Vienna effect

The Wien effect (after Max Wien ) describes the behavior of dissolved ions in electrolytes at high electrical field strengths .

overview

In an electrolyte solution, ions are surrounded by oppositely charged ions, such as cations by several anions , which in turn are surrounded by several cations. This environment around the central ion is also called an ion cloud or ion atmosphere.

If there is no electric field , the ion atmosphere is approximately spherically symmetrical , so that the centers of charge coincide. However, if an electric field is applied, the charge distribution is deformed and the ions are attracted to the oppositely charged electrode . So the centers of charge are no longer together, and the resulting electric field slows down the movement of ions, the charge transport and thus the flow of current .

Both Wien's effects cannot be observed separately from one another; rather, they seem to occur simultaneously. The First Vienna Effect, which predominates in the case of weaker fields, changes into the predominant Second Vienna Effect in the case of stronger fields.

First Vienna effect

The Erste Wien effect explains that the conductivity of an electrolyte solution increases again at high field strengths and hardly differs from an infinitely dilute solution:

{\ displaystyle \ Lambda _ {E} \ vert _ {E \ to \ infty} = \ Lambda _ {\ infty}}

With

${\ displaystyle \ Lambda _ {E}}$ : Molar conductivity in the field strength E .
${\ displaystyle \ Lambda _ {\ infty}}$ : molar conductivity at infinite dilution.

The cause is believed to be that the braking effects of the ion atmosphere are eliminated by accelerating the ions so much that the ion atmosphere cannot even form completely or the relaxation time of the ion atmosphere is too long to allow it during the Set the electrolysis process . Typical field strengths for this are above 10,000 V / cm; here the ion velocity is 10 cm / s.

It must be noted that if the voltages at the electrodes are too high, the speeds of the ions are too high, so that Stokes' law of friction must be modified, which is included in the calculation.

Second Vienna effect

The second Wien effect describes the behavior of the electrolysis of weak electrolytes in a strong field. Theoretically, the effect was investigated by Lars Onsager .

In a strong field, weak acids and salts (electrolytes) have effects that are about five to ten times larger than that of strong acids. The strength of an acid is related to the acid constant and the associated value. These effects can no longer be explained by ion cloud effects. The effect of increased dissociation of weak acids and salts in strong fields occurs. The effect is based on the increase in the number of ions, i.e. a resulting increase in the ion concentration. ${\ displaystyle pK_ {s}}$

It is either an ionization of the electrolyte molecules by ion impact, or the strong field causes a separation of the ions in the molecules, which are already in a loosened connection due to the movement of heat .

Onsager's theory assumes that the dissociation of a weak electrolyte occurs in two steps:

Breaking of the covalent bond with the formation of a Bjerrum ion pair (see ion association ) ${\ displaystyle HA \ rightleftharpoons (H ^ {+} A ^ {-})}$

Breaking up of the Bjerrum ion pair

{\ displaystyle (H ^ {+} A ^ {-}) \ rightleftharpoons H ^ {+} + A ^ {-}}

The second Wien effect states (according to Onsager) that the dissociation constant for a weak 1.1 electrolyte in the presence of an electric field is given by: ${\ displaystyle {\ vec {E}}}$

{\ displaystyle K_ {E} = K_ {0} \ cdot \ left ({\ sqrt {s / \ pi}} (8b) ^ {- 3/4} \ exp (8b) ^ {1/2} \ right )}

with and the dissociation constant without an electric field.

{\ displaystyle b = {\ frac {e ^ {3} | {\ vec {E}} |} {8 \ pi \ varepsilon _ {0} \ varepsilon _ {r} k _ {\ mathrm {B}} ^ { 2} T ^ {2}}}}

{\ displaystyle K_ {0}}

literature

Hans Falkenhagen : Electrolytes , Leipzig 1932

Individual evidence

^ Karl Willy Wagner: Max Vienna on the 70th birthday. In: The natural sciences. 25, 1937, p. 65, doi : 10.1007 / BF01493271 .
^ Max Wien: (1) Ann. Physics. 85: 795 (1928); (2) Phys. Line 29, 751 (1928); (3) Ann. Physics. 1,400 (1929); (4) Phys. Z. 32, 545 (1931); (5) J. Malsch and M. Wien, Ann. Physics. 83, 305 (1927).
^ Walter J. Moore, Dieter O. Hummel: Physikalische Chemie . Ed .: Dieter O. Hummel. 4th edition. Walter de Gruyter, 1986, ISBN 3-11-010979-4 , p. 561 .
^ Walter J. Moore, Dieter O. Hummel: Physikalische Chemie . Ed .: Dieter O. Hummel. 4th edition. Walter de Gruyter, 1986, ISBN 3-11-010979-4 , p. 562 .

[1] Karl Willy Wagner: Max Vienna on the 70th birthday. In: The natural sciences. 25, 1937, p. 65, doi : 10.1007 / BF01493271 .

[2] Max Wien: (1) Ann. Physics. 85: 795 (1928); (2) Phys. Line 29, 751 (1928); (3) Ann. Physics. 1,400 (1929); (4) Phys. Z. 32, 545 (1931); (5) J. Malsch and M. Wien, Ann. Physics. 83, 305 (1927).

[3] Walter J. Moore, Dieter O. Hummel: Physikalische Chemie . Ed .: Dieter O. Hummel. 4th edition. Walter de Gruyter, 1986, ISBN 3-11-010979-4 , p. 561 .

[4] Walter J. Moore, Dieter O. Hummel: Physikalische Chemie . Ed .: Dieter O. Hummel. 4th edition. Walter de Gruyter, 1986, ISBN 3-11-010979-4 , p. 562 .