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THE GEOCHEMISTRY OF NATURAL WATERS

2. LEARNING OBJECTIVES. Learn to use some tools of thermodynamics.Become acquainted with equilibrium and the equilibrium constant.Become acquainted with activity and activity coefficients.Learn to calculate and use IAP and SI.Learn to calculate variation of the equilibrium constant with temperat

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THE GEOCHEMISTRY OF NATURAL WATERS

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    1. 1 THE GEOCHEMISTRY OF NATURAL WATERS CHEMICAL EQUILIBRIUM CHAPTER 2c - Kehew (2001)

    2. 2 LEARNING OBJECTIVES Learn to use some tools of thermodynamics. Become acquainted with equilibrium and the equilibrium constant. Become acquainted with activity and activity coefficients. Learn to calculate and use IAP and SI. Learn to calculate variation of the equilibrium constant with temperature. See and be able to calculate the effect of activity coefficients on mineral solubility. See and be able to calculate the effect of complexation on mineral solubility. In this lecture we will cover very briefly some aspects of thermodynamics and chemical equilibrium that will be of most use to us in understanding what controls the geochemistry of natural waters. The goal of this lecture is not for the student to come away with a deep understanding of the science of thermodynamics. That is certainly beyond the scope of this course, and may not even be possible in an entire semester course in thermodynamics. As J. Willard Gibbs, whom many consider to be the father of chemical thermodynamics, apparently said: “The first time I studied thermodynamics, I thought I would never understand it. The second time I studied thermodynamics, I thought I understood it. The third time I studied thermodynamics, I knew I would never understand it.” Thermodynamics is just one of those things that one must study numerous times to fully come to grips with. However, it is fully possible to use thermodynamics to do useful things in aqueous geochemistry, without actually fully understanding all the background and details. That is our goal for this lecture, to learn how to use thermodynamics to make some basic calculations. We will be introduced to the concepts of equilibrium, the equilibrium constant, activity and the activity coefficient. We will learn how to use thermodynamics to determine whether a water might be in equilibrium with a particular mineral. We will learn how to calculate the effect of temperature on mineral solubilities and other relevant reactions. We will also learn how to take into account activity coefficients and complexes when calculating mineral solubilities. In this lecture we will cover very briefly some aspects of thermodynamics and chemical equilibrium that will be of most use to us in understanding what controls the geochemistry of natural waters. The goal of this lecture is not for the student to come away with a deep understanding of the science of thermodynamics. That is certainly beyond the scope of this course, and may not even be possible in an entire semester course in thermodynamics. As J. Willard Gibbs, whom many consider to be the father of chemical thermodynamics, apparently said: “The first time I studied thermodynamics, I thought I would never understand it. The second time I studied thermodynamics, I thought I understood it. The third time I studied thermodynamics, I knew I would never understand it.” Thermodynamics is just one of those things that one must study numerous times to fully come to grips with. However, it is fully possible to use thermodynamics to do useful things in aqueous geochemistry, without actually fully understanding all the background and details. That is our goal for this lecture, to learn how to use thermodynamics to make some basic calculations. We will be introduced to the concepts of equilibrium, the equilibrium constant, activity and the activity coefficient. We will learn how to use thermodynamics to determine whether a water might be in equilibrium with a particular mineral. We will learn how to calculate the effect of temperature on mineral solubilities and other relevant reactions. We will also learn how to take into account activity coefficients and complexes when calculating mineral solubilities.

    3. 3 IONIC STRENGTH - I Recall that activity and concentration are related through the activity coefficient according to: ai = ?i·Mi Activity coefficients different from unity arise because of the interaction of ions as concentration rises. The degree of ion interaction depends on ionic charge as well as concentration.

    4. 4 IONIC STRENGTH - II Ionic strength (I ) is a quantity that is required to estimate activity coefficients. It takes into account both concentration and charge: The calculation of ionic strength must take into account all major ions:

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