6 Intro to Entropy

# 6 Intro to Entropy - The Second Law The Concepts Chapter 4...

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The Second Law: The Concepts Sections 4.1 and 4.2, both 6th & 7th editions Sections 3.1 and 3.2, 8th edition Direction of Spontaneous Change Dispersal of Energy Entropy Thermodynamic defintion Entropy as a State Function Thermodynamic Temperature (skip) The Clausius Inequality Chapter 4 of Atkins: The Second Law: The Concepts Last updated: Minor update, slide 1; added slides 25-29 (taken from Lecture 11)

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First vs. Second Law First Law of Thermodynamics : The energy of the universe is constant - energy is conserved # This law tells us nothing about the spontaneity of physical and chemical transformations Consider the formation of water: H 2 (g) + ½O 2 (g) 6 H 2 O(l) ) f H o = -286 kJ mol -1 Using the first law, it is easy to calculate ) U and ) H associated with this spontaneous reaction - we can also calculate ) U and ) H for the reverse reaction, which we know does not occur spontaneously (thankfully!!) Neither the sign nor the magnitude of ) U and ) H tell us which way the reaction will go - however, we can get this information from the Second Law of Thermodynamics What about gases? We know that gases expand spontaneously to fill a container - we know that the opposite does not happen - unless some sort of work is done to bring about this reverse change
First vs. Second Law The first law gives us no clue what processes will actually occur and which ones will not. For that matter, why does anything ever happen at all? The universe is an isolated system after all. There is no change in internal energy. There is no heat transferred in or out and no work is done on or by the system as a whole. q =0; w =0; ) U =0. Without only the first law, and without the second law: The universe is a boring place Yet, the universe is not a boring place: # Stars come into existence and blow up into oblivion # Planets are created and hurl around stars # Life evolves amid all of this turmoil on these planets Why? What is driving all of this? These processes do not lead to a lower energy for the universe as a whole. So what is going on?

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Statements of the Second Law Elements in a closed system tend to seek their most probable distribution; in a closed system entropy always increases. 1. Clausius (1822-1888) It is impossible that, at the end of a cycle of changes, heat has been transferred from a colder to a hotter body without at the same time converting a certain amount of work into heat. 2. Lord Kelvin (1824-1907) In a cycle of processes, it is impossible to transfer heat from a heat reservoir and convert it all into work, without at the same time transferring a certain amount of heat from a hotter to a colder body. 3. Ludwig Boltzmann (1844-1906) For an adiabatically enclosed system, the entropy can never decrease. Therefore, a high level of organization is very improbable. 4. Max Planck (1858-1947) A perpetual motion machine of the second kind is impossible.
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