Chem_208_04_2007_Web

Chem_208_04_2007_Web - system and 125 J of heat escapes...

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“Discovery of Phosphorus”, by Hennig Brand 1669 PS1 Due Tomorrow, 2PM PS2 Distributed Today- Problems 1 and 2 require material to be covered on Tuesday. You can go ahead with Problem 3. Read to end of 7.8; 7.9 is optional
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Pressure-Volume Work W = -P ex V
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State Function (Depends only on state): Altitude (h) , Position (x,y), Path Dependent Function: Distance traveled (d) Internal Energy Change: U = U f – U i Internal Energy (U) Heat released or absorbed (q), Work done (w)
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Bomb: V = 0; w = 0
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Coffee-Cup Calorimeter Reaction occurs under the constant pressure of the atmosphere P is constant, so if gases are formed, V will change: U = q + w = q - P V
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Bomb: Const. V (w = 0) Open Cup: Const. P
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State Function (Depends only on state): Altitude (h) , Position (x,y), Path Dependent Function (depends upon path taken): Distance traveled (d) Internal Energy Change: U = U f – U i = q + w Internal Energy (U) Heat released or absorbed (q), Work done (w)
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Poll • In compressing a gas, 355 J of work is done on the
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Unformatted text preview: system, and 125 J of heat escapes. What is ∆ U for the system? a) -480 J b) -230 J c) +230 J d) -480 J ∆ U = q + w = -125 J +355 J = +230 J For reactions in open containers, we are interested in the heat of reaction at constant pressure (q p ): q p = ∆ U + P ∆ V Enthalpy (H) We introduce a new state function, enthalpy (H): H = U + PV Note that U, P and V are all state functions. At constant P: ∆ H = ∆ U + P ∆ V = q p Thus, the heat of reaction measured in an open container (q p ) is equal to the change in enthalpy ( ∆ H). Standard States and Standard Enthalpy Changes Standard State: P = 1 bar (10 5 Pa) Note that 1 atm. = 1.01325 bar T = 298.15K (25 o C), unless stated otherwise. The following are in their standard states: H 2 O (l), Hg (l), CO 2 (g), P(white), Fe(s), C(s,graphite) The standard enthalpy of reaction ( ∆ H o ) is the enthalpy change when reactants and products are in their standard states, unless otherwise noted....
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