1 Dissociation (Chemistry)
Dissociation (chemistry)
From Wikipedia, the free encyclopedia
Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes,
or salts) separate or split into smaller particles, ions, or radicals, usually in a reversible manner. For
instance, when a Bronsted-Lowry acid is put in water, a covalent bond between an electronegative atom
and a hydrogen atom is broken by heterolytic fission, which gives a proton and a negative ion.
Dissociation is the opposite of association and recombination. The process is frequently confused
with ionization.
Contents

1 Dissociation constant

2 Dissociation degree

3 Salts

4 Gases

5 Acids
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6 Fragmentation
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7 Receptors
Dissociation constant
For reversible dissociations in a chemical equilibrium
AB
A+B
the dissociation constant Kd is the ratio of dissociated to undissociated compound
where the brackets denote the equilibrium concentrations of the species.
Dissociation degree
The dissociation degree is the fraction of original solute molecules that have dissociated. It is usually
indicated by the Greek symbol α. There is a simple relationship between this parameter and the van 't
Hoff factor i. If the solute substance dissociates into n ions, then
i = 1 + α(n − 1)
For instance, for the following dissociation
KCl
+
K + Cl
-
2 Dissociation (Chemistry)
As n = 2, we would have that i = 1 + α
Salts
The dissociation of salts by solvation in a solution like water means the separation of
the anions and cations. The salt can be recovered by evaporation of the solvent.
An electrolyte refers to a substance that contains free ions and can be used as an electrically conductive
medium. Most of the solute does not dissociate in a weak electrolyte whereas in a strong electrolyte a
higher ratio of solute dissociates to form free ions.
A weak electrolyte is a substance whose solute exists in solution mostly in the form of molecules, with
only a small fraction in the form of ions. Simply because a substance does not readily dissolve does not
+
make it a weak electrolyte. Acetic acid (CH3COOH) and ammonium (NH4 ) are good examples. Acetic
acid is extremely soluble in water, but most of the compound dissolves into molecules, rendering it a
weak electrolyte. Weak bases and weak acids are generally weak electrolytes. In an aqueous solution
-
+
there will be some CH3COOH and some CH3COO and H .
A Strong electrolyte is a solute that exists in solution completely or nearly completely as ions. Again, the
strength of an electrolyte is defined as the percentage of solute that is ions, rather than molecules. The
higher the percentage, the stronger the electrolyte. Thus, even if a substance is not very soluble, but what
does dissolve dissociates completely into ions, the substance is defined as a strong electrolyte. Similar
logic applies to a weak electrolyte. Strong acids and bases are good examples such as HCl, H2SO4,
NaCl. These will all exist as ions in an aqueous media.
Gases
The degree of dissociation in gases is denoted by the symbol α where α refers to the percentage of gas
molecules which dissociate. Various relationships between Kp and α exist depending on the stoichiometry
of the equation. The example of dinitrogen tetroxide (N2O4) dissociating to nitrogen dioxide (NO2) will be
taken.
N2O4
2NO2
If the initial concentration of dinitrogen tetroxide is 1 mole per litre, this will decrease by α at equilibrium
giving, by stoichiometry, 2α moles of NO2. The equilibrium constant (in terms of pressure) is given by
the equation;
3 Dissociation (Chemistry)
Where p represents the partial pressure. Hence, through the definition of partial pressure and using
pT to represent the total pressure and x to represent the mole fraction;
The total number of moles at equilibrium is (1-α)+(2α) which is equivalent to 1+α. Thus, substituting the
mole fractions with actual values in term of alpha and simplifying;
This equation is in accordance with Le Chatelier's Principle. Kp will remain constant with temperature. The
addition of pressure to the system will increase the value of pT so α must decrease to keep Kp constant. In
fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of
dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure is proportional
to number of moles) hence decreasing the extent of dissociation α.
Acids
+
The dissociation of acids in a solution means the split-off of a proton H . This is an equilibrium process,
meaning that dissociation and recombination takes place at the same time. The acid dissociation
constant Ka is an indicator of the acid strength: stronger acids have a higher Ka value (and a lower
pKa value).
Fragmentation
Fragmentation of a molecule can take place by a process of heterolysis or homolysis
Receptors
Receptors are proteins that bind small ligands. The dissociation constant Kd is used as indicator of
the affinity of the ligand to the receptor. The higher the affinity of the ligand for the receptor the lower
the Kd value (and the higher the pKd value).