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Biogeochemistry of Estuaries$
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Thomas S. Bianchi

Print publication date: 2006

Print ISBN-13: 9780195160826

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195160826.001.0001

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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: 02 December 2021

Physical Properties and Gradients

Physical Properties and Gradients

(p.57) Chapter 4 Physical Properties and Gradients
Biogeochemistry of Estuaries

Thomas S. Bianchi

Oxford University Press

Before discussing the chemical dynamics of estuarine systems it is important to briefly review some of the basic principles of thermodynamic or equilibrium models and kinetics that are relevant to upcoming discussions in aquatic chemistry. Similarly, the fundamental properties of freshwater and seawater are discussed because of the importance of salinity gradients and their effects on estuarine chemistry. Stumm and Morgan (1996) described how different components of laboratory- and field-based measurements in aquatic chemistry are integrated. Basically, observations from laboratory experiments are made under well-controlled conditions (focused on a natural system of interest), which can then be used to make predictions and models, which are ultimately used to interpret complex patterns in the natural environment. Due to the complexity of natural systems, equilibrium models can tell you something about how chemical constituents (gases, dissolved species, solids) under well-constrained conditions (no change over time, fixed temperature and pressure, and homogeneous distribution of constituents). Equilibrium models will tell you something about the chemistry of the system at equilibrium but will not tell you anything about the kinetics with which the system reached equilibrium state. The laws of thermodynamics are the foundation for chemical systems at equilibrium. The basic objectives in using equilibrium models in estuarine/aquatic chemistry is to calculate equilibrium compositions in natural waters, to determine the amount of energy needed to make certain reactions occur, and to ascertain how far a system may be from equilibrium (Stumm and Morgan, 1996). The first law of thermodynamics states that energy cannot be created or destroyed (i.e., the total energy of a system is always constant). This means that if the internal energy of a reaction increases then there must be a concomitant uptake of energy usually in the form of heat. Enthalpy (H) is a parameter used to describe the energy of a system as heat flows at a constant pressure; it is defined by the following equation: . . . H = E + PV (4.1) . . . where: E = internal energy; P = pressure; and V = volume.

Keywords:   Arrhenius equation, Davies equation, Guntleberg equation, dissolved salts, entropy, hydrogen bonding

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