Читать книгу Geochemistry - William M. White - Страница 40

We have seen that work is the integral of force applied through a distance. Force times distance has units of energy (mass-velocity²), thus work is a form of energy. The SI (Système Internationale) unit of energy is the joule = 1 kg-m²/s². Conversion factors for energy and other variables as well as values of important constants are listed in Appendix I.

There are several kinds of work of interest to thermodynamics, the most important of which is that involved in chemical reactions (later, when we consider oxidation and reduction reactions, we will be concerned with electrochemical work). One of the most important forms of work in classical thermodynamics is ‘PV’ work: expansion and contraction. Expressing eqn. 2.4 in differential form:

(2.23)

Pressure is force per unit area, and therefore has units of mass-distance^–1-time^–2, while volume has units of distance³. The product of P and V therefore has units of energy: mass-(distance/time)².* The negative sign arises because, by convention, we define energy flowing into the system as positive. Work done by the system is thus negative, while work done on the system is positive. This conforms to a 1970 IUPAC (International Union of Pure and Applied Chemistry) recommendation.

While PV work is not as important in geochemistry as in other applications of thermodynamics, it is nevertheless of significant interest. There is, of course, a great range of pressures within the Earth.

Systems rising within the Earth, such as magma, a hydrothermal fluid, or upwelling water in the ocean or air in the atmosphere, will thus do work on their surroundings, and systems sinking, such as sediments being buried or lithosphere being subducted, will have work done on them.

We mentioned the concept of reversible and irreversible reactions and stated that a reversible reaction is one that occurs in sufficiently small steps that equilibrium is maintained. In an expansion or contraction reaction, equilibrium is maintained, and the reaction is reversible if the external pressure is equal to the internal pressure. The work done under these conditions is said to be reversible:

(2.24)