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Thermodynamics in GeochemistryThe Equilibrium Model$
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Greg M. Anderson and David A. Crerar

Print publication date: 1993

Print ISBN-13: 9780195064643

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195064643.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 27 July 2021

Applications to Simple Systems

Applications to Simple Systems

(p.184) 8 Applications to Simple Systems
Thermodynamics in Geochemistry

Greg M. Anderson

David A. Crerar

Oxford University Press

Thus far we have developed just about all the thermodynamic concepts required by Earth scientists with the exception of those needed to deal with solutions. Since all naturally occurring substances are solutions of one kind or another (although some can usefully be treated as pure substances), this is quite an important limitation, and we will proceed to discuss the treatment of solutions in Chapter 10. However, a great deal can be done with the thermodynamics of pure systems, and in this chapter we discuss a couple of applications of the concepts so far developed which are of particular interest to Earth scientists—the thermal effects associated with adiabatic volume changes, and the T-P phase diagrams of pure minerals. All systems experience a change in volume in response to changes in pressure. We have discussed this mostly from the point of view of the work accomplished by isobaric volume changes in Chapter 4, but it is even more informative to consider the temperature changes accompanying volume changes. The best way to do this is to consider only cases uncomplicated by heat entering or leaving the system, i.e., adiabatic processes. Such processes, although yet another "hypothetical limiting case," serve as useful end-members in considering actual processes in real systems. The most familiar everyday example is the hand-held bicycle pump, which most cyclists at least know gets quite warm during pumping (compressing air). This process, while not strictly adiabatic (bicycle pumps are not well insulated) is sufficient to show that volume changes can be associated with temperature changes, and it is not difficult to see in this case why—a great deal of energy in the form of work is being added to the gas, and some of it is being used to warm the gas. It seems reasonable to suppose, too, that by reversing the process—suddenly expanding the gas—it would experience a temperature decrease. This much may seem intuitively reasonable, perhaps even obvious. What is not so obvious is the fact, first investigated by Joule and Thompson in 1853, that some substances do not warm but cool during compression, and that in fact all substances have a range of conditions where they warm on expansion and another where they cool on expansion. When the expansions are at constant enthalpy, these two ranges are separated by the Joule-Thompson inversion curve.

Keywords:   Clapeyron equation, Joule expansion, Joule-Thompson expansion, LeChatelier's principle, adiabatic processes, boiling, isenthalpic, isenthalpic boiling, isentropic volume changes

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