Physical chemistry

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The physical chemistry (in short: PC or . Phys.Chem , including: physical chemistry ) in addition to the inorganic and organic chemistry of the "classical" branches of chemistry . It deals with the border area between physics and chemistry, in particular the application of methods of physics to objects of chemistry, which is why the term chemical physics is sometimes used. While preparative chemistry focuses on the methodology of the chemical synthesis of known and new substances, physical chemistry tries to describe the properties of substances and their transformation with the help of theoretical and experimental methods, with the aim of creating generally applicable mathematical processes for all relevant processes Set up formulas with clearly defined units and exact numerical values.

Naturally, there is a close proximity to physics (especially to molecular physics ), and the classification of a research topic as “physics” or “chemistry” is often not very clear. Nevertheless, depending on the focus, a distinction is sometimes made between physical chemistry and chemical physics. Physical chemistry provides the theoretical basis for technical chemistry and process engineering . Chemists who work primarily in the field of physical chemistry are known as physical chemists. Physical chemistry is a compulsory part of every chemistry degree .

history

Svante Arrhenius (1909)

The first courses on subjects from physical chemistry were held around 1752 at the Lomonossow University in Moscow by Michail Lomonossow . In 1890, Svante Arrhenius , Jacobus Henricus van 't Hoff , Wilhelm Ostwald and Walther Nernst first introduced physical chemistry as an independent subject at universities. Josiah Willard Gibbs is considered the founder of physical chemistry in the Anglo-Saxon region with his article "On the Equilibrium of Heterogeneous Substances" published in 1867, in which he developed the basic concepts of free energy , chemical potential and phase rule . The work of Gibbs, Robert Mayer , Hermann Helmholtz , Jacobus Henricus van 't Hoff formed an important link between the concept of energy from a chemical point of view for Wilhelm Ostwald.

Gustav Wiedemann received the first German chair for physical chemistry in Leipzig in 1871 . It was not until 1887 that physical chemistry could manifest itself in research after the chair was filled with Wilhelm Ostwald . Ostwald became the first editor of the journal for physical chemistry, stoichiometry and kinship theory, which was founded in 1887 together with van 't Hoff .

At the suggestion of his pupil Walther Nernst, other institutes specifically dedicated to physical chemistry followed in rapid succession in Göttingen (1891), Dresden (1900), Karlsruhe (1900), Berlin (1905), Aachen (1906), Breslau (1910) and elsewhere .

Wilhelm Ostwald founded the German Electrochemical Society in 1894 , which was renamed the German Bunsen Society for Applied Physical Chemistry in 1901 . In England, the Faraday Society (now the Faraday Division of the Royal Society of Chemistry ) was founded in 1903 . In the meantime, countless university and several Max Planck institutes are concerned with physical chemistry.

An overview article by the Bunsen Society gives a detailed overview of the origin and development of physical chemistry. Further details can be found under History of Chemistry , a list of important physical chemists at all German universities can be found here .

Sub-areas

Physical chemistry is divided into different sub-areas in which different phenomena are examined. The most important are theoretical chemistry , thermodynamics , kinetics , spectroscopy and electrochemistry .

Theoretical chemistry

In theoretical chemistry one tries to predict the properties of single molecules or macroscopic amounts of substance with the help of mathematics or computer simulations and calculations. The quantum mechanics provides the basis for understanding the structure of matter and the chemical bond , while the statistical thermodynamics provides the interconnection with the macroscopic thermodynamics.

Chemical thermodynamics

Chemical thermodynamics unified the energy terms of electrochemical work ( electromotive force ), thermal energy by increasing the temperature of a substance, work with gas expansion ( steam engine , combustion engine ) and heat energy with the conversion of substances ( enthalpy , e.g. combustion of coal or gasoline).

Chemical thermodynamics also enables statements to be made as to whether material conversions are possible, which energies develop or have to be added during a reaction, which substance concentrations are to be expected with regard to products for starting materials (starting materials) according to the law of mass action , whether an increase in temperature or pressure promotes or attenuates which redox potential or which ion concentrations of individual substances is to be expected.

