to a couple of other fuels.
Let’s start with gasoline.
A gallon of gasoline weighs about 6 pounds. Burning a gallon of gasoline produces about 19 pounds of carbon dioxide. How is that possible? How can the weight of the output, carbon dioxide, be more than the weight of the input, gasoline? The reason is that burning combines oxygen from the air with elements of the gasoline.
Gasoline is mostly a chemical called octane. The oil companies throw in a bunch of other ingredients and additives, but basically it’s octane. The chemical formula for octane is C8H18. A molecule of octane contains eight carbon atoms and eighteen hydrogen atoms. When you burn gasoline, the octane is broken down and combined with oxygen from the air to produce water vapor and carbon dioxide.
Here’s the actual chemical reaction showing the combustion of octane:
2 C8H18 + 25 02 –> 16 CO2 + 18 H2O
Two molecules of octane plus twenty five oxygen molecules produces sixteen molecules of carbon dioxide and eighteen molecules of water. Now this isn’t a totally accurate representation of what happens when gasoline is burned because of those other ingredients and additives and because combustion isn’t ever fully complete, which is how you also get the poisonous gas carbon monoxide coming out of your tailpipe. But for our purposes it’s close enough.
Now to figure out the weights of octane and carbon dioxide we just add up the weights of the carbon, oxygen and hydrogen atoms. Strictly speaking, we should use the term mass rather than weight here.
Here are the masses of these three elements. Again, these are approximations but accurate enough for our purposes.
Carbon – 12
Oxygen – 16
Hydrogen – 1
Similarly a molecule of carbon dioxide has one carbon atom and two oxygen atoms for a total of 44 (12 + 2 X 16 = 44).
Now in the chemical reaction above, two molecules of octane having a total atomic mass of 228 combine with oxygen to produce sixteen molecule of carbon dioxide with a total mass of 704. So the atomic mass of the carbon dioxide is a hair over three times the atomic mass of the octane (704 / 228 = 3.0877).
So that’s how burning six pounds of gasoline produces three times its weight, roughly nineteen pounds, of carbon dioxide
Natural gas is basically methane, or CH4. The gas company adds something really stinky to the methane so you can smell a leak, but that’s not significant in terms of carbon dioxide emissions.
When you burn natural gas to heat your home or cook your dinner, you’re combining methane with oxygen from the air to produce carbon dioxide and water vapor. Here’s the chemical reaction:
CH4 + 2 O2 –> CO2 + 2 H2O
A molecule of methane and two oxygen molecules produce one molecule of carbon dioxide and two molecules of water. This is a much simpler reaction. Maybe we should have started with this one!
Let’s compare the weights again. A molecule of methane has an atomic mass of 16 (12 + 4 X 1 = 16) and as before a molecule of carbon dioxide has a mass of 44. The carbon dioxide molecule has 2.75 times the mass of the methane molecule, so burning a pound of natural gas would produce about 2.75 pounds of carbon dioxide.
Actually natural gas is usually measure in cubic feet and converting that to weight depends on a bunch of other factors like temperature and atmospheric pressure which I’m not going to bother with here.
Lastly, let’s take a look at the propane you might burn in your barbecue. It’s formula is C3H8.
And the chemical reaction is:
C3H8 + 5 O2 –> 3 CO2 + 4 H2O
The atomic mass of one molecule of propane is 44 (3 X 12 + 8), and the three carbon dioxide molecules have a total mass of 132. The carbon dioxide produced by burning propane weighs three times as much as the propane itself. So for every pound of propane burned, three pounds of carbon dioxide are produced.
So that’s how burning stuff produces so much carbon dioxide.