Читать книгу Geochemistry - William M. White - Страница 31

In the next two chapters we will be using a number of variables, or properties, to describe thermodynamic systems. Some of these will be quite familiar to you, others less so. Volume, pressure, energy, heat, work, entropy, and temperature are the most fundamental variables in thermodynamics. As all other thermodynamic variables are derived from them, it is worth our while to consider a few of these properties.

Energy is the capacity to produce change. It is a fundamental property of any system, and it should be familiar from physics. By choosing a suitable reference frame, we can define an absolute energy scale. However, it is changes in energy that are generally of interest to us rather than absolute amounts. Work and heat are two of many forms of energy. Heat, or thermal energy, results from random motions of molecules or atoms in a substance and is closely related to kinetic energy. Work is done by moving a mass, M, through some distance, x = X, against a force F:

(2.1)

where w is work and force is defined as mass times acceleration:

(2.2)

(the minus sign is there because of the convention that work done on a system is positive, work done by a system is negative). This is, of course, Newton's first law. In chemical thermodynamics, pressure–volume work is usually of more interest. Pressure is defined as force per unit area:

(2.3)

Since volume is area times distance, we can substitute eqn. 2.3 and dV = Adx into eqn. 2.1 and obtain:

(2.4)

Thus, work is also done as a result of a volume change in the presence of pressure.

Potential energy is energy possessed by a body by virtue of its position in a force field, such as the gravitational field of the Earth, or an electric field. Chemical energy will be of most interest to us in this book. Chemical energy is a form of potential energy stored in chemical bonds of a substance. Chemical energy arises from the forces involved in the interaction between atoms and electrons. Internal energy, which we denote with the symbol U, is the sum of the potential energy arising from these forces as well as the kinetic energy of the atoms and molecules (i.e., thermal energy) in a substance. It is internal energy that will be of most interest to us.

We will discuss all these fundamental variables in more detail in the next few sections.