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physical chemistry

 
American Heritage Dictionary:

physical chemistry


n.
Scientific analysis of the properties and behavior of chemical systems primarily by physical theory and technique, as, for example, the thermodynamic analysis of macroscopic chemical phenomena.


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Britannica Concise Encyclopedia:

physical chemistry

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Branch of chemistry concerned with interactions and transformations of materials. Unlike other branches, it deals with the principles of physics underlying all chemical interactions (e.g., gas laws), seeking to measure, correlate, and explain the quantitative aspects of reactions. Quantum mechanics has clarified much for physical chemistry by modeling the smallest particles ordinarily dealt with in the field, atoms and molecules, enabling theoretical chemists to use computers and sophisticated mathematical techniques to understand the chemical behaviour of matter. Chemical thermodynamics deals with the relationship between heat and other forms of chemical energy, kinetics with chemical reaction rates. Subdisciplines of physical chemistry include electrochemistry, photochemistry (see photochemical reaction), surface chemistry, and catalysis.

For more information on physical chemistry, visit Britannica.com.

The branch of chemistry that deals with the interpretation of chemical phenomena and properties in terms of the underlying physical processes, and with the development of techniques for their investigation. The term chemical physics is often employed to denote a branch of physical chemistry where the emphasis is on the interpretation and analysis of the physical properties of individual molecules and bulk systems, instead of their reactions. Theoretical chemistry is another major branch, where the emphasis is on the calculation of the properties of molecules and systems, and which used the techniques of quantum mechanics and statistical thermodynamics. It is convenient to regard physical chemistry as dealing with three aspects of matter: its equilibrium properties, structure, and ability to change.

Equilibrium properties

The study of matter in a state of equilibrium constitutes the field of chemical thermodynamics. In particular, chemical thermodynamics provides a technique for discussing the response of a system to a change in the external conditions (such as the shift in the boiling and freezing point of either a pure substance or a mixture when the applied pressure is changed, or when the composition of the mixture is modified), and for rationalizing the energy changes that occur in the course of a chemical reaction. The branch of thermodynamics dealing with the latter is called thermochemistry. Chemical thermodynamics also provides a framework for the determination of the maximum amount of work that may be generated by a system undergoing a specified change, and it therefore provides a way of establishing bounds for the efficiencies of a variety of devices, including engines, refrigerators, and electrochemical cells. Thermodynamics is used in chemistry to assess the position of equilibrium of a chemical reaction (that is, how far it will proceed), and to determine what conditions are necessary in order to optimize the yield of a particular product. The branch of chemical thermodynamics dealing with ionic reactions occurring in the presence of electrodes constitutes the field of equilibrium electrochemistry. See also Chemical equilibrium; Chemical thermodynamics; Electrochemistry; Enthalpy; Entropy; Free energy.

Structure

The principal role of quantum mechanics in chemistry is in the discussion of atomic and molecular structure, and in the interpretation of spectroscopic data. In the branch of physical chemistry known as computational quantum chemistry, interest centers on the numerical solution of the Schrödinger equation in order to obtain wave functions and geometries of molecules. Computational quantum chemistry is so developed that it is capable of being used to map the changes in the structures of molecules while they are in the course of reaction, when atoms and groups of atoms are being transferred from one molecule to another. See also Quantum chemistry.

Spectroscopic techniques are used not only to identify molecules present in a sample, but also to determine their shape, size, and electron distribution. The techniques fall into four categories: absorption spectroscopy, emission spectroscopy, Raman spectroscopy, and resonance techniques. See also Electron paramagnetic resonance (EPR) spectroscopy; Electron spectroscopy; Molecular structure and spectra; Mössbauer effect; Nuclear magnetic resonance (NMR); Photochemistry; Spectroscopy.

Techniques for the investigation of molecular structure based on diffraction depend on the observation of the direction through which radiation and particles are scattered when they impinge on a sample. Other techniques for investigating structure include the electric and magnetic properties of molecules, in particular, the determination of electric polarizabilities and dipole moments, magnetic properties, and the properties based on optical birefringence, such as optical activity and the Faraday effect.

Structural properties and thermodynamic properties are brought together by statistical thermodynamics. This major theoretical procedure gives a way of predicting the thermodynamic properties of assemblies of molecules in terms of their individual energy levels.

Physical and chemical change

The third major branch of physical chemistry is concerned with change: physical change and chemical change. In particular, it is concerned with the rate of change. Physical change includes the diffusion of one substance into another, or the migration of ions in an electrode solution. The application of thermodynamics to change in general constitutes the field of nonequilibrium thermodynamics. See also Gas; Transport processes.

