Understanding Molecules

Lectures on Chemistry for Physicists and Engineers

Franco Battaglia, Thomas F. George

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9780429827174
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Chemistry is a subject that many students with differing goals have to tackle. This unique general chemistry textbook is tailored to more mathematically-oriented engineering or physics students. The authors emphasize the principles underlying chemistry rather than chemistry itself and the almost encyclopedic completeness appearing in a common textbook of general chemistry is sacrificed for an emphasis to these principles. Contained within 300 pages, it is suitable for a one-semester course for students who have a strong background in calculus. Over 200 problems with answers are provided so that the students can check their progress.

1 From the Chemical System to the Atomic Nucleus

Chemistry, like other disciplines such as physics, biology, and geology, is a natural science. We could define it as the science that studies matter (its properties, constitution, and transformations) as constituted by atoms and molecules. However, as happens with any definition, also this might sound unsatisfactory. The best course of action is to dive directly into the subject. In fact, the separation between the several natural sciences is itself not sharp or distinct, and arises from our need to simplify (subdividing into parts) the complexity of the universe around us, which, at the end, is the object of all natural sciences. This approach is what one can refer to as reductionist, whose great success is validated by both the deep understanding we have reached about Nature per se and by its use in applications.

The chemist, as any other natural scientist, relies on the Galilean method, with the experimental observation at the center, which is the ultimate judgmental criterion for any statement: a conjecture proposed on the system under study but not verified by a specific experiment, no matter how intuitive it could sound, stays at the level of conjecture. And no matter how daring and counterintuitive might a hypothesis sound, if it defeats any attempt to falsify it, then it cannot be ignored.

In this regard, it is important to recognize that Nature is, indeed, counterintuitive: whoever studies it must be prepared for this. An example should suffice: what is more “obvious” than the Earth at the center of the Universe? It is “obvious” that the Sun revolves around the Earth, to which each body falls, attracted by that center that appears to be its natural place. As we know, the actual situation is different from that. Nowadays, this might sound trivial, but here is another example: the speed we measure for a light beam is the same regardless of the speed of the light source.¹

To comfortably move in a world that is anything but intuitive, we have equipped ourselves with a powerful tool that cannot be ignored by anybody who studies science: mathematics. This is much more than the language of science: it is language and logic. If there is something in science that cannot be true, it is the one that is logically impossible. Anything else, with permission of the experiment, is possible.

At the beginning of the previous century, the Universe was seen as constituted of two fundamental and distinct entities: matter and radiation. The former, constituted by atoms and molecules, was supposed to obey the laws of Newtonian mechanics; the latter, constituted by electromagnetic fields, follows the Maxwell laws of electromagnetism. Chemistry focuses its interests on matter but does not neglect its interaction with radiation. As a matter of fact, as we shall see in Chapter 8, it is by studying the matter–radiation interaction that we are able to have information on matter itself.

1.1 CHEMICAL SYSTEM

The first step taken by chemists in their investigation is to identify the object of study: a chemical system is a well-defined portion of the world around us, enclosed within walls, ideal or real as they may be. It can be a virus, a butterfly, a mineral sample, the book you are reading, or the terrestrial atmosphere. Two observations arise immediately. First, matter presents itself to us in several, qualitatively distinct, aggregation states, of which one can readily recognize at least three: solids, which have their own volume and shape; liquids, which have their own volume, but shape of their container; and gases, which have volume and shape of the container. We shall see that to consider matter from the point of view of its microscopic constitution will induce us to modify these definitions.

The second observation is that, in general, a system is composed of qualitatively distinct portions. For instance, if you look at (and disassemble) the pen you are holding, you recognize in it metallic, plastic, rubbery, and liquid materials. Such a system is said to be heterogeneous. According to the already-mentioned reductionist approach, the chemist starts by separating a system into its homogeneous parts: we shall then call homogeneous a system whose properties are uniform at every point within it, and heterogeneous a system composed of several homogeneous parts (phases). Among heterogeneous systems, we include also those where there is dispersion within parts otherwise homogeneous, such as dispersion of liquids in liquids (emulsions), solids in gases (mists), and gases in liquid or solids (foams): with suitable instruments, it is possible to distinguish the composing parts.

There are plenty of laboratory techniques able to separate a heterogeneous system into its homogeneous parts. Suspensions may be decanted, i.e., the solid suspended particles, in the absence of stirring, tend to sediment because of gravity. This is what slowly happens, for instance, to dust in the air in the absence of ventilation. If the particles are very thin, natural gravitation may not be sufficient, and one can resort to centrifugation, which may reach 70,000 cycles/minute (ultracentrifuges), where much stronger forces are acting on the particles. Another technique is filtration: only some of the phases go through the pores of appropriate filters (for instance, liquids and/or gases, whereas solid particles get stuck on the filter). None of these techniques induce transformations in the nature of the components: remixing them together and stirring gives back the original system. The separation has taken place by making use of physical techniques only.

