Geoscience Reference
In-Depth Information
Chapter 2
Selected Geochemical Processes
In physical chemistry, the world is divided in two parts: the system, containing the
portion of the world of particular interest, and the surroundings, comprising the
region outside the system (Atkins and de Paula
2002
). A geochemical system is an
open system that may be studied within two basic frameworks:
1. Thermodynamic system, which is a state of equilibrium where environmental
parameters, such as pressure and temperature, are imposed on the bulk com-
position of the system. This approach is used to predict the system stability and
the impact of changing environmental conditions.
2. Kinetic system, wherein the pathways along the system are moving toward
some state of local equilibrium, which in turn determines the rate of change
along the pathway. In the context of a kinetic approach, which is relevant to
geochemical processes, dissolution-precipitation, exchange-adsorption, oxida-
tion-reduction, vaporization, and formation of new phases are discussed here.
2.1 Thermodynamics and Equilibrium
When a thermodynamic approach is used to describe geochemical phenomena in
the subsurface, it is necessary to define the solids, liquids, gases, and soluble
species that exist at equilibrium.
2.1.1 Enthalpy, Entropy, and the Laws of Thermodynamics
In thermodynamics, the total energy of a system is given by the sum of the total
kinetic and potential energies of the molecules in the system.
The first law of thermodynamics is the application of the conservation of energy
principle. In geochemistry, the first law considers that the change in internal
