Page 87 - Geothermal Energy Renewable Energy and The Environment

72 Geothermal Energy: Renewable Energy and the Environment

is ephemeral, with water molecules aligning with different neighbors very quickly. This property
has a strong influence on the way in which the internal energy of a volume of water changes with
temperature.
Many solutes are themselves charged (Table 5.3), in which case they are called ions. Positively
charged ions are called cations and negatively charged ions are called anions. Since it is a matter
of universal human experience that water is not electrically charged, it is clear that the dissolved
load is exactly electrically balanced. In other words, the positive electrical charge of the total dis-
solved load of cations exactly neutralizes the negative electrical charge of the total load of dissolved
anions. This charge balance requirement, along with a variety of other chemical constraints that are
discussed below, determine the outcome of water interacting with rock. The end result is recorded
in the broad range of compositions of geothermal waters found around the world (Table 5.1). In the
remainder of this chapter we will examine the concepts of saturation, reaction rates, equilibrium,
and other processes that determine what is contained in an aqueous solution and how those con-
centrations can be affected by the use and development of geothermal resources. But first, we must
develop the means for defining and describing a chemical system.

componenTs and chemIcal sysTems
Any collection of compounds, whether they be metals, individual elements, mixed gases, mixed
liquids, condensed solids, or any combination of these, are composed of chemical constituents. The
minimum number of these chemical constituents that are needed to fully describe the collection
of substances are defined as the components of the system. How these components are identified
depends upon the way in which the system is to be analyzed. In developing our list of components
it is important that they describe the system completely, and are not themselves composed of two or
more entities that occur in our system. The entities that makeup the system are the phases, which
can be minerals, gases, or liquids.
As an example, consider the minerals quartz, cristobalite, and chalcedony, all of which have the
chemical formula SiO . These three phases are the system we will consider. The difference between
2
them is how the atoms in the mineral structures are arranged. Three chemical reactions can be writ-
ten that completely describe the possible interactions of the minerals

quartz < = > tridymite

tridymite < = > chalcedony

quartz < = > chalcedony.

There are two ways in which the components of this system can be defined. One way is to note
that the elements Si and O compose all of the minerals in this system and can therefore completely
describe the chemical properties. It is also possible, however, to use SiO as a chemical component
2
since it, too, provides a complete description of this system. In fact, it is the preferable and neces-
sary way to describe this system since it allows the smallest number of components to be used
for defining the system. If, however, we were interested in the dissociation of these minerals into
their respective atomic constituents, then we would have to use Si and O as the components, rather
than SiO .
2

chemical poTenTials, μ, and Gibbs enerGy
Once the components are identified for a system it is possible to describe such things as what min-
eral or collections of minerals will be the stable phases in the system under a given set of pressure
and temperature conditions. Each component in a mineral has a chemical potential, μ, which has