chem456_ch2lecture - First Law of Thermodynamics Internal...

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2-1 First Law of Thermodynamics Internal Energy Heat and Work Getting work from a heat engine – Thermodynamic cycles Our Strategy about heat and work, they get us started and they are measures of processes or paths. They teach us about state functions of Energy. We discovery entropy from them. Then we can bypass them except for measurement.
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2-1 U ( V,T ) , U ( P,T ) , or U ( P,V ), can take on a number of forms •Kinetic energy of molecules in system. •the potential energy of the constituents of the system relative to the center of mass of the system. For example, a crystal consisting of dipolar molecules will experience a change in its potential energy as an electric field is applied to the system. •the internal energy stored in the form of molecular vibration and rotation. the internal energy stored in the form of chemical bonds that can be released through a chemical reaction. Internal Energy (U)
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2-2 First Law of Thermodynamics The First Law of Thermodynamics is based on our experience that energy can neither be created nor destroyed, if both the system and the surroundings are taken into account. The internal energy is U or Usystem. 0 total system surroundings UU U Δ= Δ + Δ = system surroundings Δ Uq w Δ =+ U can be changed by either heat transfer or by work “flow”
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2-3 Work: any quantity of energy that “flows” across the boundary between the system and surroundings that can be used to change the height of a mass in the surroundings. •Work only appears during a change in state of the system and surroundings. Only energy, and not work, is associated with the initial and final states of the systems. piston piston cylinder cylinder mass mass mechanical stops Initial State Final State p i ,V i p f ,V f
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2-4 •Net effect of work is to change U in accordance with the First Law. If the only change in the surroundings is that a mass in the surroundings has been raised or lowered, work has flowed across the system between system and surroundings. •The quantity of work can be calculated from the change in potential energy of the mass, where g is the gravitational acceleration and h is the change in the height of the mass, m . •The sign convention for work is as follows. If the height of the mass in the surroundings is lowered, w is positive, and if the height is raised, w is negative. In short, w > 0 if Δ U > 0 . It is common usage to say that if w is positive, work is done on the system by the surroundings. If w is negative, work is done by the system on the surroundings. potential Ew F h m g h Δ == =
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2-5 How to calculate work •Expansion work •Electrical work external external w F dl P dAdl P dV =• = = ∫∫∫ G G electrical wq I t φ =⋅ mass Initial State Final State p i ,V i p i ,V f electrical generator +- H 2 O H 2 O 2 mass electrical generator H 2 O H 2 O 2 mass mass Is the electrical potential, usually in Volts.
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2-6 EP2.1a) Calculate the work involved in expanding 20.0 L of an ideal gas to a final volume of 85.0 L against a constant external pressure of 2.50 bar .
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This note was uploaded on 02/21/2010 for the course CHEM 11773 taught by Professor Robinson during the Spring '10 term at University of Washington.

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chem456_ch2lecture - First Law of Thermodynamics Internal...

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