Relaxation method

Relaxation methods (or relaxation processes ) are used in chemistry to study the kinetics of very fast reactions . These were first introduced around 1950 by Manfred Eigen . In 1967 the Nobel Prize in Chemistry was awarded to Manfred Eigen, Ronald George Wreyford Norrish and George Porter for developing this process .

The temporal course (the “kinetics”) of chemical reactions with characteristic reaction times in the millisecond range or less can hardly be observed with conventional methods such as the mixing process ; relaxation methods can be used instead. The reactants are first mixed, after which a reaction equilibrium is quickly established. Then the equilibrium of the system is disturbed by a sudden change in a physical quantity. After this sudden change, the system strives to reach the new equilibrium, i.e. to relax (with a speed characteristic of the respective process). The establishment of the new equilibrium can be followed by suitable means (e.g. spectroscopically ).

Concentration course

If the shift in the equilibrium is small, the establishment of the new equilibrium always follows a law of the first order (regardless of the actual kinetics of the back and forth reaction !). The rate constant of the examined reaction can therefore be determined from the measurement results.

If the concentration of a characteristic of the balance component at the time to move, then for the shift at the time (the deviation from equilibrium unless low, the "jump" was thus not too large): ${\ displaystyle t = 0}$ ${\ displaystyle \ Delta c_ {0}}$ ${\ displaystyle t}$

{\ displaystyle \ Delta c (t) = \ Delta c_ {0} \ cdot e ^ {- t / \ tau}}

Where is the relaxation time of the system. The speed coefficients of the forward and reverse reactions of extremely fast processes can be calculated from. ${\ displaystyle \ tau}$ ${\ displaystyle \ tau}$

Processes used

In principle, physical quantities that can be changed abruptly and on which the equilibrium constant of the system depends are suitable for the relaxation method . Examples of sizes used are: ${\ displaystyle K}$

The temperature : Very rapid temperature jumps can be generated, for example, by strong electrical discharges through the reaction mixture; this process is known as the temperature jump method.

The pressure : Rapid changes in pressure, for example due to the reaction space bursting into a larger reaction space, are used in the so-called pressure jump method.

The electric field : Sudden field changes, for example by switching off a voltage source in an electrolyte, are used in the field jump method .

literature

Walter J. Moore: Fundamentals of physical chemistry . Walter de Gruyter, Berlin New York 1990, ISBN 3-11-009941-1 , p. 331–334 (English: Basic physical chemistry . Translated by Wolfgang Paterno). ( limited preview in Google Book search)

Peter W. Atkins, Julio de Paula: Short textbook physical chemistry . 4th completely revised edition. Wiley-VCH, Weinheim 2008, ISBN 3-527-31807-0 , pp. 477-481 (English: Elements of physical chemistry . Translated by Ralf Ludwig, Andreas Appelhagen). ( limited preview in Google Book search)

Manfred Eigen: Methods for investigation of ionic reactions in aqueous solutions with half-times as short as 10 ⁻⁹ sec. Application to neutralization and hydrolysis reactions . In: Discussions of the Faraday Society . tape 17 , 1954, pp. 194-205 , doi : 10.1039 / DF9541700194 .

Manfred Eigen: Immeasurably Fast Reactions . Nobel Lecture, December 11, 1967. In: Nobel Lectures, Chemistry 1963-1970 . Elsevier Publishing Company, Amsterdam 1972, p. 170-203 ( PDF, 1.3 MiByte ).