lecturetopic2

lecturetopic2 - Our Progress in the Course 2) The 1st Law...

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Our Progress in the Course 2) The 1st Law of Thermodynamics — Conservation of Energy. a. Terminology, Definitions, and Open and Closed Systems. b. Heat and Work; Energy and Enthalpy c. Enthalpy, and how to keep track of Breaking and Forming Bonds. d. Combustion and Metabolism: mechanical work from Chemical Bonds. READ CHAPTER 4 Our future goals: Entropy and The Second Law of Thermodynamics .
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It matters greatly whether or not MASS can leave or enter the system, or whether HEAT can enter or leave the system Terminology, Definitions, and Open and Closed Systems OPEN systems allow CLOSED systems ISOLATED systems MASS and ENERGY allow only ENERGY allow NO flow (eg: not flow (eg: most cells) (some membranes) much, unfortunately)
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Diathermic vs. Adiabatic We also need to learn the terminology for the walls of the system : DIATHERMIC—allows energy to leave the system as heat. ADIABATIC—does NOT allow energy to leave the system as heat So: a system with no walls is OPEN a system with diathermic walls is CLOSED a system with adiabatic walls is ISOLATED
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Heat as Energy A chemical reaction in this test tube can release 4200 J of energy. What is the temperature change of the 100 g water (all liquid) surrounding the test tube? Specific heat of water is ~ 4.2 J g -1 K -1 -1 1 ! " " " " q = m x s x Δ T Δ T = 10 K ( Δ T K)
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ADIABATIC container: How can energy be transferred to the surroundings ? The container can expand, and a weight could be raised to increase its potential energy (work = mgh). So, the system can transfer energy to the surroundings with WORK Work as Energy
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We can still think of these effects in terms of collisions of molecules: When we transfer HEAT When we transfer WORK the molecules move in the molecules move in different directions the same direction
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We can still think of these effects in terms of collisions of molecules: Work is the transfer of energy that makes use of organized motion Heat is the transfer of energy that makes use of chaotic molecular motion
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Definition: + and – of heat and work Work Heat + done on the system + adding to the system - done by system to surr. - adding to the surr.
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Expansion/compression work in an ideal gas this work can also be expressed as an expansion against const P work = opposing force x d = -(P ext x A) x d = - P ext x A x h = - P ext Δ V (units of Pa m 3 ) we now have some more units for work: 1 J = 1 Pa m 3 d work = - P ext dV
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expansion work done by the system = - P ext Δ V Equals the area under the Force-Distance curve: work = - P ext dV Chang eq. 4.3 w = - P EX dV Expansion against constant Pressure
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How much work do we do in exhaling 0.5 L of air from our lungs against 1 atm (101 kPa). Question
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Free Expansion (no Pressure) expansion work done by the system = - P ext Δ V = 0
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A: maximum work can be done when P ext = P . We can do this by ‘matching’ the pressures as we expand = VARIABLE external pressure A REVERSIBLE system is balanced exactly between expansion and compression (it is in a mechanical equilibrium with elastic collisions) Isothermal Reversible Expansion
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work = - P ext dV
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work = - P ext dV
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work = - P ext dV
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lecturetopic2 - Our Progress in the Course 2) The 1st Law...

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