Dissociation (chemistry)

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Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes, molecules, or salts) separate or split into smaller molecules, ions, or radicals, usually in a reversible manner. 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.

1 Dissociation constant
2 Salts
3 Gases
4 Acids
5 Fragmentation
6 Receptors
7 See also

Dissociation constant

For reversible dissociations in a chemical equilibrium

$AB \leftrightarrow A + B$

the dissociation constant K_d is the ratio of dissociated to undissociated compound

$K_d = \frac{[A][B]}{[AB]}$

where the brackets denote the equilibrium concentrations of the species.

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. See also: Solubility equilibrium

An electrolyte refers to a substance that contains free ions and can be used as an electrically conductive medium. Most of the solute doesnot dissociate in a weak electrolyte whereas in a strong electrolyte a higher ratio of solute dissociates to form free ions.

A Weak Electrolyte does not completely dissociate. Acetic Acid (CH3COOH) and Ammonium (NH3) are good examples. 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 completely dissociates. Stong 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 K_p and α exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide (N₂O₄) dissociating to nitrogen dioxide (NO₂) will be taken.

$N_2O_4 \leftrightarrow 2NO_2$

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;

$K_p = \frac{P(NO_2)^2}{P(N_2O_4)}$

Where P represents the partial pressure. Hence, through the definition of partial pressure and using P_t to represent the total pressure and X to represent the mole fraction;

$K_p = \frac{(P_t)^2(X(NO_2))^2}{(P_t)(X(N_2O_4))} = \frac{(P_t)(X(NO_2))^2}{(X(N_2O_4))}$

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;

$K_p = \frac{(P_t)(4\alpha^2)}{(1+\alpha)(1-\alpha)} = \frac{(P_t)(4\alpha^2)}{(1-\alpha^2)}$

This equation is in accordance with Le Chatelier's Principle. K_p will remain constant with temperature. The addition of pressure to the system will increase the value of P_t so α must decrease to keep K_p 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⁺, see Acid-base reaction theories. This is an equilibrium process, meaning that dissociation and recombination takes place at the same time. The acid dissociation constant K_a is an indicator of the acid strength: stronger acids have a higher K_a value (and a lower pK_a 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 K_d 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 K_d value (and the higher the pK_d value).

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