110112 - First Law of Thermodynamics(Chapter 3 p 39-49 1st...

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1 CHM3400 - Lecture 4 Jan 12 First Law of Thermodynamics (Chapter 3 p. 39-49) 1 st Law of Thermodynamics State functions vs. path functions Enthalpy
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2 Energy Forms of energy : 2 mc E = Mass (defect) energy: 2 2 1 mv E kin = Kinetic energy: Gravitational energy: mgh E = T k E B thermal 2 3 = Thermal energy: Photon energy: ν h E = Units?
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3 First Law of Thermodynamics “Energy cannot be created or destroyed, it can only be converted from one form to another.” Law of conservation of energy : “The total energy of a closed system is conserved.” “The total energy of the Universe is conserved.” ( ) More difficult to prove
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4 Energy transfer: w and q Transfer of energy can appear in two forms: Either through work ( w ) or heat ( q ) By convention, if : w,q = 0 : no energy transfer w,q > 0 : heat absorbed by the system ( endothermic reaction), work done on the system by the surroundings w,q < 0 : heat emitted by the system ( exothermic reaction), work done by the system Heat transfer = temperature change Work = ………………. . e.g. mechanical work here: (gas) expansion
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5 Isothermal expansion A gas at initial state ( P 1 , V 1 , T ) is allowed to expand to a final state ( P 2 , V 2 , T ) while lifting a weight (mass = m ) over a distance Δ h . Δ h What is the work performed by the system? Actually very confusing example! Air???
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6 Work performed for weighted piston The system is the gas that performs the work The environment is the weight on which the work is performed h mg h F env Δ = Δ = ϖ h mg env sys Δ = = w sys < 0: what does that mean?
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110112 - First Law of Thermodynamics(Chapter 3 p 39-49 1st...

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