Chemical change may be studied at a variety of levels. Empirical chemical kinetics is the study of reactions in order to determine how their rates depend on the concentrations of the participants in the reaction and on the conditions, mainly the temperature. Investigation of the time dependence of reactions yields a detailed picture of the sequence of molecular transformations involved in a complex chemical reaction. See also Chemical dynamics; Shock tube; Ultrafast molecular processes.

An important extension of chemical kinetics is to the reactions that occur on surfaces; these are the processes involved in heterogeneous catalysis. A special application of surface chemistry is to the stability of colloidal suspensions of species in fluids, and another is to the processes that occur at the interface between an electrode and the solution in which it is immersed. See also Adsorption; Colloid; Heterogeneous catalysis.


Columbia Encyclopedia:

physical chemistry

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physical chemistry, branch of science that combines the principles and methods of physics and chemistry. It provides a fundamental theoretical and experimental basis for all of chemistry, including organic, inorganic, and analytical chemistry. In addition, it is the foundation of chemical engineering. Topics of interest are chemical equilibrium, reaction rates, solutions, molecular weights, molecular structure, and the properties of gases, liquids, crystals, and colloids. Among other factors, the influence of temperature, pressure, electricity, light, concentration, and turbulence are considered. There are three principal approaches in physical chemistry: thermodynamics, involving large numbers of molecules in equilibrium; kinetics, involving chemical changes with time; and molecular structure, involving the electronic and atomic arrangements that follow from the quantum theory. The latter approach is primarily theoretical and provides an understanding of the chemical bonds which are responsible for the structure of all materials.

Bibliography

See I. N. Levine, Physical Chemistry (4th ed. 1995); G. M. Barrow, Physical Chemistry (6th ed. 1996); P. W. Atkins, Physical Chemistry (6th ed. 1997); D. A. McQuarrie and J. D. Simon, Physical Chemistry: A Molecular Approach (1997).


Oxford Dictionary of Biochemistry:

physical chemistry

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the branch of chemistry concerned with the relationship between the physical properties of substances and their chemical properties, reactions, and structures.

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Random House Word Menu:

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Wikipedia on Answers.com:

Physical chemistry

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Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts. It applies the principles, practices and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mechanics and dynamics.

Physical chemistry, in contrast to chemical physics, is predominantly (but not always) a macroscopic or supra-molecular science, as the majority of the principles on which physical chemistry was founded, are concepts related to the bulk rather than on molecular/atomic structure alone. For example, chemical equilibrium, and colloids.

Some of the relationships that physical chemistry strives to resolve include the effects of:

  1. Intermolecular forces that act upon the physical properties of materials (plasticity, tensile strength, surface tension in liquids).
  2. Reaction kinetics on the rate of a reaction.
  3. The identity of ions on the electrical conductivity of materials.
  4. Surface chemistry and electrochemistry of membranes.[1]
Contents

Key concepts

The key concepts of physical chemistry are the ways in which pure physics is applied to chemical problems.

One of the key concept in chemistry is that all chemical compounds can be described as groups of atoms bonded together and chemical 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 the nuclei of the atoms are, and how electrons are distributed around them.[2] 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,[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 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 a combustion engine and which provides links between properties like the thermal expansion coefficient and change of rate in entropy with pressure for a gas or a liquid.[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 and non-equilibrium thermodynamics can describe irreversible changes.[5] However, classical thermodynamics is mostly concerned with systems in equilibrium and 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 reactants to react and form products, most chemical species must go through transition states which are higher in energy 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 of elementary 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 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 millions of billions of billions 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,[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]

History

Fragment of M. Lomonosov's manuscript 'Physical Chemistry' (1752)

The term "physical chemistry" was coined by Mikhail Lomonosov in 1752, when he presented a lecture course entitled "A Course in True Physical Chemistry" (Russian: «Курс истинной физической химии») before the students of Petersburg University.[9]

Modern physical chemistry originated in the 1860s to 1880s with work on chemical thermodynamics, electrolytes in solutions, chemical kinetics and other subjects. One milestone was the publication in 1876 by Josiah Willard Gibbs of his paper, On the Equilibrium of Heterogeneous Substances. This paper introduced several of the cornerstones of physical chemistry, such as Gibbs energy, chemical potentials, Gibbs phase rule.[10] Other milestones include the subsequent naming and accreditation of enthalpy to Heike Kamerlingh Onnes and to macromolecular processes.[citation needed]

The first scientific journal specifically in the field of physical chemistry was the German journal, Zeitschrift für Physikalische Chemie, founded in 1887 by Wilhelm Ostwald and Jacobus Henricus van 't Hoff. Together with Svante August Arrhenius,[11] these were the leading figures in physical chemistry in the late 19th century and early 20th century. All three were awarded with the Nobel Prize in Chemistry between 1901-1909.

