MCTP
Maryland Collaborative
for Teacher Preparation
COUNTING BONDS AND CALORIES:
A Molecular View of Reaction Energy
Thomas C. O'Haver
Department of Chemistry and Biochemistry
University of Maryland
College Park, MD 20742
(301) 4051831
to2@umail.umd.edu
NSF Cooperative Agreement No. DUE 9255745
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Chemistry 121/122 Name________________________________
Fall, 1994
Chapter 4 Class Work Partner ______________________________
COUNTING BONDS AND CALORIES:
A Molecular View of Reaction Energy
This is not a quiz, but rather a class exercise. The papers will
be collected and graded. You may talk to your classmates and you
may refer to your textbook, but you must write your answers to each
question in your own words. (Don't just copy things right out of
the textbook).
1. Look at the table of "bond energies" on page 99*.
a. Which are the two strongest bonds on this table?
b. Which are the two weakest bonds on this table?
c. The complete combustion of hydrocarbons (compounds of carbon and
hydrogen, such as methane and octane) in air produces carbon
dioxide and water. Show that the bond energies in this table would
generally lead you to expect that such reactions would be
exothermic. (Hint: what the energies of the bonds in hydrocarbons
and in oxygen, compared to those of the bonds in water and carbon
dioxide?)
d. Figure 4.10 (page 104) show that the energy released by burning
(the "heat of combustion") of carbon to carbon dioxide is greater
than that of carbon to carbon monoxide. How can this be, when
table 4.2 says that the CO TRIPLE bond energy is greater (1072
KJoules/mole) that the CO DOUBLE bond energy (799 KJoules/mole)?
2. a. Here is a partial table of bond energies, showing how to compute
the heat of reaction for the synthesis of water
2H2 + O2 --> 2H2O
which can we viewed as
H-H + O=O --> H-O-H
H-H H-O-H
to show the bonds more clearly.
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Bond | Bond energy| Number of | Energy | Number of | Energy |
| (KJ/mole) | bonds broken| required| bonds formed| released |
-----+------------+-------------+---------+-------------+-----------
H-H | 432 | 2 | 864 | | |
H-C | 411 | | | | |
C-C | 346 | | | | |
H-O | 459 | | | 4 | 1836 |
C-O | 359 | | | | |
C=O | 799 | 1 | 494 | | |
O=O | 494 | | | | |
N#N | 942 | | | | |
C-N | 305 | | | | |
N-O | 201 | | | | |
O-O | 142 | | | | |
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Total | 1358 | | 1836 |
(KJ/mole) ----------- ------------
Since the energy released by forming bonds is greater than the energy
required to break bonds, the reaction is predicted to be EXOTHERMIC
(energy producing). The "heat of reaction" is conventionally
calculated as:
Heat of Reaction = Energy required - Energy released
so that the heat of reaction turns out to negative for an exothermic
reaction. In this case the Heat of Reaction = 1358 - 1836 = _______
b. Using the table below as a worksheet, compute the heat of
combustion (KJoules/mole) of ethyl alcohol (C2H5OH), whose structure
is shown on page 100. How does this compare to the heat of
combustion of propane, also given on page 100?
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Bond | Bond energy| Number of | Energy | Number of | Energy |
| (KJ/mole) | bonds broken| required| bonds formed| released |
-----+------------+-------------+---------+-------------+-----------
H-H | 432 | | | | |
H-C | 411 | | | | |
C-C | 346 | | | | |
H-O | 459 | | | | |
C-O | 359 | | | | |
C=O | 799 | | | | |
O=O | 494 | | | | |
N#N | 942 | | | | |
C-N | 305 | | | | |
N-O | 201 | | | | |
O-O | 142 | | | | |
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Total | | | |
(KJ/mole) ----------- ------------
c. Do you think it is likely that someone could invent an engine
that would burn nitrogen (N2) as a fuel? Explain on the basis on
bond energies in the above table?
3. a. The reaction for the combustion of glucose (a type of sugar)
is given on page 102. The structural formula for glucose is given
on page 335 (upper left corner of the blue box). Use this
information and the table below as a worksheet to compute the heat
of combustion of glucose in KJoules/mole and compare to the
measured value given on page 102.
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Bond | Bond energy| Number of | Energy | Number of | Energy |
| (KJ/mole) | bonds broken| required| bonds formed| released |
-----+------------+-------------+---------+-------------+-----------
H-H | 432 | | | | |
H-C | 411 | | | | |
C-C | 346 | | | | |
H-O | 459 | | | | |
C-O | 359 | | | | |
C=O | 799 | | | | |
O=O | 494 | | | | |
N#N | 942 | | | | |
C-N | 305 | | | | |
N-O | 201 | | | | |
O-O | 142 | | | | |
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Total | | | |
(KJ/mole) ----------- ------------
b. Convert the heat of combustion of glucose into KJoules per
gram. You will need to compute the molecular weight (grams/mole)
of glucose; the atomic weights are: H =1 gram/mole; C = 12
gram/mole; O - 16 gram/mole.
c. Chemically, wood consists of organic compounds which are long
chains of simple sugar units like glucose. Therefore wood is
expected to contain many of the same chemical bonds as glucose.
The "heat content" of wood, in KJoules per gram, is given in Table
4.3. How does this compare to your calculated heat of combustion
of glucose in KJoules per gram?
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Bibliography
* American Chemical Society, "Chemistry in Context: Applying
Chemistry to Society", Wm. C. Brown Publishers, 1994.
Copyright 1994, Maryland Collaborative for Teacher Preparation