How does one gallon produce 19 pounds of carbon dioxide?

Answers to your questions about the news.
Nov. 1 2006 7:05 PM

How Gasoline Becomes CO2

A gallon turns into 19 pounds?

Download the MP3 audio version of this story here, or sign up for The Explainer's free daily podcast on iTunes.

Last week, Slate published the first installment of the "Green Challenge," a program that helps participants reduce the amount of carbon dioxide they put into the atmosphere. We started by asking people to think about the effects their cars have on the environment: "For each gallon of gas your car burns, it releases about 19 pounds of carbon dioxide." Explainer readers wondered about this statistic: If a gallon of gasoline weighs about 6 pounds, how can it produce three times that much greenhouse gas?

The carbon from the gasoline mixes with oxygen from the air. Gasoline consists mostly of hydrocarbons—chains of carbon encircled by atoms of hydrogen. When the hydrocarbons burn, they break apart and recombine with the air. This reaction produces heat, as well as two chemical byproducts: water and carbon dioxide.

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

Advertisement

For example, consider a single molecule of octane—a typical hydrocarbon that you'd find in gasoline. Octane consists of eight atoms of carbon and 18 atoms of hydrogen, written as C8H18. If you break down the octane and mix it with enough oxygen (O2), you've got the ingredients—i.e., the atoms of carbon, hydrogen, and oxygen—to make eight molecules of carbon dioxide (CO2) and nine molecules of water (H2O). The eight molecules of CO2 weigh about three times more than the one molecule of octane you started with. That doesn't mean you've violated the law of conservation of mass; instead, you've added the weight of the oxygen from the air to the weight of the carbon from the gasoline. (For a more in-depth discussion of this reaction, click here.)

This reaction gives only a general sense of what happens when you burn a gallon of gas. First, the combustion that occurs in a car engine doesn't work perfectly, which means not every hydrocarbon gets converted into carbon dioxide and water vapor. Sometimes there's not enough oxygen available to complete the reaction, in which case hydrocarbons can be converted into poisonous carbon monoxide (CO). Burning gasoline can also release nitrous oxide and other gases.

Second, gasoline consists of octane along with many other kinds of hydrocarbons. You'll also find additives like surfactants, freezing-point depressants, corrosion inhibitors, and dyes. These nonhydrocarbon additives might make up half a percent of the total composition of the gasoline. There are also differences between winter and summer blends, low- and high-octane, and leaded and unleaded.

Thus, any estimate of the amount of carbon dioxide that comes from a gallon of gas must be based on some assumptions. The Environmental Protection Agency starts with a guess for how many grams of carbon are in each gallon of gas. First, they determine how much carbon is in each particular kind of gasoline, and then they come up with a weighted average based on consumption levels for each variety. Using this method, they estimate that a gallon of gas contains, on average, 2,421 grams of carbon. That's enough to make 8,877 grams of CO2. They multiply that number by 0.99 to account for the carbon that doesn't react fully with the oxygen. Their result: 8,788 grams, or about 19.4 pounds. (The Energy Information Administration gives a slightly higher number—19.564.)

Got a question about today's news? Ask the Explainer.

Explainer thanks Roxanne Smith from the Environmental Protection Agency. 

TODAY IN SLATE

Sports Nut

Grandmaster Clash

One of the most amazing feats in chess history just happened, and no one noticed.

The Extraordinary Amicus Brief That Attempts to Explain the Wu-Tang Clan to the Supreme Court Justices

Amazon Is Officially a Gadget Company. Here Are Its Six New Devices.

The NFL Explains How It Sees “the Role of the Female”

Amazon Is Officially a Gadget Company

Sports Nut

How Much Should You Loathe NFL Commissioner Roger Goodell?

Here are the facts.

Food

How to Order Chinese Food

First, stop thinking of it as “Chinese food.”

Scotland Is Inspiring Secessionists Across America

The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant

The XX Factor
Sept. 18 2014 11:40 AM The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant
Moneybox
Sept. 17 2014 5:10 PM The Most Awkward Scenario in Which a Man Can Hold a Door for a Woman
  News & Politics
The World
Sept. 18 2014 1:34 PM Americans Fault Obama for Giving Them Exactly the Anti-ISIS Strategy They Want
  Business
Business Insider
Sept. 17 2014 1:36 PM Nate Silver Versus Princeton Professor: Who Has the Right Models?
  Life
The Eye
Sept. 18 2014 12:47 PM How One of the Most Prolific Known Forgers in Modern History Faked Great Works of Art
  Double X
The XX Factor
Sept. 18 2014 12:03 PM The NFL Opines on “the Role of the Female”
  Slate Plus
Behind the Scenes
Sept. 18 2014 1:23 PM “It’s Not Everyday That You Can Beat the World Champion” An exclusive interview with chess grandmaster Fabiano Caruana.
  Arts
Brow Beat
Sept. 18 2014 1:15 PM Beyoncé’s 10 Greatest Live Performances
  Technology
Future Tense
Sept. 18 2014 1:24 PM Can the Celebrities Whose Photos Were Stolen Really Sue Apple? It may be harder to prove “harm” than it seems.
  Health & Science
Bad Astronomy
Sept. 18 2014 7:30 AM Red and Green Ghosts Haunt the Stormy Night
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.