ln24f15 - LECTURE 24 ENTROPY THE TRUTH BEHIND SPONTANEITY...

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L ECTURE 24: E NTROPY T HE T RUTH B EHIND S PONTANEITY We have defined spontaneity through G: If Δ G = (–) Reaction is spontaneous If Δ G = (+) Reaction is non-spontaneous And we have seen that reactions can occur: Whether a reaction is endothermic or exothermic. Whether the entropy of a system increases or decreases. Clearly something more profound is at work here than just the first law’s concept of energy conservation. So what’s the deeper insight? The second law of thermodynamics says that: “A reaction is spontaneous if the entropy of the isolated system increases.” (Note: in most examples, “the isolated system” is the universe .) We’re already familiar with seeing physical processes gradually increase in disorder: Food coloring distributes itself throughout a beaker of water. A hot block of metal cools to room temperature. But we are less accustomed to understanding this in chemical processes, especially when considering reactions in which the system’s energy increases (endothermic), or ones in which its entropy decreases (the system gets more ordered). Like, why do we exist? The answer rests in a deeper appreciation of entropy on a global level. But first, we need a quantitative measure of entropy.
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E NTROPY D EFINED Q
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