Behavior of gases with changes in temperature, volume and pressure

When the temperature changes and the external pressure remains constant, the volume of a gas changes proportionally to the temperature change ( Gay-Lussac law ). When the temperature rises , the gas expands ; when it cools, it contracts. In the case of ideal gases, the volume is proportional to the absolute temperature ; This assumption is often a good approximation even for real gases . If a gas is compressed under high pressure, the temperature and internal energy of the gas rise. This internal energy of a gas can also give work by the gas expanding. This process was used, for example, to drive steam engines. If you expand a gas very quickly in a cylinder with a piston to a larger volume, the gas cools down. This process is used, for example, in refrigerators or air liquefaction systems .

In a steam engine, only a certain part of the thermal energy is converted into mechanical energy. The heat energy becomes work, but the total energy of a closed system does not change. The quotient of the share of thermal energy that is released unused into the environment during this process to the temperature is called entropy . The discharge of a gas into a vacuum is also associated with an increase in entropy; the process does not take place voluntarily in the opposite direction.

Chemical conversions of substances

Chemical conversions of substances, changes in the physical state or the dissolution of salts or concentrated acids or bases in water are often associated with a heat release or absorption. Chemists used to believe that heat generation is the basis for chemical reactions between substances to occur. However, reactions in which cooling occurred were also found later. Scientists recognized that in the case of material conversions with a decrease in heat, the entropy for chemical processes had to play an important role.

The amount of energy of each metabolism can be related to a mole of substance so that the results can be compared. When 12 g of carbon are burned to form 48 g of carbon dioxide , a different amount of heat ( enthalpy ) is released than when 12 g of carbon are burned to form 28 g of carbon monoxide . Each material compound can be assigned a certain amount of energy (standard enthalpy of formation) based on the measured thermal energies - in relation to one mole. Unknown amounts of energy, e.g. B. the formation of carbon dioxide from carbon monoxide and oxygen can be determined by a total. From the knowledge of the standard enthalpies of formation, the chemist can determine how much heat energy is required for a substance conversion or is released during a reaction.

When hydrogen gas and oxygen gas are burned , water and thermal energy are produced. At the same time, the gas volume is reduced. The gas reduction in this reaction is an energy quantity ( entropy ), the energy content of which results from the change in the gas volume as described above. The standard entropy of formation can also be determined for the majority of substances. In terms of energy, the standard entropy of formation can be determined by multiplying it by the absolute temperature (in K). Standard enthalpy of formation and entropy of formation are linked by the free enthalpy. If one forms the differences from the free enthalpy of the end products to the starting materials, one obtains the free enthalpy of reaction . The free enthalpy of reaction must always be negative so that a reaction is possible; if it is positive, the chemical reaction is impossible.

Law of mass action

The law of mass action - or more precisely the chemical equilibrium with the equilibrium constant K - describes the multiplicative link between the concentrations of the products and the starting materials. The free enthalpy of reaction is linked to the equilibrium constant of the law of mass action by a simple formula. If the free enthalpy of reaction is negative, the products are mainly formed in equilibrium from the starting materials; if the enthalpy of reaction is positive, almost no conversion takes place. The equilibrium of a chemical conversion can often be changed by changes in temperature or pressure. Sometimes, however, catalysts are also needed to achieve the desired equilibrium.

Before the development of the Haber-Bosch process for the production of ammonia, it was known from thermodynamics that the formation of ammonia from hydrogen and nitrogen should be possible. For a long time, however, the formation failed, only with catalysts and under higher temperatures and pressure did the reaction proceed as desired. The pressure was necessary to compensate for the decrease in entropy; although a high temperature had a negative effect on the entropy, it was advantageous for the catalytic activation.

