Physical chemistry, in contrast tochemical physics, is predominantly (but not always) a supra-molecular science, as the majority of the principles on which it was founded relate to the bulk rather than the molecular or atomic structure alone (for example, chemical equilibrium andcolloids).
Some of the relationships that physical chemistry strives to understand include the effects of:
The key concepts of physical chemistry are the ways in which purephysics is applied to chemical problems.
One of the key concepts in classical chemistry is that allchemical compounds can be described as groups ofatomsbonded together andchemical reactions can be described as the making and breaking of those bonds. Predicting the properties of chemical compounds from a description of atoms and how they bond is one of the major goals of physical chemistry. To describe the atoms and bonds precisely, it is necessary to know both where thenuclei of the atoms are, and how electrons are distributed around them.[2]
Quantum chemistry, a subfield of physical chemistry especially concerned with the application ofquantum mechanics to chemical problems, provides tools to determine how strong and what shape bonds are,[2] how nuclei move, and how light can be absorbed or emitted by a chemical compound.[3]Spectroscopy is the related sub-discipline of physical chemistry which is specifically concerned with the interaction ofelectromagnetic radiation with matter.
Another set of important questions in chemistry concerns what kind of reactions can happen spontaneously and which properties are possible for a given chemical mixture. This is studied inchemical thermodynamics, which sets limits on quantities like how far a reaction can proceed, or how muchenergy can be converted into work in aninternal combustion engine, and which provides links between properties like thethermal expansion coefficient and rate of change ofentropy withpressure for agas or aliquid.[4] It can frequently be used to assess whether a reactor or engine design is feasible, or to check the validity of experimental data. To a limited extent,quasi-equilibrium andnon-equilibrium thermodynamics can describe irreversible changes.[5] However, classical thermodynamics is mostly concerned with systems inequilibrium andreversible changes and not what actually does happen, or how fast, away from equilibrium.
Which reactions do occur and how fast is the subject ofchemical kinetics, another branch of physical chemistry. A key idea in chemical kinetics is that forreactants to react and formproducts, most chemical species must go throughtransition states which are higher inenergy than either the reactants or the products and serve as a barrier to reaction.[6] In general, the higher the barrier, the slower the reaction. A second is that most chemical reactions occur as a sequence ofelementary reactions,[7] each with its own transition state. Key questions in kinetics include how the rate of reaction depends on temperature and on the concentrations of reactants andcatalysts in the reaction mixture, as well as how catalysts and reaction conditions can be engineered to optimize the reaction rate.
The fact that how fast reactions occur can often be specified with just a few concentrations and a temperature, instead of needing to know all the positions and speeds of every molecule in a mixture, is a special case of another key concept in physical chemistry, which is that to the extent an engineer needs to know, everything going on in a mixture of very large numbers (perhaps of the order of theAvogadro constant, 6 x 1023) of particles can often be described by just a few variables like pressure, temperature, and concentration. The precise reasons for this are described instatistical mechanics,[8] a specialty within physical chemistry which is also shared with physics. Statistical mechanics also provides ways to predict the properties we see in everyday life from molecular properties without relying on empirical correlations based on chemical similarities.[5]
Fragment of M. Lomonosov's manuscript 'Physical Chemistry' (1752)
The term "physical chemistry" was coined byMikhail Lomonosov in 1752, when he presented a lecture course entitled "A Course in True Physical Chemistry" (Russian:Курс истинной физической химии) before the students ofPetersburg University.[9] In the preamble to these lectures he gives the definition: "Physical chemistry is the science that must explain under provisions of physical experiments the reason for what is happening in complex bodies through chemical operations".
Further development in physical chemistry may be attributed to discoveries innuclear chemistry, especially in isotope separation (before and during World War II), more recent discoveries inastrochemistry,[12] as well as the development of calculation algorithms in the field of "additive physicochemical properties" (practically all physicochemical properties, such as boiling point, critical point, surface tension, vapor pressure, etc.—more than 20 in all—can be precisely calculated from chemical structure alone, even if the chemical molecule remains unsynthesized),[citation needed] and herein lies the practical importance of contemporary physical chemistry.
Historical journals that covered both chemistry and physics includeAnnales de chimie et de physique (started in 1789, published under the name given here from 1815 to 1914).
