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
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 44), so that a kilogram of carbon
produces 44/12 = 3.66 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?
To answer this, you first have to work out the reaction of the fuel with oxygen. For example, burning
wood looks like this
C6H12O6 + 12O = 6CO2 + 6 H2O (The number of C’s, H’s, and O’s are the same on each side)
The molecular weight of C6H12O6 is 6*12 + 12*1 + 6*16 = 180 grams/mole, and burning it produces 6
moles of CO2 = 6*44 g/(mole of wood) = 264 g/mole, so we get 264/180 = 1.47 kg of CO2 per kg of
wood. Since the kg of wood is worth 15 MJ, we get 1.47/15 MJ = 0.098 kg of CO2 per MJ.
Your turn! Fill in the table below.
©J. P. Straley
ChemistryQdoc
1
Formula
At. Wt.
Number
of C atoms
C
12
1
g/mole
CH4
H2
C2H6O
(CH2)
(C6H12O6) 180
6
g/mole
Grams
of CO2
CO2 per
kg
Energy density
(per kilogram)
CO2 per MJ
44
3.66
30 megaJoules/kg
0.122 kg/MJ
1.47
56 megaJoules/kg
142 MegaJoules/kg
30 megaJoule/kg
45 megaJoules/kg
17 megaJoules/kg
0.098 kg/MJ
264
2. The amount of energy obtainable from a kilogram of the fuel varies quite a bit. As part of an
explanation, assume you have a mole of each material, and calculate how many carbon atoms, how
many hydrogen atoms, and how many oxygen atoms it contains (measured in moles. A mole of
hydrogen atoms is one gram, a mole of carbon atoms is 12 grams, and a mole of oxygen atoms is 16
grams), and how many moles of carbon dioxide and water molecules will be produced when it is burned.
Multiplying the energy density (MJ/kg) by the molecular weight (kg/mole) tells you the energy per mole,
which is proportional to the energy per molecule.
Formula
At. Wt. C
g/mole
12
1
C
CH4
H2
C2H6O
(CH2)
(C6H12O6) 180
6
H
O
0
0
12 6
CO2 H2O Energy density
(per kg)
1
0
30 megaJoules/kg
56 megaJoules/kg
142 MegaJoules/kg
30 megaJoule/kg
45 megaJoules/kg
6
6
17 megaJoules/kg
Energy density
(per mole)
0.36 MJ/mole
3.06 MJ/mole
Now the puzzle is to figure out how is the energy density is related to the atomic composition. Which
elements raise the energy per molecule, and which lower it? Can we write a formula
Energy density per mole = A*(# of C atoms) + B*(# of H atoms) + C*(# of O atoms) ?
©J. P. Straley
ChemistryQdoc
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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? You will need to know that it takes 4186 J to raise the temperature of 1 kg of
water by 1 Celsius degree; hot water is 50 C, some 40 C warmer than water from the water company.
4. Hydrogen and methane are gases, which makes it hard to store them in a small space. Gasoline and
other forms of oil have high energy density, and this goes a long way to explaining why they are the
fuels for transportation. But a car on the moon would have to carry along its oxygen, too. How many
kg of oxygen do we need per kg of gasoline? What is the energy density (MJ per kg) when we include
the mass of the oxygen?
5. Plants store energy from sunlight in the form of sugars (C6H12O6), perhaps polymerized into
cellulose (wood) or other carbohydrates. After being buried for a long time under high pressure and
temperature, they turn into coal, oil, and gas. The oxygen atoms end up either as CO2 or H2O. Write a
reaction that will turn sugar into pure carbon, and a reaction that will turn sugar into methane. Show
that in the process, the fuel value decreases (this has to be true, or energy would be created out of
nothing).
©J. P. Straley
ChemistryQdoc
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