L5A Free Energy and Chemical Thermodynamics.pdf

L5A Free Energy and Chemical Thermodynamics.pdf - Chapter...

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Chapter 5A Free Energy and Chemical Thermodynamics PHYS 4050 Thermodynamics and Statistical Physics 1
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Outline of this Chapter 1. Enthalpy 2. Helmholtz Free Energy and Gibbs Free Energy 3. Free Energy as Available Work 4. Free Energy and Equilibrium 5. Phase Transformations PHYS 4050 Thermodynamics and Statistical Physics 2
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Microcanonical Ensemble By far we have studied isolated system , from which we have developed the concepts of internal energy and entropy . And by maximizing the total entropy in the interactions of two small systems (energy/volume/particle exchanges), we have (re)defined temperature, pressure and chemical potential. The collection of the isolated systems in different microstates with the specified values of ( ?, ?, ? ) is called the microcanonical ensemble . PHYS 4050 Thermodynamics and Statistical Physics 3
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Interaction with Environment The analysis of the interaction of a system with the environment (much larger system, e.g. the atmosphere) is somehow different. We may still maximize the total entropy of the system and the environment according to the second law. However, the change to the environment is extremely small so that its temperature, pressure and chemical potential are almost unchanged by the system. PHYS 4050 Thermodynamics and Statistical Physics 4
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Thermodynamic Potentials Therefore in equilibrium, the thermodynamic variables (i.e. ? , ? , 𝜇 ) of the system are the same as the environment. We may define some new energy functions called thermodynamic potentials (includes internal energy) based on this property. These new energy functions are equivalent to the internal energy, except that they will be easier to handle in different situations. PHYS 4050 Thermodynamics and Statistical Physics 5
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Enthalpy The enthalpy is defined as: PHYS 4050 Thermodynamics and Statistical Physics 6 ? ≡ ? + ?? The term ?? is the work needed to make room for an object in a constant pressure environment.
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Helmholtz Free Energy Similarly, for constant temperature environments: PHYS 4050 Thermodynamics and Statistical Physics 7 ? ≡ ? − ?? To create a system in a constant temperature environment, we need not provide the entire energy ? . Since heat energy ?? flows in automatically from the environment, we only need to provide the Helmholtz free energy :
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