Dissociation constant

In chemistry, the dissociation constant  K _d is a measure of where an equilibrium is established in a dissociation reaction. It indicates "on which side" of the reaction the equilibrium lies or in which form (dissociated or undissociated) the substance is preferably present: the greater K _d (the dissociation constant), the further the equilibrium lies in the dissociated form. It is assumed that this is an ideal solution, i.e. that it is dilute enough that the forces of attraction of the particles can be neglected. If you want to use the law of mass action for real solutions (> 0.1  molar ), you have to work with a correction factor. If the binding molecule is an inhibitor , the dissociation constant is also referred to as the inhibition constant .

${\displaystyle {\ce {AB <=> A + B}}}$

The dissociation constant is a special case of the equilibrium constant from the law of mass action :

${\displaystyle K_{d}={\frac {c(A)\cdot c(B)}{c(AB)}}={\frac {k_{1}}{k_{-1}}}}$

With

• the concentrations c (A), c (B), and c (AB) of substances A, B and AB in mol per liter (mol / l)
• the rate constants for the dissociation reaction as a forward reaction and for the association reaction as a reverse reaction. ${\displaystyle k_{1}}$ ${\displaystyle k_{-1}}$

Instead of K _d , it is also possible to specify a degree of dissociation α in percent. If this is done with acids, we speak of weak acids if α <1%, of medium-strength acids if α> 1% and of strong acids if α ≈ 100%. The values ​​refer to a one-molar solution.

In reactions in solutions , the dissociation constant in the thermodynamic sense is practically only dependent on the temperature. Theoretically, it is also influenced by pressure, but this only plays a role in the case of gases.

Example: thermal dissociation of dinitrogen tetroxide

The thermal decomposition of dinitrogen tetroxide to nitrogen dioxide is an equilibrium reaction :

${\displaystyle {\ce {N2O4<=>2\,NO2}}}$

It is often demonstrated as a demonstration experiment, as equilibrium is established quickly enough and changes in a moderate temperature range. An ampoule filled with dinitrogen tetroxide / nitrogen dioxide shows the equilibrium with red-brown nitrogen dioxide in warm water; in ice water, discoloration occurs as a result of the equilibrium shift towards colorless dinitrogen tetroxide.

The dissociation constant can be reproduced here using the partial pressures proportional to the concentration :

${\displaystyle K_{d}={p^{2}(NO_{2}) \over p(N_{2}O_{4})}}$

The value of the dissociation constant depends significantly on the temperature.

T in ° C	0	8.7	25th	35	45	50	86.5	101.5	130.8
K d in atm	0.0177	0.0374	0.147	0.302	0.628	0.863	7.499	16.18	59.43

See also

• PH value
• Acid constant

Individual evidence

1. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 196.
2. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 197.
3. ↑ ^{a b} J. Chao; RC Wilhoit; BJ Zwolinski: Gas phase chemical equilibrium in dinitrogen trioxide and dinitrogen tetroxide in Thermochim. Acta  10 (1974) 359-371, doi: 10.1016 / 0040-6031 (74) 87005-X .