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Lecture13A - Welcome to Entropy(20.1-3 1st Law tells us...

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Lecture 13 1 Welcome to Entropy (20.1-3) 1 st Law tells us whether energy or enthalpy is released or absorbed in a chemical process. It does not tell us whether a process will occur spontaneously. Well, what does make a process spontaneous? 1 st thought: energy (or enthalpy) – based, i.e. exothermic process would seem reasonable. Products have lower energy (enthalpy) than reactants. 2 nd thought: what would we expect based on mechanical systems? Balls run downhill, springs unwind, effects of gravity. All tend to minimize energy. 1) Consider expansion of a gas against P ext =0 in an isolated system. P ext =0 so w=0. Isolated so q=0. Therefore: ΔU=0=ΔH. Yet the gas will spontaneously expand (and the opposite doesn’t happen – i.e. gas will not spontaneously occupy less than the full V of the system!)
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Lecture 13 2 2) Mixing two gases; again ΔU=0=ΔH but gases will mix. Opposite process does not occur spontaneously (N 2 and O 2 don’t spontaneously separate!) 3) Phase changes: solid liquid (at T>T m ), liquid gas (at T>T b ). Both are spontaneous, both are endothermic!
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