What is Physical Chemistry?

Physical chemistry is an empirical science.  A science is a set of constructs, called theories, that link fragments of experience into a consistent description of natural phenomena.  The adjective “empirical” refers to the common experiences from which the theories grow, that is, to experiments.  Simple working hypotheses are guessed by imaginative insight or intuition or luck, usually from a study of experiments.  This repetitive interplay in time leads to the formulation of theories that correlate the accumulated experimental information and that can predict new phenomena with accuracy.  (Berry, Rice and Ross)

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 kinetics (which concerns the rates of chemical reactions).  (McQuarrie and Simon)

Physical chemistry is the branch of chemistry that establishes and develops the principles of the subject.  Its concepts are used to explain and interpret observations on the physical and chemical properties of matter.  Physical chemistry is also essential for developing and interpreting the modern techniques used to determine the structure and properties of matter, such as new synthetic materials and biological membranes.  (Atkins)

Physical chemistry is the study of the physical basis of phenomena related to the chemical composition and structure of substances.  It has been pursued from two levels, the macroscopic and the molecular.  Knowledge in physical chemistry available today provides a rich, comprehensive view of the world of atoms and molecules that connects their nature with macroscopic properties and phenomena of materials and substances.  A starting point for an introduction to physical chemistry is the concept of energy levels in atoms and molecules, distributions among these energy levels, and something familiar, temperature.  (Dykstra)

Physical chemistry is the study of the underlying physical principles that govern the properties and behavior of chemical systems.  (Levine)

Physical chemistry, like a table with four legs, is built upon four major theoretical areas:  thermodynamics, kinetics (or, more generally, transport processes), quantum mechanics, and statistical mechanics  This is not all of physical chemistry, no more than a table is only legs.  Physical chemistry is a widely diverse subject that cannot be summarized adequately in any brief definition, and there are important parts of physical chemistry that do not fit neatly into this quadrivium.  (Noggle)

Physical Chemistry is a fascinating field of study.  It can reasonably be claimed that many parts of physics and all parts of chemistry are included within physical chemistry and its applications.  Furthermore, it is the course in which most chemistry students first have the opportunity to synthesize what they have learned in mathematics, physics, and chemistry courses into a coherent pattern of knowledge.  (Mortimer)

We see it as the quantitative interpretation of the macroscopic world in terms of the atomic-molecular world.  To achieve this interpretation, we must organize our observations of macroscopic phenomena, as we do in thermodynamics and in parts of kinetics.  We must advance our studies of atoms and molecules, as we do, for example, in quantum mechanics and spectroscopy.  Then we must bring these studies together.  This coming together is woven into much of the fabric of a modern physical chemistry course.  (Barrow)

Physical chemistry is the application of the methods of physics to chemical problems.  It includes the qualitative and quantitative study, both experimental and theoretical, of the general principles determining the behavior of matter, particularly the transformation of one substance into another.  Although the physical chemist uses many of the methods of the physicist, he applies them to chemical structures and chemical processes.  Physical chemistry is not so much concerned with the description of chemical substances and their reactions-this is the concern of organic and inorganic chemistry-as with theoretical principles and with quantitative problems.  (Laidler & Meiser)

It is said that there are more than four million chemical compounds.  If you add to this list composite materials like alloys and minerals and intermediate species like the free radicals, it becomes truly staggering.  The list of properties that interest scientists, even though modest compared to the above list, is also vast.  The fascinating aspect of science is that only a few principles are needed to understand the behavior of the huge number of substances and their properties.  Physical chemistry is the study of these principles.  (Vemulapalli)

More Detailed Descriptions

Our goal is the presentation of the three major areas of physical chemistry: molecular structure, the equilibrium properties of systems, and the kinetics of transformations of systems.  The theoretical foundations of these subjects are respectively, quantum mechanics, thermodynamics and equilibrium statistical mechanics, and chemical kinetics and kinetic theory.  These theories, firmly based on experimental findings, constitute the structure required for the understanding of past accomplishments and a basis for recognition and development of significant new areas in physical chemistry.  (Berry, Rice and Ross)

Physical Chemistry encompasses the structure of matter at equilibrium and also the processes of chemical change.  Its three principal subject areas are thermodynamics, quantum chemistry, and chemical kinetics; other topics such as electrochemistry, have aspects that lie in all of these three categories. 

