Need help understanding a chemical reaction/equation?

Question

I'm a little confused about something I read in an article in Motor Trend (May 2007). I also heard a reference to this in a news story on the radio as well.

This is with reference to using Gasoline as an automotive fuel and greenhouse gas emissions:
"Hydrocarbons and NOx emissions can be further reduced, but each gallon produces 19.4 pounds of CO2, so improved efficiency is the only CO2 abatement option, and diesel and hybrid technologies are more efficient."

What I don't understand is, if a gallon of gas weighs ~8 pounds, how does burning 1 gallon of gas result in 19.4 pounds of Carbon Dioxide? What are the other inputs (and outputs) of the combustion of gasoline that can result in 19.4 pounds of CO2 resulting from burning only 8 pounds of gasoline?

Please don't get overly technical in your explanation, I don't have a science or engineering background.

Thanks!

Answer 1

Let's select pentane as a light hydrocarbon that may be present in petrol (what you call gasoline - to us down under, gas is LPG, also used as a fuel for cars - taxis/cabs in particular)
Pentane is C5H12
If we round off carbon weighing 12 and hydrogen 1, then a molecule of pentane weighs 60 + 12 =72 which makes it 60*100/72 percent by mass = 83% [I have chosen to not use a unit of mass, as it doesn't matter to your explanation]

So 83% of pentane weighs about 6.6 pounds

NOW, the product CO2 "weighs" 12 + 2x16 = 44, so carbon is 27% of CO2 - a much smaller proportion of the CO2.

If we assume that all of the 6.6 pounds of carbon were totally converted to CO2 (it actually doesn't burn cleanly or completely), then 6.6 pounds of C is also in the CO2.

But 27% of CO2 = 6.6 pounds (or 83% of pentane = 8)
so CO2 = 6.6/27% = 24.4 pounds THE THEORETICAL MAXIMUM MASS OF CO2 from one gallon of PENTANE.

Now, the real constituency of gasoline is not exactly 5:12 for C:H as there are other molecules making up gasoline, and these do include compounds that contain Oxygen (e.g. ethanol - sugar alcohol or whatever is the local brand name for it), so the exact proportion of carbon in gasoline is NOT 83% - using pentane is for illustrative purposes only. Further, combustion is incomplete, resulting in the formation of carbon monoxide which is probably worse than CO2 because it is poisonous and gets involved in ozone depletion and then adds to the amount of CO2 when it has converted to CO2.

I would suggest, as a lay person on the matter, that more complete burning of gasoline will reduce the wasted hydrocarbons, producing more CO2.

Remember that C compounds contribute to the greenhouse effect; be they methane, CO, CO2, hydrocarbons, etc, so CO2 is not the only offender. Which is the least of a number of evils? I leave that to the more expert contributors.

If you could get complete combustion and reduce NOx emissions, then photochemical smog risk is significantly reduced. Others can explain how to reduce NOx better than me.

Use of hydrogen fuel instead will eliminate C, CO, CO2 emissions; but because H burns very hot, it may result in large increases in NOx compared to gasoline. Unless you use pure oxygen as well as hydrogen as your FUELS (plural); then you only get water as a product. Extreme explosive risk, though.

Hope this is not too technical and descriptive enough without being my usual long-winded self.

BG

Answer 2

The extra amount of "stuff" that makes up the difference in weight is probably just the oxygen, O(2), that is consumed and required for the combustion of the gasoline. For the most part, gasoline is composed of hydrocarbons, which are compounds composed of primarily carbon and hydrogen, and can be considered to be kind of like the propane you might you to barbecue with. The combustion reaction is literally the chemical reaction between the hydrocarbons (think propane) and oxygen in the atmosphere (just like the kind you breathe.) The result of the reaction is water, which forms from the combination of the hydrogens and oxygen, carbon dioxide, which forms from the combination of the carbons and oxygen, and whole bunch of heat. To a certain extent, you could crudely visualize the result of the combustion as the replacement of the hydrogen atoms in the hydrocarbon being burned with oxygen atoms. These oxygen atoms are about 16 times heavier than the hydrogen atoms. The rough ratio between the masses of the released carbon dioxide and the starting hydrocarbon should be about
(12 + 2*(16))/(12+2*1) ~ 3. The 12 comes from the relative mass of the carbon atom. So in a perfect combustion reaction, about 24 pounds of carbon dioxide might be expected from one pound of gasoline. The number you listed is quite a bit less, but that's probably due to several reasons, not the least of which is that this is an oversimplified treatment. Take home message though, the extra weight comes from the heavier oxygen atoms in carbon dioxide compared to the light hydrogen atoms in the gasoline.

Answer 3

One way to think about it is looking at the chemical structure of gasoline. gasoline is a long carbon chain with oxygens attached, so essentially there are many CO2 within one strand of gasoline. Therefore due to mass conservation, it is possible that 1 pound of gasoline will produce more than one pound of CO2. Also, the mass of gasoline (not the 1 pound) is much greater than the mass of CO2, since they have to be equal (can't create nor destroy matter) in the reaction, you have to produce much more CO2 than gasoline used. Hope this helps.

Answer 4

In combustion reactions hydrocarbon reacts with oxygen from the air to give carbon di-oxide. The quantity of carbon di-oxide produced is dependent upon the number of carbon atoms in the hydrocarbon for example if say 10 carbon atoms are present in one molecule of the gasolene ,then it will produce 10 molecule of carbon di oxide by burning one molecule of the gasolene. For 8pounds gasolene approx. number of moles of 10 carbon HC is about :

8*453.6/142 =25.55 Gm moles;

Carbon dioxide produced = 10*25.55 Gm moles
=255.5 Gm moles;
which is equivalent to =255.5*44 Gm;

or = 255.5*44/453.6 =24.78 pounds;
Thus a gasolene of 10 carbon will produce this much gas .

Therefore,there is no magic in increase in quantity of carbon di oxide produced by combustion of gasolene.The basic reason is quantity of carbon present in the fuel which will burn and produce Carbon dioxide ..
For 10 carbon HC it is 90.9 % carbon ;
20carbon it is 85.2% carbon
so higher the number of carbon in the fuel higher will be Carbon di oxide generation.