Physical Chemistry: A Molecular Approach
To the Student
You are about to begin your study of physical chemistry. You may have been told that physical chemistry is the most difficult chemistry course that you will take, or you may have even seen the bumper sticker that says “Honk if you passed P Chern.” The anxiety that some students bring to their physical chemistry course has been eloquently addressed by the British professor E. Brian Smith in the preface of his introductory text, Basic Chemical Thermodynamics, Oxford University Press: _—..The first time I heard about Chemical Thermodynamics was when a secondyear undergraduate brought me the news in my freshman year. He told a spine-chilling story of endless lectures with almost three hundred numbered equations, all of which, it appeared, had to be committed to memory and reproduced in exactly the same form in subsequent examinations. Not only did these equations contain all the normal algebraic symbols but in addition they were liberally sprinkled with stars, daggers, and circles so as to stretch even the most powerful of minds. Few would wish to deny the mind-improving and indeed character-building qualities of such a subject! However, many young chemists have more urgent pressures on their time.”
We certainly agree with this last sentence of Professor Smith’s. The fact is, however, that every year thousands upon thousands of students take and pass physical chemistry, and many of them really enjoy it. You may be taking it only because it is required by your major, but you should be aware that many recent developments in physical chemistry are having a major impact in all the areas of science that are concerned with the behavior of molecules. For example, in biophysical chemistry, the application of both experimental and theoretical aspects of physical chemistry to biological problems has greatly advanced our understanding of the structure and reactivity of proteins and nucleic acids. The design of pharmaceutical drugs, which has seen great advances in recent years, is a direct product of physical chemical research.
Traditionally, there are three principal areas of physical chemistry: thermodynamics (which concerns the energetics of chemical reactions), quantum chemistry (which concerns the structures of molecules), and chemical kinetic~ (which concerns the rates of chemical reactions). Many physical chemistry courses begin with a study of thermodynamics, then discuss quantum chemistry, and treat chemical kinetics last. This order is a reflection of the historic development of the field. Today, however, physical chemistry is based on quantum mechanics, and so we begin our studies with this topic. We first discuss the underlying principles of quantum mechanics and then show how they can be applied to a number of model systems. Many of the rules you have learned in general chemistry and organic chemistry are a natural result of the quantum theory. In organic chemistry, for example, you learned to assign molecular structures using infrared spectra and nuclear magnetic resonance spectra, and in Chapters 13 and 14 we explain how these spectra are governed by the quantum-mechanical properties of molecules.
Your education in chemistry has trained you to think in terms of molecules and their interactions, and we believe that a course in physical chemistry should reflect this viewpoint. The focus of modem physical chemistry is on the molecule. Current experimental research in physical chemistry uses equipment such as molecular beam machines to study the molecular details of gas-phase chemical reactions, high vacuum machines to study the structure and reactivity of molecules on solid interfaces, lasers to determine the structures of individual molecules and the dynamics of chemical reactions, and nuclear magnetic resonance spectrometers to learn about the structure and dynamics of molecules. Modem theoretical research in physical chemistry uses the tools of classical mechanics, quantum mechanics, and statistical mechanics along with computers to develop a detailed understanding of chemical phenomena in terms of the structure and dynamics of the molecules involved. For example, computer calculations of the electronic structure of molecules are providing fundamental insights into chemical bonding and computer simulations of the dynamical interaction between molecules and proteins are being used to understand how proteins function.
In general chemistry, you learned about the three laws of thermodynamics and were introduced to the quantities, enthalpy, entropy, and the Gibbs energy (formerly called the free energy). Thermodynamics is used to describe macroscopic chemical systems. Armed with the tools of quantum mechanics, you shall learn that thermodynamics can be formulated in terms of the properties of the atoms and molecules that make up macroscopic chemical systems. Statistical thermodynamics provides a way to describe thermodynamics at a molecular level. You shall see that the three laws of thermodynamics can be explained simply and beautifully in molecular terms. We believe that a modem introduction to physical chemistry should, from the outset, develop the field of thermodynamics from a molecular viewpoint.
Our treatment of chemical kinetics, which constitutes the last five chapters, develops an understanding of chemical reactions from a molecular viewpoint. For example, we have devoted more than half of the chapter of gas-phase reactions (Chapter 28) to the reaction between a fluorine atom and a hydrogen molecule to form a hydrogen fluoride molecule and a hydrogen atom. Through our study of this seemingly simple reaction, many of the general molecular concepts of chemical reactivity are revealed. Again, quantum chemistry provides the necessary tools to develop a molecular understanding of the rates and dynamics of chemical reactions.
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