Editing Physical chemistry (section)

== Disciplines ==
[[Quantum chemistry]], a subfield of physical chemistry especially concerned with the application of [[quantum mechanics]] to chemical problems, provides tools to determine how strong and what shape bonds are,<ref name=Atkins249 /> how nuclei move, and how light can be absorbed or emitted by a chemical compound.<ref>Atkins, Peter and Friedman, Ronald (2005). ''Molecular Quantum Mechanics'', p. 342. Oxford University Press, New York. {{ISBN|0-19-927498-3}}.</ref> [[Spectroscopy]] is the related sub-discipline of physical chemistry which is specifically concerned with the interaction of [[electromagnetic 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 in [[chemical thermodynamics]], which sets limits on quantities like how far a reaction can proceed, or how much [[energy]] can be converted into work in an [[internal combustion engine]], and which provides links between properties like the [[thermal expansion coefficient]] and rate of change of [[entropy]] with [[pressure]] for a [[gas]] or a [[liquid]].<ref>Landau, L.D. and Lifshitz, E.M. (1980). ''Statistical Physics'', 3rd Ed. p. 52. Elsevier Butterworth Heinemann, New York. {{ISBN|0-7506-3372-7}}.</ref> 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]] and [[non-equilibrium thermodynamics]] can describe irreversible changes.<ref name=Hill1>Hill, Terrell L. (1986). ''Introduction to Statistical Thermodynamics'', p. 1. Dover Publications, New York. {{ISBN|0-486-65242-4}}.</ref> However, classical thermodynamics is mostly concerned with systems in [[Thermodynamic equilibrium|equilibrium]] and [[reversible process (thermodynamics)|reversible changes]] and not what actually does happen, or how fast, away from equilibrium.

Which reactions do occur and how fast is the subject of [[chemical kinetics]], another branch of physical chemistry. A key idea in chemical kinetics is that for [[reagent|reactants]] to react and form [[Product (chemistry)|products]], most chemical species must go through [[transition state]]s which are higher in [[Thermodynamic free energy|energy]] than either the reactants or the products and serve as a barrier to reaction.<ref>Schmidt, Lanny D. (2005). ''The Engineering of Chemical Reactions'', 2nd Ed. p. 30. Oxford University Press, New York. {{ISBN|0-19-516925-5}}.</ref> In general, the higher the barrier, the slower the reaction. A second is that most chemical reactions occur as a sequence of [[elementary reaction]]s,<ref>Schmidt, Lanny D. (2005). ''The Engineering of Chemical Reactions'', 2nd Ed. pp. 25, 32. Oxford University Press, New York. {{ISBN|0-19-516925-5}}.</ref> 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 and [[catalysts]] 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 the [[Avogadro constant]], 6 x 10<sup>23</sup>) of particles can often be described by just a few variables like pressure, temperature, and concentration. The precise reasons for this are described in [[statistical mechanics]],<ref name=Chandler>Chandler, David (1987). ''Introduction to Modern Statistical Mechanics'', p. 54. Oxford University Press, New York. {{ISBN|978-0-19-504277-1}}.</ref> 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.<ref name=Hill1 />