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hand04 - Chemistry 008 Handout#4 Mrs Harriet Smithline...

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Chemistry 008 Handout #4 Mrs. Harriet Smithline Spring 2005 Chapters 12 and 13 Review Guide Lecture 2/1 and 2/3 Chemical Energetucs and Spontaneity of Chemical Processes I. Enthalpy of a Reaction (H) A. E rxn = heat + expansion work B. Most reactions are at constant pressure (subscript p) E rxn = q p + w p C. Since w = -P V, E rxn = q p - P V and rearranging gives q p = E rxn + P V D. Enthalpy (H) is the measure of heat flow experienced by a system at constant pressure. q p = H = E + P V. E. For expansion and contraction work we are interested in the change in amount of gas in the system; from the ideal gas law PV = nRT and P V = n (RT) F. Enthalpy and energy are state functions. Only the initial and final states, but not the pathway, are of interest. II. Calculating Enthalpy A. From Calorimetry B. From Hess’ Law 1.Add reactions of known H values. Multiply or reverse to get appropriate amounts and to cancel unwanted substances Given: A + B C H = -x reverse reactions C A + B reverse H
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