Chemistry 008 Handout #4Mrs. Harriet SmithlineSpring 2005Chapters 12 and 13 Review Guide Lecture 2/1 and 2/3Chemical Energetucs and Spontaneity of Chemical ProcessesI. Enthalpy of a Reaction (H)A. ∆Erxn= heat + expansion workB.Most reactions are at constant pressure (subscript p) ∆Erxn= qp+ wpC. Since w = -P∆V, ∆Erxn= qp- P∆V and rearranging gives qp= ∆Erxn + P∆VD. Enthalpy (H) is the measure of heat flow experienced by a system at constant pressure.qp= ∆H = ∆E + P∆V.E. For expansion and contraction work we are interested in the change in amount of gas inthe 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 thepathway, are of interest.II. Calculating EnthalpyA.From CalorimetryB.From Hess’ Law1.Add reactions of known ∆H values. Multiply or reverse to get appropriateamounts and to cancel unwanted substances Given:A + B →C ∆H = -xreverse reactionsC →A + B reverse ∆H∆
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