Studio_7_fuels_FALL_2006_FINAL - Chem. 25: Studio #7...

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Thermochemistry: Fuels NAME:____________________________ STUDIO:_______________________ Thermochemistry: The Relative Energy Content of Fuels Material for this studio comes from Chapter 7: Sections 7:3, 7:6, 7:8 and 7:9 in the text and the data in AppendixD2: Thermodynamic Properties of Substances. In addition we’ll pull into this discussion the information of bond energies and chemical reactions in Chapter 10: Section 10.9. A fuel is anything that is burned to produce heat. When this process takes place in a furnace or an automobile engine, it is called combustion; when our bodies burn food to produce energy, we call it metabolism. The net process is the same in both cases: materials react with oxygen and release energy. In this laboratory, we concentrate on the combustion of simple organic molecules that contain carbon, hydrogen and perhaps oxygen, too. The complete combustion of such molecules produces carbon dioxide and water as products in addition to energy. For the purposes of this laboratory, we assume the combustion of the fuels we are evaluating is complete. You know from your own experience that combustion is frequently incomplete and results in the formation of products like carbon monoxide and soot (carbon). Incomplete combustion frequently arises because of a limited supply oxygen. Most fires in buildings, for example, produce abundant of soot and other byproducts because sufficient oxygen is unavailable to sustain the combustion. In describing the combustion of gasoline in an engine, the terms ‘lean’ and ‘rich’ are used to describe the relative amounts of fuel and oxygen present. A lean fuel condition exists when a relative excess of oxygen over fuel exists; chemists say oxygen is in excess and the fuel is the limiting reagent. This situation is associated with complete combustion characterized by the production of only CO 2 and H 2 O. A rich fuel condition is the opposite; fuel is in excess and oxygen is the limiting reagent. This situation results in incomplete combustion; carbon monoxide, soot and other products are formed. We can use some of the concepts we’ve developed to understand why combustion reactions release the tremendous amounts of energy we associate with the fuels that heat our buildings and power our cars. Remember that a reaction is a making and breaking of bonds. Section 10:9 in the text describes a way we can use bond energies to estimate the heats of reaction. In essence, you simply calculate all the energy in all the bonds in molecules on the reactant side of an equation (you can think of all these bonds as being broken) and subtract from it the amount of energy in all the bonds on the product side (these are all the new bonds being formed). Said symbolically -- H rxn = Σ BE (reactants) – Σ BE (products) where BE is the bond energy per mole of bonds; BE always has a positive sign. 1
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This note was uploaded on 02/27/2008 for the course CHEM 025 taught by Professor X during the Fall '06 term at Lehigh University .

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Studio_7_fuels_FALL_2006_FINAL - Chem. 25: Studio #7...

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