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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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The Second Law of Thermodynamics

The Second Law of Thermodynamics

5 The Second Law of Thermodynamics
Thermodynamics in Geochemistry

Greg M. Anderson

David A. Crerar

Oxford University Press

We have seen that the first great principle of energy transfers is that energy is conserved. The essence of the second principle or law has to do with energy availability, or with the "directionality" of these transfers (processes). In other words, it is observed that once the constraints on the beginning and ending states are decided upon, processes can only proceed spontaneously in one direction between these states and are never observed to proceed in the other direction unless they are "pushed" with an external energy source. For example, a brick can fall off a table onto the floor. The potential energy it has on the table is converted to kinetic energy and then to a certain amount of heat and mechanical deformation (work) upon impact. According to the first law, the energy expended on impact will exactly equal the energy the brick had on the table. Bricks have never been observed to spontaneously cool themselves, convert this heat energy into kinetic energy, and fly from the floor to the table. Such events could exactly satisfy the first law, which clearly has nothing to say about why they don't happen—just that if they did happen, energy would be conserved. It would obviously be very useful to have a method of predicting which way a given process would go under given conditions. It would open the way towards systematizing chemical and mineral reactions, for one thing. We could start to predict which minerals would form under given metamorphic conditions, for example, and perhaps even predict their compositions, because all such changes are simply processes that can be considered to go from one equilibrium state to another. Possibly the greatest single step forward in the history of the development of thermodynamics was the recognition and definition of a parameter, the entropy, which enables such predictions and systematizations to be made. deduced on the basis of years of experience, and then show through use of the Carnot cycle the logical consequences (such as the existence of the entropy and an absolute scale of temperature).

Keywords:   Duhem's law, affinity, differential inequalities, fundamental equations, progress variable, second law of thermodynamics, thermodynamic potentials

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