PC1Set3 - 3. The Second Law Motivation of the Second Law :...

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Unformatted text preview: 3. The Second Law Motivation of the Second Law : In the following table, the first column shows a particular state A of the sys- tem, as defined by its temperature, volume and com- position, and the second column shows another state B, characterized by different values of some or all of these variables. Table 1 (All systems are in an adiabatic enclosure) State A State B Two equal blocks of cop- per are connected by a wire. One block is at 20 C and the other at 30 C . The blocks are each at 25 C . A dilute gas at a tem- perature T occupies one half of a vessel and the other half is vacuum. The gas at the same temperature T occupies the whole of the vessel 1 K. Denbigh: The Principles of Chemical Equilibrium (Cambridge UP, Cam- bridge 1966) PChem I 3.1 A dilute gas X occupies one half of a vessel and a dilute gas Y occupies the other half. The tem- perature is T . The gases are uniformly mixed in the vessel and the temperature has the same value T . A vessel contains hydro- gen and oxygen and a catalyst. The volume is V and the temperature is T . The vessel contains the same amount of hydro- gen and oxygen, com- bined as water, togeth- er with the catalyst. The volume is V and the temperature exceeds T by an amount corre- sponding to the heat of reaction. Question: Are the processes A- B and B- A al- lowed? Answer: Energy is conserved for all A- B and B- A processes. The processes are allowed by the First Law. PChem I 3.2 Question: Are the processes A- B and B- A ob- served? Answer: Only the processes A- B are observed to occur. = spontaneous direction Spontaneity or nonspontaneity of the processes A- B and B- A depends entirely on the nature of the states A and B. = We expect that there exists a state function, S , that characterizes the spontaneous direction of a pro- cess. Second Law of Thermodynamics : Heat flows sponta- neously from a hot object to a cold object. Equivalent form: No process is possible in which the sole result is the absorption of heat from a reservoir and its com plete conversion into work. need mathematical form parallel to First Law PChem I 3.3 First Law (permissible change) 1) There exists a state function U 2) d U = d q + d w Second Law (spontaneous change) ( 1 ) There exists a state function S , the entropy , for the system. ( 2 ) If the state of the system changes, then d S = d q rev T derive the mathematical form consider a classical ideal gas, n = const First Law: d U = d q rev + d w rev = d q rev- P d V , reversible change, no extra work Joule experiment: d U = C V d T C V d T = d q rev- P d V PChem I 3.4 d q rev = C V d T + P d V q rev T V = C V , q rev V T = P are mixed second derivatives equal?...
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This note was uploaded on 12/24/2011 for the course CH 440 taught by Professor Staff during the Spring '08 term at Oregon State.

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PC1Set3 - 3. The Second Law Motivation of the Second Law :...

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