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 -------------------------------------------------------------------- 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. -------------------------------------------------------------------- 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 | | | | | -------------------------------------------------------------------- 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? -------------------------------------------------------------------- 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 | | | | | -------------------------------------------------------------------- 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. -------------------------------------------------------------------- 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 | | | | | -------------------------------------------------------------------- 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? -------------------------------------------------------------------- Bibliography * American Chemical Society, "Chemistry in Context: Applying Chemistry to Society", Wm. C. Brown Publishers, 1994. Copyright 1994, Maryland Collaborative for Teacher Preparation