Chemical Fundamentals of Geology and Environmental Geoscience

Robin Gill

• Editore: Wiley-Blackwell
• Anno: 2014
• ISBN:
  9781118957943
• Edizione: 3
• Argomento: Scienze fisiche
• Categoria: Mineralogia

Chemical principles are fundamental to the Earth sciences, and geoscience students increasingly require a firm grasp of basic chemistry to succeed in their studies. The enlarged third edition of this highly regarded textbook introduces the student to such 'geo-relevant' chemistry, presented in the same lucid and accessible style as earlier editions, but the new edition has been strengthened in its coverage of environmental geoscience and incorporates a new chapter introducing isotope geochemistry.

The book comprises three broad sections. The first (Chapters 1–4) deals with the basic physical chemistry of geological processes. The second (Chapters 5–8) introduces the wave-mechanical view of the atom and explains the various types of chemical bonding that give Earth materials their diverse and distinctive properties. The final chapters (9–11) survey the geologically relevant elements and isotopes, and explain their formation and their abundances in the cosmos and the Earth. The book concludes with an extensive glossary of terms; appendices cover basic maths, explain basic solution chemistry, and list the chemical elements and the symbols, units and constants used in the book.

1
ENERGY IN GEOCHEMICAL PROCESSES

Introduction

The purpose of this book is to introduce the average Earth science student to chemical principles that are fundamental to the sciences of geology and environmental geoscience. There can be no more fundamental place to begin than with the topic of energy (Box 1.1), which lies at the heart of both geology and chemistry. Energy plays a role in every geological process, from the atom-by-atom growth of a mineral crystal to the elevation and subsequent erosion of entire mountain chains. Consideration of energy provides an incisive intellectual tool for analysing the workings of the complex geological world, making it possible to extract from this complexity a few simple underlying principles upon which an orderly understanding of Earth processes can be based.

---

Box 1.1 What is energy?

The concept of energy is fundamental to all branches of science, yet to many people the meaning of the term remains elusive. In everyday usage it has many shades of meaning, from the personal to the physical to the mystical. Its scientific meaning, on the other hand, is very precise.

To understand what a scientist means by energy, the best place to begin is with a related – but more tangible – scientific concept that we call work. Work is defined most simply as motion against an opposing force (Atkins, 2010, p. 23). Work is done, for example, when a heavy object is lifted a certain distance above the ground against the force of gravity (Figure 1.1.1). The amount of work this involves will clearly depend upon how heavy the object is, the vertical distance through which its centre of gravity is lifted (Figure 1.1.1b), and the strength of the gravitational field acting on the object. The work done in this operation can be calculated using a simple formula:

Figure 1.1.1 Work done in raising an object: (a) an object of mass m resting on the ground; (b) the same object elevated to height h; (c) the object elevated to height 2 h; (d) another object of mass 3 m elevated to height h. Note: elevation is measured between each object's centre of gravity in its initial and final positions (note the centre of gravity of the larger weight is slightly higher than the smaller one).

(1.1.1)

where m represents the mass of the object (in kg), h is the distance through which its centre of gravity is raised (in m – see footnote)², and g, known as the acceleration due to gravity (metres per second per second = m s^–2), is a measure of the strength of the gravitational field where the experiment is being carried out; at the Earth's surface, the value of g is 9.81 m s^–2. The scientific unit that we use to measure work is called the joule (J), which as Equation 1.1.1 shows is equivalent to kg × m × m s^–2 = kg m² s^–2 (see Table A2, Appendix A). Alternative forms of work, such as cycling along a road against a strong opposing wind, or passing an electric current through a resistor, can be quantified using comparably simple equations, but whichever equation we use, work is always expressed in joules.

The weight suspended in its elevated position (Figure 1.1.1b) can itself do work. When connected to suitable apparatus and allowed to fall, it could drive a pile into the ground (this is how a pile-driver works), hammer a nail into a piece of wood, or generate electricity (by driving a dynamo) to illuminate a light bulb. The work ideally recoverable from the elevated weight in these circumstances is given by Equation 1.1.1. If we were to raise the object twice as far above the ground (Figure 1.1.1c), we double its capacity for doing work:

(1.1.2)

Alternatively if we raise an object three times as heavy to a distance h above the ground (Figure 1.1.1d), the amount of work that this new object could perform would be three times that of the original object in Figure 1.1.1b:

(1.1.3)

The simple mechanical example in Figure 1.1.1 shows only one, simply understood way of doing work. Mechanical work can also be done by an object's motion, as a demolition crew's ‘wrecking ball' illustrates. Electric current heating the element of an electric fire represents another form of work, as does an explosive charge used to blast a rock face in a mine.

Energy is simply the term that we use to describe a system's capacity for doing work. Just as we recognize different forms of work (mechanical, electrical, chemical …), so energy exists in a number of alternative forms, as will be illustrated in the following pages. The energy stored in an electrical battery, for example, represents the amount of work that it can generate before becoming exhausted. A system's capacity for doing work is necessarily expressed in the units of work (just as the capacity of a bucket is expressed as the number of litres of water it can contain), so it follows that energy is also expressed in joules = kg m² s⁻¹. When discussing large amounts of energy, we use larger units such as kilojoules (kJ = 10³ J) or megajoules (MJ = 10⁶ J).

---

Many n...