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Dissociation (chemistry)
 
 
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Dissociation in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 and biochemistry
Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
 is a general process in which ionic compounds (complexes
Complex (chemistry)

In chemistry, a complex, also called a "coordination compound" or "metal complex", is a structure consisting of a central atom or molecule connected to surrounding atoms or molecules....
, molecules, or salts) separate or split into smaller molecules, ions, or radicals
Radical (chemistry)

In chemistry, radicals are atoms, molecules or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly chemical reaction, so radicals are likely to take part in chemical reactions....
, usually in a reversible manner. When a Bronsted-Lowry acid
Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
 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
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
. Dissociation is the opposite of association and recombination.






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Dissociation in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 and biochemistry
Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
 is a general process in which ionic compounds (complexes
Complex (chemistry)

In chemistry, a complex, also called a "coordination compound" or "metal complex", is a structure consisting of a central atom or molecule connected to surrounding atoms or molecules....
, molecules, or salts) separate or split into smaller molecules, ions, or radicals
Radical (chemistry)

In chemistry, radicals are atoms, molecules or ions with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly chemical reaction, so radicals are likely to take part in chemical reactions....
, usually in a reversible manner. When a Bronsted-Lowry acid
Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
 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
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
. Dissociation is the opposite of association and recombination. The process is frequently confused with ionization
Ionization

Ionization is the physics process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions....
.

Dissociation constant


For reversible dissociations in a chemical equilibrium
Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
the dissociation constant
Dissociation constant

In chemistry and biochemistry, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate reversibly into smaller components, as...
 Kd is the ratio of dissociated to undissociated compound


where the brackets denote the equilibrium concentrations of the species.

Salts


The dissociation of salts by solvation
Solvation

Solvation, commonly called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute....
 in a solution
Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
 like water
Water (molecule)

File:Blue-water-pool.jpgWater is the most abundant molecule on Earth's surface, constituting about 70% of the Earth's surface in liquid, solid, and gaseous states....
 means the separation of the anions and cations. The salt can be recovered by evaporation
Evaporation

Evaporation is the slow vaporization of a liquid and the reverse of condensation. A type of phase transition, it is the process by which molecules in a liquid State of matter spontaneously become gaseous ....
 of the solvent. See also: Solubility equilibrium
Solubility equilibrium

Solubility equilibrium is any type chemical equilibrium between solid and dissolved states of a compound at saturation .Solubility equilibria involve application of chemical principles and constants to predict solubility of substances under specific conditions ....


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 comletely 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 a where a refers to the percentage of gas molecules which dissociate. Various relationships between Kp and a exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide
Dinitrogen tetroxide

Dinitrogen tetroxide is the chemical compound N2O4. It forms an Chemical equilibrium with nitrogen dioxide; some call this mixture dinitrogen tetroxide, some call it nitrogen dioxide....
 (N2O4) dissociating to nitrogen dioxide
Nitrogen dioxide

Nitrogen dioxide is the chemical compound with the chemical formula NitrogenOxygen2. One of several nitrogen oxides, NO2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year....
 (NO2) will be taken.

If the initial concentration of dinitrogen tetroxide is 1 mole per litre, this will decrease by a at equilibrium giving, by stoichiometry, 2a moles of NO2. The equilibrium constant
Equilibrium constant

For a general chemical equilibriumthe equilibrium constant can be defined bywhere is the activity of the chemical species A etc . It is conventional to put the activities of the products in the numerator and those of the reactants in the denominator....
 (in terms of pressure) is given by the equation;

Where P represents the partial pressure. Hence, through the definition of partial pressure
Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
 and using Pt to represent the total pressure and X to represent the mole fraction;

The total number of moles at equilibrium is (1-a)+(2a) which is equivalent to 1+a. 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 a 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 a.

Acids


The dissociation of acids in a solution means the split-off of a proton
Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
 H+, see Acid-base reaction theories
Acid-base reaction theories

An acid-base reaction is a chemical reaction that occurs between an acid and a base . Several concepts exist which provide alternative definitions for the reaction mechanisms involved and their application in solving related problems....
. This is an equilibrium process, meaning that dissociation and recombination takes place at the same time. The acid dissociation constant
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
 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
Heterolysis

In chemistry, heterolysis or heterolytic fission is chemical bond cleavage of a neutral molecule generating a cation and an anion. In this process the two electrons that make up the bond are assigned to the same fragment....
 or homolysis
Homolysis

In chemistry, homolysis or homolytic fission is chemical bond dissociation of a neutral molecule generating two free radicals. That is, two electrons that are involved in the bond are distributed one by one to the two species....


Receptors


Receptors
Receptor (biochemistry)

In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or cytoplasm of a cell, to which a mobile signaling molecule may attach....
 are protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s that bind small ligands. The dissociation constant Kd is used as indicator of the affinity
Affinity

Affinity, in etymology affinity is the opposite of infinity . These two words have the same root coming from the Latin: finis = end....
 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).

See also

  • Ionization
    Ionization

    Ionization is the physics process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions....
  • Acid
    Acid

    An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
  • Base (chemistry)
    Base (chemistry)

    In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
  • Thermal decomposition
    Thermal decomposition

    Thermal decomposition, also called thermolysis, is defined as a chemical reaction when a chemical substance breaks up into at least two chemical substances when heated....