Lec03_drobny_11 - Lecture 3:Enthalpy Reading Zumdahl 9.2...

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Lecture 3:Enthalpy • Reading: Zumdahl 9.2, 9.3 Recommended Problems: 9.11,9.23-25, 9.27, 9.29 • Outline – Definition of Heat Capacity (C V ) – Definition of Enthalpy ( Δ H and C P ) – Calculating Δ E and Δ H using C V and C P
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Facts: Heat Transfer at Constant P Many (most!) chemical reactions occur at constant pressure (P), not constant volume V. In most chemical reactions of interest, volume changes and heat is transferred at constant pressure P. • Now Δ E=nC v Δ T=q-P Δ V for ideal gases. • If Δ V=0, Δ E=q v . So q=nC v Δ T only for heat transfer at constant volume. We need a new state function to express heat transfer at constant P!
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Introducing Enthalpy Thermodynamic Definition of Enthalpy (H): H = E + PV E = energy of the system P = pressure of the system V = volume of the system
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Definition of Enthalpy (cont.) Consider a process carried out at constant pressure. If work is of the form -P Δ V, then: Δ E = q P + w = q P - P Δ V Δ E + P Δ V = q P q P is heat transferred at constant pressure.
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Definition of Enthalpy (cont.)
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