THE GEOCHEMISTRY OF NATURAL WATERS

1. 1 THE GEOCHEMISTRY OF NATURAL WATERS CHEMICAL KINETICS CHAPTER 2e - Kehew (2001)

2. 2 LEARNING OBJECTIVES Be introduced to some of the basic principles of chemical kinetics. Learn what is meant by reaction order. Become familiar with some basic rate-law expressions. Learn about the Arrhenius equation. Gain an appreciation for the factors controlling mineral precipitation and dissolution rates. The first part of this lecture deals with Chemical Kinetics, or the study of the rates of chemical reactions. We will cover only some of the very basic principles here, including learning about rate laws, reaction order, the Arrhenius equation (which governs how rates depend on temperature), and the kinetics of mineral precipitation and dissolution. In the second part of the lecture, we will start discussing acid-base reactions and the carbonate system. The first part of this lecture deals with Chemical Kinetics, or the study of the rates of chemical reactions. We will cover only some of the very basic principles here, including learning about rate laws, reaction order, the Arrhenius equation (which governs how rates depend on temperature), and the kinetics of mineral precipitation and dissolution. In the second part of the lecture, we will start discussing acid-base reactions and the carbonate system.

3. 3 THERMODYNAMICS VS. KINETICS These two subjects are intertwined: thermodynamics tells us where the system should go, and kinetics tells us how fast. Many reactions among species in solution, e.g., complexation reactions and acid-base reactions are quite fast and almost always attain equilibrium. Mineral dissolution/precipitation reactions and oxidation/reduction reactions are much slower. A number of geochemically important reactions, such as acid-base reactions and the formation of complexes of metal ions, are sufficiently rapid that thermodynamics and the concepts of chemical equilibrium which we covered in Lecture 2 can be employed to predict successfully how natural systems will behave. However, mineral dissolution/precipitation reactions and oxidation/reduction reactions can be much slower and may not always attain equilibrium in natural systems. To help us understand such processes, we need to use a kinetic approach. The sciences of kinetics and thermodynamics are very closely connected. Thermodynamics tells us the directions in which natural processes proceed, but nothing about how fast they will proceed. Kinetics helps us understand how fast reactions may occur. However, we cannot measure how fast we are reaching a goal, unless we know what our goal is. Thermodynamics provides the map that tells us where the goal is, and kinetics is the speedometer that tells how fast we are getting to that goal. Another connection between thermodynamics and kinetics is that, the farther away from equilibrium a system is, the faster the rate of the reaction will be, all other things being equal. As we approach equilibrium, the overall rate of the reaction decelerates until at equilibrium, the overall rate of reaction becomes zero. This is part of the definition of equilibrium, that there be no net change in the system, and so the rate of change must be zero. A number of geochemically important reactions, such as acid-base reactions and the formation of complexes of metal ions, are sufficiently rapid that thermodynamics and the concepts of chemical equilibrium which we covered in Lecture 2 can be employed to predict successfully how natural systems will behave. However, mineral dissolution/precipitation reactions and oxidation/reduction reactions can be much slower and may not always attain equilibrium in natural systems. To help us understand such processes, we need to use a kinetic approach. The sciences of kinetics and thermodynamics are very closely connected. Thermodynamics tells us the directions in which natural processes proceed, but nothing about how fast they will proceed. Kinetics helps us understand how fast reactions may occur. However, we cannot measure how fast we are reaching a goal, unless we know what our goal is. Thermodynamics provides the map that tells us where the goal is, and kinetics is the speedometer that tells how fast we are getting to that goal. Another connection between thermodynamics and kinetics is that, the farther away from equilibrium a system is, the faster the rate of the reaction will be, all other things being equal. As we approach equilibrium, the overall rate of the reaction decelerates until at equilibrium, the overall rate of reaction becomes zero. This is part of the definition of equilibrium, that there be no net change in the system, and so the rate of change must be zero.

4. 4 REACTION RATE LAWS Consider the irreversible reaction: A + B + C ? P The rate of such a reaction is generally proportional to the concentrations of the reactants raised to a power, i.e., where k is the rate constant (dependent on T), and nA, nB and nC are the orders of the reaction with respect to A, B, and C, respectively.