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. Sticking
more units together changes the freezing and boiling
temperatures for the compound (and its tendency to evaporate),
but so far as the energy content of a gallon or kilogram is
concerned, they are all pretty much the same: “oil.”
Type
Coal
Natural Gas
Hydrogen
ethanol
Oil
Sugar
Wood
Chemical formula
C
CH4
H2
C2H6O
(CH2)
(C6H12O6)
(C6H12O6)
Energy density
30 megaJoules/kg
56 megaJoules/kg
142 MegaJoules/kg
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
C
12
83 0 0
83
0
CH4
H2
C2H6O
(CH2)
(C6H12O6) 180
33 33 5.5 33
16.5
Energy density
30 megaJoules/kg
56 megaJoules/kg
142 MegaJoules/kg
30 megaJoule/kg
45 megaJoules/kg
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
©J. P. Straley
ChemistryQdocx
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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?
©J. P. Straley
ChemistryQdocx
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