Once a system has been separated into its homogeneous parts, the chemist turns the attention to each one of them. In general, a homogeneous system (i.e., a one-phase system) may still be decomposed into several other homogeneous parts by means of physical techniques alone. A simple example would be a glass of seawater: it is a homogeneous system, indeed. However, if one allows the water to evaporate and recondense into another container, on the bottom of the glass remains a solid crust of sodium chloride and other salts that were dissolved into the seawater to begin with. By blending together again the crust and the evaporated water, the initial seawater sample is reconstituted.

A solution is the name for a one-phase system which is separable into several other systems by means of physical methods only, i.e., without resorting to any chemical transformation (as clarified at the end of this section). In a solution, the component that happens to be present in quantities much larger than the other components is called the solvent, in contrast to solutes, which are the components present in smaller quantities. Typically, the solvent is a liquid, and the solute(s) may be solid (as in the above example of seawater), liquid (a glass of wine is a solution of ethanol and water plus other solutes as well), or gas (sparkling water is a solution of carbon dioxide gas and water plus other solutes). An alloy can be often considered a solution between solids (some of them, though, form inter-metallic compounds, as happens between copper and zinc in brass). A mixture of gases is always a homogeneous system, because gases are totally miscible with each other.

The separation of a solution into its components requires specific laboratory techniques. Distillation of solutions made of liquid components takes advantage of their different volatility: the most volatile liquid evaporates in larger quantity than those which are less volatile and, made to condense in a separate container, the condensate will have a fraction of the most volatile component larger than the one in the original sample; repeating the procedure, it could be possible to sometimes obtain an almost complete separation. In the seawater example above, a distillation would be appropriate. The evaporated water would be recondensed and collected into an appropriate container, whereas the dissolved salts start to precipitate when their concentration overcomes a specific value—the solubility. This is defined as the solute concentration above which it starts to precipitate.

Precipitation may be obtained by cooling as well. For instance, by preparing a sodium chloride solution which is saturated at, say, 90°C, and letting it cool at the laboratory temperature, say 20°C, most of the solute precipitates because its solubility at 20°C is lower than at 90°C: we then have recrystallized the sodium chloride. Recrystallization is also a method to eliminate impurities: present in tiny quantities in the original solid, they stay almost totally in solution, while the salt precipitates into purer crystals.

A homogeneous system which cannot be separated into parts that would reconstitute the system by simple mixing is called a pure substance. For instance, from a seawater sample, the water separated by distillation and collected into a container appropriately sealed so as to prevent any contact with other species that could dissolve in it (even the air, then), is a pure substance.

However, by means of appropriate laboratory experiments, it is possible to observe that some pure substances appear to be constituted by other pure substances. For instance, it is possible to perform a transformation on pure water and obtain from it two other gaseous pure substances, one of which (oxygen) is in every respect identical to another pure substance separable from samples of air, and with properties very different from those of water; the other (hydrogen) has properties different from those of both water and oxygen. Moreover, by simply mixing the two gases, one does not bring back the water. In order to obtain only water, it is necessary, first, that the two gases be mixed into a specific relative proportion and, second, that a chemical reaction occurred, i.e., not a simple mixing. It is in this respect that we say chemical methods have occurred in the separation process, in contrast to physical methods, as when a heterogeneous system is separated into its homogeneous parts or a solution into its composing pure substances. In terms of a formula, we write:

H2O→H2+12O2.(1.1)

Some pure substances, however, do not appear to be further decomposable. For instance, one does not obtain other pure substances from a sample of pure...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• About the Authors
• Chapter 1 From the Chemical System to the Atomic Nucleus
• Chapter 2 Stoichiometry
• Chapter 3 Nucleus and Radioactivity
• Chapter 4 Interlude of Physics I: Quantum Mechanics
• Chapter 5 Atoms and the Periodic Table of the Elements
• Chapter 6 Chemical Bond I: Lewis Scheme
• Chapter 7 Chemical Bond II: Molecular Orbitals
• Chapter 8 Chemical Bond III: Complements
• Chapter 9 Interlude of Physics II: Thermodynamics
• Chapter 10 Phase Equilibria
• Chapter 11 Chemical Equilibrium
• Chapter 12 Ionic Equilibria in Aqueous Solutions
• Chapter 13 Chemical Kinetics
• Chapter 14 Electrochemistry
• Appendix A A Concise Guide to the Vocabulary of Chemistry
• Appendix B Tables
• Index