bond - One doesn't discover new lands without consenting to...

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One doesn't discover new lands without consenting to lose sight of the shore for a very long time. - - A n d r é G i d e Bond Breaking and the Heat of Reaction All chemical reactions take place with either an absorption or release of energy. Generally this energy is in the form of heat, but in some processes it may take the form of mainly light, or a mixture of forms including some mechanical energy such as sound. Whatever the case, the conclusion we could draw from this aspect of chemical change is that energy is somehow stored in the chemicals and during a transformation there is sometimes excess energy (in which case it will be released) or sometimes not enough energy (in which case it will be absorbed from the surroundings). The change in energy is due mainly to bond rearrangement during a reaction. Whether there is a net absorption or release of energy depends on the number of bonds broken and formed as well as the strength of those bonds. For example, in the combustion of methane: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(g) we could imagine the reaction taking place by all of the reactant bonds breaking to make individual atoms and then these separated atoms later recombining to form the products. While it is unlikely that the actual chemical process happens in this way, theory and experiment show that the net energy difference between reactants and products is the same regardless of the pathway. Therefore, using bond energy values from your text book (p. 291) we would have: 4(C-H) + 2(O=O) = 4(414 kJ) + 2(499 kJ) = energy to break bonds (+) and 2(C=O) + 4(H-O) = 2(799 kJ) + 4(460 kJ) = energy to make bonds (-) The sum of these two values is -784 kJ--in other words, 784 kJ is released when one mole of methane is burned. The experimental heat of combustion is -803 kJ. So it is possible to estimate the energy change using a table of experimentally determined bond energies, but such estimates are limited by a number of factors. The most important limitation is the fact that the bond energies have been measured or calculated for gaseous species. In addition, the energy required to break several similar bonds in a molecule is not the same. For example, the energy needed to break one of the hydrogen- carbon bonds in methane is not the same as the energy needed to break the second, third, or fourth one (although they are similar). Calorimeter design based on Heat Effects , CHEM Study , 1960 In this experiment you will be looking at a series of hydrocarbons from the alkane family. Alkanes have the general formula C n H 2n+2 where n is an integer. Thus methane (CH 4 ) is the first member of the family. When you use your bunsen burner in the lab you are burning methane and the heat given off is the heat of combustion (or, more generally, the heat of reaction ). Quantifying how much methane is burned in a given amount of time is difficult. However, as the number of carbon atoms in alkanes increases, the compounds eventually become liquids at room temperature and the mass of a compound burned is more easily measured. Three liquid alkanes will be used in this experiment. They are hexane (C
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This note was uploaded on 10/27/2010 for the course CHEM 337 taught by Professor Kim during the Fall '10 term at CSU Sacramento.

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bond - One doesn't discover new lands without consenting to...

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