Thermodynamics, as applied to chemical problems, is primarily concerned with the position of chemical equilibrium, with the direction of chemical change, and with the associated changes in energy.

Quantum chemistry theoretically describes the bonding at a molecular level.  In its exact treatments, it deals only with the simplest of atomic and molecular systems, but it can be extended in an approximate way to deal with bonding in much more complex molecular structures.

Chemical kinetics is concerned with the rates and mechanisms with which processes occur as equilibrium is approached.

An intermediate area, known as statistical thermodynamics, links the three main areas of thermodynamics, quantum chemistry, and kinetics and also provides a basic relationship between the microscopic and macroscopic worlds.  Related to this are is nonequilibrium statistical mechanics, which is becoming an increasingly important part of modern physical chemistry.  This area includes problems in such areas as the theory of dynamics in liquids and light scattering.

(Laidler and Meiser)

Two approaches are possible in a physicochemical study.  In what might be called a systemic approach, the investigation begins with the very basic constituents of matter-the fundamental particles-and proceeds conceptually to construct larger systems from them.  The adjective microscopic(Greek micros, small) is used to refer to these tiny constituents.  In this way, increasingly complex phenomena can be interpreted on the basis of the elementary particles and their interactions.

In the second approach, the study starts with investigations of macroscopic material (Greek macros, large), such as a sample of liquid or solid that is easily observable with the eye.  Measurements are made of macroscopic properties such as pressure, temperature, and volume.  In the phenomenological approach, more detailed studies of microscopic behavior are made only insofar as they are needed to understand the macroscopic behavior in terms of the microscopic.

(Laidler & Meiser)

The topics of  the traditional physical chemistry course can be grouped into several areas:  (1) the study of the macroscopic properties of systems of many atoms or molecules; (2) the study of the processes which systems of many atoms or molecules can undergo; (3) the study of the properties of individual atoms and molecules; and (4) the study of the relationship between molecular and macroscopic properties.  (Mortimer)

Additional Perspectives

Physical chemistry provides the theoretical basis for all of chemistry and many subjects related to it.  Hence I feel, along with many instructors, that the first course in physical chemistry should lead to a critical understanding of primary theoretical concepts and their use in explaining crucial experiments.  (Vemulapalli)

Universal lessons of physical chemistry:  quantitative reasoning, problem solving, rigorous and exact thinking.  Many students may never directly use the factual knowledge gained in a course of physical chemistry, but all can benefit from the skills and habits learned.  In the opinion of some, this emphasis leads to excessive detail in certain places.  But attention to detail is the essence of good science and, in any case, it is easier to ignore unnecessary details than to add vital ones that are missing.  (Noggle)

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 modern 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.  Modern 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. (McQuarrie and Simon)

Physical chemists are inordinately proud of the "back of the envelope" calculations; napkins and paper place mats have also proved useful.  (Noggle)

Everyone makes mistakes, but smart people find those mistakes before they cause harm.  (Noggle)

Chemistry works with an enormous number of substances . . .; it is an extensive science.  Physics on the other hand works with rather few substances . . .; it is an intensive science.  Physical chemistry is the child of these two sciences; it has inherited the extensive character from chemistry.  Upon this depends its all-embracing feature, which has attracted so great admiration.  Physical chemistry may be regarded as an excellent school of exact reasoning for all students of natural sciences.  (Svante Arrhenius, Theories of Solutions (1912))

Honk If You Passed PChem (Bumper sticker sold by the ACS)