Chemistry questions
1. Here is a table of some fuels. The chemical formulas are characteristic values: coal is a complex mixture, but is high in carbon; oil, sugar, and wood are polymers, which means the molecules consist of many of the indicated building blocks. For example, gasoline is a mixture of molecules similar to heptane, which is seven (CH2) units stuck together (with two extra hydrogens): C7H16,
Type
Coal
Chemical formula Energy density
C 30 megaJoules/kg
Natural Gas CH4
Hydrogen H2
56 megaJoules/kg
142 MegaJoules/kg ethanol
Oil
Sugar
Wood
C2H6O
(CH2)
(C6H12O6)
(C6H12O6)
30 megaJoule/kg
45 megaJoules/kg
17 megaJoules/kg
15 megaJoules/kg
We note that the amount of energy obtainable from a kilogram of the fuel varies quite a bit. As part of an explanation, assume you have 1000 atomic units of each material, and calculate how many carbon atoms, how many hydrogen atoms, and how many oxygen atoms it contains, and how many carbon dioxide and water molecules will be produced when it is burned. As a first step, work out the atomic weight of each formula. In the table below, the first and last row have been worked out for you.
Formula At. Wt. C H O CO2 H2O Energy density
C 12 83 0 0 83 0 30 megaJoules/kg
CH4
H2
56 megaJoules/kg
142 MegaJoules/kg
C2H6O
(CH2)
(C6H12O6) 180
30 megaJoule/kg
45 megaJoules/kg
33 33 5.5 33 16.5 17 megaJoules/kg
How is the energy density related to the atomic composition? Which elements raise the energy density, and which lower it? (It might be worth learning how to use a spreadsheet program. Here is an Excel version of the table, to get you started).
2. Burning carbon-containing fuels is an important part of our energy economy. But this produces carbon dioxide, which (according to some people) might cause global warming. Each atom of carbon
(atomic weight 12) produces a molecule of carbon dioxide (molecular weight 45), so that a kilogram of carbon produces 45/12 = 3.75 kg of carbon dioxide. For each of the fuels in the table, work out how many kilograms of carbon dioxide are produced per megajoule. Obviously, using hydrogen would the best way to decrease carbon dioxide production; unfortunately, there are no hydrogen wells. How do the other fuels rank?

3. There is an active movement to ban the plastic bags that stores give out for free. There are many motivations for this. One of them is the claim that use of plastic bags increases our use of fossil fuels.
Given that a plastic bag has mass 6 grams and that it is a polymer of CH2, how much hot water could we make by burning one?