notes_Lecture_02_100410

notes_Lecture_02_100410 - Lecture 2 Enthalpy Reading...

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Lecture 2: Enthalpy • Reading: Zumdahl 9.2, 9.3 • Outline – Define a new thermo variable: enthalpy ( H ), the heat transferred at constant pressure – Define the manner in which substances “store” heat energy: heat capacity ( C V , C P ) – Mathematical form for Δ E and Δ H using C V and C P
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Heat and Work Sign Conventions • If system releases heat q < 0 (q is negative) • If system absorbs heat q > 0 (q is positive) • If system does work w < 0 (w is negative) • If work done on system w > 0 (w is positive) Δ E = q + w
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Δ E = q + w If system loses energy: E final < E initial E final -E initial = Δ E < 0. If system gains energy: E final > E initial E final -E initial = Δ E > 0. E n e rg y E n e rg y E initial E final Δ E > 0 E in E initial E final Δ E < 0 E out
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We would like to know about the internal energy change accompanying chemical processes: Consider a process carried out at constant P: The work term at constant pressure represents the energy required to change the volume of the system…this is “PV-work.” The remainder of the internal energy is transferred as heat energy. This heat energy is given subscript “P” to indicate that it is transferred at constant pressure. Measuring the heat flow at const P gives you the internal energy minus PV-work.
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