Developments in the following decades include the application of statistical mechanics to chemical systems and work on colloids and surface chemistry, where Irving Langmuir made many contributions. Another important step was the development of quantum mechanics into quantum chemistry from the 1930s, where Linus Pauling was one of the leading names. Theoretical developments have gone hand in hand with developments in experimental methods, where the use of different forms of spectroscopy, such as infrared spectroscopy, microwave spectroscopy, EPR spectroscopy and NMR spectroscopy, is probably the most important 20th century development.

Further development in physical chemistry may be attributed to discoveries in nuclear chemistry, especially in isotope separation (before and during World War II), more recent discoveries in astrochemistry,[12] as well as the development of calculation algorithms in field of "additive physicochemical properties" (practically all of physicochemical properties, as: boiling point, critical point, surface tension, vapor pressure etc. - more than 20 in all, can be precisely calculated from chemical structure, even if such chemical molecule is still non existent), and in this area is concentrated practical importance of contemporary physical chemistry.

See Group contribution method, Joback method, QSPR, QSAR

Journals

Some journals that deal with physical chemistry include:

Historical journals that covered both chemistry and physics include Annales de chimie et de physique (started in 1789, published under the name given here from 1815–1914).

Branches and related topics

See also

References

  1. ^ Torben Smith Sørensen (1999). Surface chemistry and electrochemistry of membranes. CRC Press. p. 134. ISBN 0824719220. http://books.google.co.in/books?id=v1oU13xAl6AC&pg=RA1-PA134. 
  2. ^ a b Atkins, Peter and Friedman, Ronald (2005). Molecular Quantum Mechanics, p. 249. Oxford University Press, New York. ISBN 0199274983.
  3. ^ Atkins, Peter and Friedman, Ronald (2005). Molecular Quantum Mechanics, p. 342. Oxford University Press, New York. ISBN 0199274983.
  4. ^ Landau, L. D. and Lifshitz, E. M. (1980). Statistical Physics, 3rd Ed. p. 52. Elsevier Butterworth Heinemann, New York. ISBN 0750633727.
  5. ^ a b Hill, Terrell L. (1986). Introduction to Statistical Thermodynamics, p. 1. Dover Publications, New York. ISBN 0486652424.
  6. ^ Schmidt, Lanny D. (2005). The Engineering of Chemical Reactions, 2nd Ed. p. 30. Oxford University Press, New York. ISBN 0195169255.
  7. ^ Schmidt, Lanny D. (2005). The Engineering of Chemical Reactions, 2nd Ed. p. 25, 32. Oxford University Press, New York. ISBN 0195169255.
  8. ^ Chandler, David (1987). Introduction to Modern Statistical Mechanics, p. 54. Oxford University Press, New York. ISBN 9780195042771.
  9. ^ Alexander Vucinich (1963). Science in Russian culture. Stanford University Press. p. 388. ISBN 0804707383. http://books.google.com/books?id=YoE1wsA6USQC&pg=PA388. 
  10. ^ Josiah Willard Gibbs, 1876, "On the Equilibrium of Heterogeneous Substances", Transactions of the Connecticut Academy of Sciences
  11. ^ Laidler, Keith (1993). The World of Physical Chemistry. Oxford: Oxford University Press. pp. 48. ISBN 0198559194. 
  12. ^ Herbst, Eric (May 12, 2005). "Chemistry of Star-Forming Regions". Journal of Physical Chemistry A 109 (18): 4017–4029. doi:10.1021/jp050461c. PMID 16833724. 

External links


 
 
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photoisomer (physical chemistry)
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American Heritage Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 1994-2012 Encyclopædia Britannica, Inc. All rights reserved.  Read more
McGraw-Hill Science & Technology Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2012, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
 Oxford Dictionary of Biochemistry. Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.  Read more
Random House Word Menu. © 2010 Write Brothers Inc. Word Menu is a registered trademark of the Estate of Stephen Glazier. Write Brothers Inc. All rights reserved.  Read more
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