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Thermodynamics lecture Kotz

Thermodynamics lecture Kotz - Thermodynamics Entropy and...

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1 Terminology: Spontaneity : The notion of whether or not a process can take place unassisted. (E.g. Rock rolling down hill; iron rusting; heat flow from hot to cold) Entropy : A measure of how energy and matter is spread out (dispersed) among the atoms and molecules of a system and its surroundings. Free Energy : A thermodynamic function that relates enthalpy and entropy to spontaneity. Thermodynamics: Entropy and Free Energy Although heat (q) is a consideration in spontaneity, it is not the only factor in its determination. Dispersal of Energy Statistical analysis of energy distribution of two quantized packets of energy Beige = Quantum of energy; Blue = No energy. Note that energy is more likely to be distributed over multiple particles (6 of 10) than concentrated on one . Energy distribution of 4 particles with a total of 6 quantized units of energy. Most probable configuration is energy distributed over a large number of particles over a large number of states. As the number of particles and the number of energy levels grows, one arrangement turns out to be vastly more probable than the others. Examples of Energy Dispersal - Heat energy in a coffee cup being distributed to the surroundings - A rock rolling down a hill distributing its energy to the air and hillside Matter Dispersal Probability of finding n molecules in the original flask when the valve is open is given by (1/2) n
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2 Matter Dispersal and Energy Dispersal Related to quantum mechanics For larger n numbers (principal quantum number) - Average radius increases - Energy levels get closer together Can be applied to macroscopic systems: - n is related to the size of the system For a vessel holding a sample of gas: - Increase in size corresponds to an increase in n number - Energy levels get closer together - Since available energy has not changed, the number of ways of distributing the total energy increases When matter is dispersed into a larger volume, energy is dispersed over more energy levels. For gases at room temperature, the entropy-driven dispersal of matter is equivalent to an increase in disorder of the system. While dispersal of gases always increases disorder, the same is not always true for solutions Increase in disorder in the distribution of potassium permanganate Decrease in disorder in the solvation of lithium hydroxide Ludwig Boltzmann S = k log W S = Entropy of system k = Boltzmann’s constant W = Number or ways energy can be distributed over the available energy levels Entropy Thermodynamics: Thermodynamics: -Determines if a process is possible and spontaneous and the equilibrium state of the system (how far the reaction will proceed) -Kinetics determines the rate of the process. Review of Thermochemistry For Pressure-Volume Work Δ E = q + P Δ V Δ E is the internal energy change of the system q p = Δ E - P Δ V (at constant pressure) q p = Enthalpy change = Δ H (negative = exothermic; positive = endothermic) Laws of Thermodynamics: 1 st Law: The energy of the universe is constant (conservation law) 2 nd Law: The entropy of the universe is always increasing 3
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