A particularly important law, the van 't Hoff equation , describes the change in equilibrium as a function of the change in temperature. Also, solubility of inorganic and organic salts in water and other liquids can be produced from the free reaction and calculate the law of mass action. In redox reactions , the Nernst equation provides a way of calculating the concentrations of ions or the electromotive potentials (for example of potassium permanganate in acidic, neutral and basic solutions).

kinetics

The kinetics is concerned with the temporal sequence of chemical reactions (reaction kinetics) or of transport processes (eg. B. diffusion , material deposition on surfaces, catalysis ). In kinetics, both the macroscopic course of a reaction (macrokinetics) and the exact course of a reaction in the individual elementary reactions (microkinetics) are examined.

Spectroscopy

Spectroscopy is a collective term for a class of experimental procedures that investigate how a sample can absorb or release energy in the form of electromagnetic radiation (radio waves, microwaves, infrared, visible light, UV, X-rays). The aim of spectroscopy is to draw conclusions about the sample from the spectrum obtained, for example about its internal structure ( intermolecular force ), material composition or dynamics.

Electrochemistry

The Electrochemistry deals with the properties of charged particles, especially ions and the effect of electric current on fabrics. The most important areas of investigation in electrochemistry are the processes in mostly aqueous solutions of ions, electrolytes and on electrodes . The interaction of these processes is crucial in display and refining electrolysis, corrosion processes and the field of electricity storage in batteries and accumulators . Other technically important applications of electrochemistry are fuel cells and the deposition of metals on surfaces in electroplating .

Relevance in technology and in everyday life

Physical chemistry deals with many objects that have great application potential or are of crucial importance for the quality of life of mankind.

literature

General textbooks

Physico-chemical journals

Journal articles

  • Paul Harteck: The quantum theory in chemistry . In: Natural Sciences . tape 38 , no. 3 , 1951, pp. 61–67 , doi : 10.1007 / BF00589913 (lecture given at the meeting of the Society of German Natural Scientists and Doctors on October 23, 1950 in Munich).

Web links

Wikibooks: Physical chemistry formulas  - learning and teaching materials
Wiktionary: physical chemistry  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ University of Leipzig Physical Chemistry, 1887 in the "Second Chemical Institute", Brüderstr. 34, and in 1898 in the new "Ostwald Institute for Physical and Theoretical Chemistry", Linnestr. 2
  2. ↑ University of Göttingen 1891 in the Physikalisches Institut, Michaelishaus am Leinekanal, and in 1896 as the new “Inst. for physical. Chemistry"
  3. ^ TH Dresden 1900 as "Electrochemical Laboratory"
  4. ^ TH Karlsruhe 1900 as "Inst. for physical. Chemistry"
  5. ↑ University of Berlin in the II. Chemical Institute (see under "History"), 1905 as "Physikalisch-Chemisches Institut"
  6. ^ TH Aachen 1897 as "Elektrochemie", 1906 with the chair as "Theoretical Metallurgy and Physical Chemistry"
  7. ^ TH Breslau 1910 as “Inst. for phys. Chemistry"; Manfred Rasch:  Schenck, Friedrich Rudolf. In: New German Biography (NDB). Volume 22, Duncker & Humblot, Berlin 2005, ISBN 3-428-11203-2 , p. 667 f. ( Digitized version ).
  8. ↑ 100 Years of Physical Chemistry in Aachen ( Memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.9 MB); 100 years of physical chemistry in Karlsruhe (PDF; 109 kB) @1@ 2Template: Webachiv / IABot / www.bunsen.de
  9. ↑ Manfred Zeidler 100 Years of Physical Chemistry at RWTH Aachen University, p. 91 ( Memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.9 MB) @1@ 2Template: Webachiv / IABot / www.bunsen.de
  10. ↑ Institutes for physical chemistry in Germany and Austria ( Memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.bunsen.de
  11. ↑ Overview of all chairs and departments for physical chemistry ( memento of the original from October 29, 2013 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 9 MB) @1@ 2Template: Webachiv / IABot / bunsen.de

See also