3b - The Enthalpy Properties H=U+PV The enthalpy is a state...

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1 The Enthalpy H=U+PV Properties • The enthalpy is a state function - since its constituents U, P and V are. • The enthalpy is an extensive property ( ) – because U ad V are • H > U – since PV>0. • For a reversible process at constant pressure ( ), in a system that only allows expansion work, the enthalpy change is equal to the heat exchange: • The enthalpy change upon heating a material at constant pressure: ex P P = ( ) f if fi ifi f i HH HU P VUP VU UP VV UPV Uwq ∆= − = + − − = − + =∆ + ∆ =∆ − = T p qH c = ∆=∆ / n = Endothermic process: 0 ; exothermic process: 0 ∆> ∆<
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2 ¾ Thermochemistry is all about calculating the enthalpy changes involved in chemical processes . ¾ These enthalpy changes correspond to the heat input/output of the process, provided that the internal pressure remains constant and equal to the external pressure (since pressure equilibration is usually a rapid process, this is often an excellent assumption). ¾ Enthalpy is a state function. We can therefore calculate the enthalpy change along the path of our choice (Hess law) . 12 ; ; ; () ; BB AA A A A II B I A B I B I B I BH H HH H AH H B H H H H B H A H A →→ →→→→ →∆= =−= = + =−+− →→→ = " " Thermochemistry- General strategy A H B H 2 I H 3 I H 1 I H All the paths correspond to the same H
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3 •The enthalpy of an ideal monoatomic gas: 35 22 H U PV nRT nRT nRT =+ = + = Enthalpy: Gases vs. condensed phases
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This note was uploaded on 06/13/2009 for the course CHEM 260 taught by Professor Staff during the Spring '08 term at University of Michigan.

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3b - The Enthalpy Properties H=U+PV The enthalpy is a state...

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