110114 - Heat Capacity and gas expansions(Chapter 3 p 49-59...

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1 CHM3400 - Lecture 5 Jan 14 Heat Capacity and gas expansions (Chapter 3 p. 49-59) • More on enthalpy • Heat capacity • Molecular basis of heat capacity • Reversible vs. irreversible expansions/contractions • Isothermal vs. adiabatic expansion/contractions
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2 Last lecture’s example We want to cool an ideal gas from its initial state (T i , V i , P i , n) to absolute zero Kelvin. We either do this by leaving the volume or the pressure constant.
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3 Constant volume vs. pressure Case 1 : constant volume i nRT U 2 3 = 0 = = V P ext sys ϖ i V nRT U U q 2 3 = = = Case 2 : constant pressure i nRT U 2 3 = i i ext sys V P V P = = i i i P nRT V P U q + = = 1 2 3 The heat transferred from the gas to the cold reservoir is larger in the constant pressure case, due to the work performed by the external pressure . H V P U q i i P = = enthalpy
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4 Summing up Case 1 : constant volume U q V = Case 2 : constant pressure H V P U q i i P = = Path function State function State function Path function How can it be that a path function (q) can be equal to a state function (U or H)? The key to fix the path ( constant pressure or constant volume ), such that the value of q is equal to either U or H.
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5 Internal energy U vs. enthalpy H U describes the internal energy of the system in terms of the kinetic and potential energy of the particles H = U + PV Enthalpy H is primarily internal energy plus a small work component that has to do with the expansion of the system U = n 3/2 RT
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This note was uploaded on 05/29/2011 for the course CHM 3400 taught by Professor Seabra during the Spring '08 term at University of Florida.

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110114 - Heat Capacity and gas expansions(Chapter 3 p 49-59...

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