- المجال: Chemistry
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The International Union of Pure and Applied Chemistry (IUPAC) serves to advance the worldwide aspects of the chemical sciences and to contribute to the application of chemistry in the service of people and the environment. As a scientific, international, non-governmental and objective body, IUPAC ...
The recombination of a pair of ions R<sup>+</sup> and Z<sup>-</sup> formed from ionization of RZ.
If the ions are paired as a tight ion pair and recombine without prior separation into a loose ion pair this is called "internal ion-pair return":
<center>R<sup>+</sup>Z<sup>-</sup> (tight ion pair) ⇌ RZ (covalent molecule)</center>
It is a special case of "primary geminate recombination".
If the ions are paired as a loose ion pair and form the covalent chemical species via a tight ion pair, this is called "external ion-pair return":
<center>R<sup>+</sup>
Industry:Chemistry
In a solution of fully dissociated electrolytes the ionic strength is defined as I = 0.5Σ<sub>i</sub>c<sub>i</sub>Z<sub>i</sub><sup>2</sup>, in which c<sub>i</sub> is the concentration and Z<sub>i</sub> the charge number of ionic species i. μ is also defined as I<sub>m</sub> = 0.5Σ<sub>i</sub>m<sub>i</sub>Z<sub>i</sub><sup>2</sup>, where m<sub>i</sub> is the molality.
Industry:Chemistry
The generation of one or more ions. It may occur, e.g. by loss of an electron from a neutral molecular entity, by the unimolecular heterolysis of such an entity into two or more ions, or by a heterolytic substitution reaction involving neutral molecules, such as
<center>CH<sub>3</sub>CO<sub>2</sub>H + H<sub>2</sub>O → H<sub>3</sub>O<sup>+</sup> + CH<sub>3</sub>CO<sub>2</sub><sup>-</sup>
Ph<sub>3</sub>CCl + AlCl<sub>3</sub> → Ph<sub>3</sub>C<sup>+</sup> + AlCl<sub>4</sub><sup>- </sup>(electrophile-assisted)
Ph<sub>3</sub>CCl → Ph<sub>3</sub>C<sup>+</sup> Cl<sup>-</sup>(ion pair, in benzene)</center>
The loss of an electron from a singly, doubly, etc. charged cation is called second, third, etc. ionization. This terminology is used especially in mass spectroscopy.
Industry:Chemistry
The minimum energy required to remove an electron from an isolated molecular entity (in its vibrational ground state) in the gaseous phase. If the resulting molecular entity is considered to be in its vibrational ground state, one refers to the energy as the "adiabatic ionization energy". If the molecular entity produced possesses the vibrational energy determined by the Franck-Condon principle (according to which the electron ejection takes place without an accompanying change in molecular geometry), the energy is called the "vertical ionization energy". The name ionization energy is preferred to the somewhat misleading earlier name "ionization potential".
Industry:Chemistry
A term to denote the tendency of a particular solvent to promote ionization of an uncharged or, less often, charged solute. The term has been used both in a kinetic and in a thermodynamic context.
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A reaction (actual or hypothetical) in which the types of bonds that are made in forming the products are the same as those which are broken in the reactants, e.g.
<center>PhCOOH + p-ClC<sub>6</sub>H<sub>4</sub>COO<sup>-</sup> → PhCOO<sup>-</sup> + p-ClC<sub>6</sub>H<sub>4</sub>COOH
ClCH=CH<sub>2</sub> + ClCH<sub>2</sub>CH<sub>2</sub>Cl → CH<sub>2</sub>=CH<sub>2</sub> + Cl<sub>2</sub>CHCH<sub>2</sub>Cl </center>
Such processes have advantages for theoretical treatment. The Hammett equation as applied to equilibria (cf. (a)) essentially deals with isodesmic processes.
Industry:Chemistry
Two or more molecular entities are described as isoelectronic if they have the same number of valence electrons and the same structure, i.e. number and connectivity of atoms, but differ in some of the elements involved. Thus
<center>CO, N<sub>2</sub> and NO<sup>+</sup> are isoelectronic
CH<sub>2</sub>=C=O and CH<sub>2</sub>=N=N are isoelectronic</center>
CH<sub>3</sub>COCH<sub>3</sub> and CH<sub>3</sub>N=NCH<sub>3</sub> have the same number of electrons, but have different structures, hence they are not described as isoelectronic.
Industry:Chemistry
A reaction series is said to be isoentropic if the individual reactions of the series have the same standard entropy of activation.
Industry:Chemistry
A relationship analogous to the isokinetic relationship but applied to equilibrium data. The equation defining the isoequilibrium temperature β is
<center>Δ<sub>r</sub>H - βΔ<sub>r</sub>S = constant</center>
where ΔH and ΔS are enthalpy and entropy of reaction, respectively.
Industry:Chemistry
When a series of structurally related substrates undergo the same general reaction or when the reaction conditions for a single substrate are changed in a systematic way, the enthalpies and entropies of activation sometimes satisfy the relation
<center>Δ<sup>†</sup>H - βΔ<sup>†</sup>S = constant</center>
where the parameter β is independent of temperature. This equation (or some equivalent form) is said to represent an "isokinetic relationship". The temperature T = β (at which all members of a series obeying the isokinetic relationship react at the same rate) is termed the "isokinetic temperature".
Supposed isokinetic relationships as established by direct correlation of Δ<sup>†</sup>H with Δ<sup>†</sup>S are often spurious and the calculated value of β is meaningless, because errors in Δ<sup>†</sup>H lead to compensating errors in Δ<sup>†</sup>S. Satisfactory methods of establishing such relationships have been devised.
Industry:Chemistry
