The
self-ionization of water (also
autoionization of water, and
autodissociation of water) is the chemical reaction in which two water molecules react to produce a
hydroniumIn chemistry, hydronium is the common name for the aqueous cation , the simplest type of oxonium ion, produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton to the surrounding water...
ion (H
3O
+) and a
hydroxideIn chemistry, hydroxide is the name for the diatomic anion OH−, consisting of oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest diatomic ions known....
ion (OH
−):
- 2 H2O (l) H3O+ (aq) + OH− (aq)
It is an example of autoprotolysis, and relies on the amphoteric nature of water.
Water, however pure, is not a simple collection of H
2O molecules.
The
self-ionization of water (also
autoionization of water, and
autodissociation of water) is the chemical reaction in which two water molecules react to produce a
hydroniumIn chemistry, hydronium is the common name for the aqueous cation , the simplest type of oxonium ion, produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton to the surrounding water...
ion (H
3O
+) and a
hydroxideIn chemistry, hydroxide is the name for the diatomic anion OH−, consisting of oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest diatomic ions known....
ion (OH
−):
- 2 H2O (l) H3O+ (aq) + OH− (aq)
It is an example of autoprotolysis, and relies on the amphoteric nature of water.
Water, however pure, is not a simple collection of H
2O molecules. Even in "pure" water, sensitive equipment can detect a very slight electrical
conductivityThe conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is Siemens per metre ....
of 0.055
µSThe siemens is the SI derived unit of electric conductance. It is equal to inverse ohm. It is named after the German inventor and industrialist Ernst Werner von Siemens, and was previously called the mho. In English, the term siemens is used both for the singular and plural...
·cm
−1. According to the theories of
Svante ArrheniusSvante August Arrhenius was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry...
, this must be due to the presence of ions.
Concentration and frequency
The preceding reaction has a chemical
equilibrium constantFor a general chemical equilibriumthe equilibrium constant can be defined bywhere {A} 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...
of
Keq = ([H
3O
+] [OH
−]) / [H
2O]
2 = 3.23 × 10
−18. So the acidity constant which is K
a = K
eq × [H
2O] = ([H
3O
+] [OH
−]) / [H
2O] = 1.8 × 10
−16. For reactions in water (or diluted aqueous solutions), the molarity (a unit of concentration) of water, [H
2O], is practically constant and is omitted from the acidity constant expression by convention. The resulting equilibrium constant is called the
ionization constant,
dissociation constant, or
self-ionization constant, or ion product
of water and is symbolized by K
w.
- Kw = Ka [H2O] = Keq [H2O]2 = [H3O+] [OH−]
- where
- [H3O+] = molarity of hydrogen or hydronium ion, and
- [OH−] = molarity of hydroxide ion.
At Standard Ambient Temperature and Pressure (SATP), about 25 °C (298 K), K
w = [H
3O
+][OH
−] = 1.0×10
−14. Pure water ionizes or dissociates into equal amounts of H
3O
+ and OH
−, so their molarities are equal:
- [H3O+] = [OH−].
At SATP, the concentrations of hydroxide and hydronium are both very low at 1.0 × 10
−7 mol/L and the ions are rarely produced: a randomly selected water molecule will dissociate within approximately 10 hours. Since the concentration of water molecules in water is largely unaffected by dissociation and [H
2O] equals approximately 56 mol/l, it follows that for every 5.6×10
8 water molecules, one pair will exist as ions. Any solution in which the H
3O
+ and OH
− concentrations equal each other is considered a
neutral solution. Absolutely pure water is neutral, although even trace amounts of impurities could affect these ion concentrations and the water may no longer be neutral.
Kw is sensitive to both pressure and temperature; it increases when either increases.
It should be noted that deionized water (also called DI water) is water that has had most
impurity ions common in tap water or natural water sources (such as Na
+ and Cl
−) removed by means of
distillationDistillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
or some other
water purificationWater purification is the process of removing undesirable chemicals, materials, and biological contaminants from raw water. The goal is to produce water fit for a specific purpose...
method. Removal of
all ions from water is next to impossible, since water self-ionizes quickly to reach equilibrium.
Dependence on temperature and pressure
By definition, p
Kw = −log
10 Kw. At SATP, p
Kw = −log
10 (1.0×10
−14) = 14.0. The value of p
Kw varies with temperature. As temperature increases, p
Kw decreases; and as temperature decreases, p
Kw increases (for temperatures up to about 250 °C). This means that ionization of water typically increases with temperature.
There is also a (usually small) dependence on pressure (ionization increases with increasing pressure). The dependence of the water ionization on temperature and pressure has been well investigated and a standard formulation exists.
Acidity
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
is a
logarithmIn mathematics, the logarithm of a number to a given base is the power or exponent to which the base must be raised in order to produce the number....
ic measure of the acidity (or alkalinity) of an aqueous solution. By definition, pH = −log
10 [H
3O
+]. Since [H
3O
+] = [OH
−] in a neutral solution, by mathematics, for a neutral aqueous solution pH = 7 at SATP.
Self-ionization is the process that determines the
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
of water. Since the concentration of hydronium at SATP (25 °C) is approximately 1.0×10
−7mol/l, the pH of pure liquid water at this temperature is 7. Since
Kw increases as temperature increases, hot water has a higher concentration of hydronium than cold water, but this does not mean it is more acidic, as the hydroxide concentration is also higher by the same amount.
Mechanism
Geissler et al. have determined that electric field fluctuations in liquid water cause molecular dissociation. They propose the following sequence of events that takes place in about 150 fs: the system begins in a neutral state; random fluctuations in molecular motions occasionally (about once every 10 hours per water molecule) produce an electric field strong enough to break an oxygen-hydrogen bond, resulting in a hydroxide (OH
−) and hydronium ion (H
3O
+); the proton of the hydronium ion travels along water molecules by the
Grotthuss mechanismThe Grotthuss Mechanism is the mechanism by which an 'excess' proton or protonic defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation/cleavage of covalent bonds....
; and a change in the hydrogen bond network in the solvent isolates the two ions, which are stabilized by solvation.
Within 1
picosecondA picosecond is 10-12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. The name is formed by the SI prefix pico and the SI unit second. It is abbreviated as ps....
, however, a second reorganization of the hydrogen bond network allows rapid proton transfer down the electric potential difference and subsequent recombination of the ions. This timescale is consistent with the time it takes for hydrogen bonds to reorient themselves in water.
Isotope effects
Heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
, D
2O, self-ionizes less than normal water, H
2O; oxygen forms a slightly stronger bond to
deuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
because the larger mass of deuterium difference results in a lower
zero-point energyIn physics, the zero-point energy is the lowest possible energy that a quantum mechanical physical system may have and is the energy of the ground state. The quantum mechanical system that encapsulates this energy is the zero-point field. The concept was first proposed by Albert Einstein and Otto...
, a quantum mechanical effect. The following table compares the values of p
Kw for H
2O and D
2O.
| T (°C) |
H2O |
D2O |
| 10 |
14.5346 |
15.439 |
| 20 |
14.1669 |
15.049 |
| 25 |
13.9965 |
14.869 |
| 30 |
13.8330 |
14.699 |
| 40 |
13.5348 |
14.385 |
| 50 |
13.2617 |
14.103 |
See also
- Chemical equilibrium
In a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction...
- 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. a pH less than 7.0...
- Base
In chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...
- Acid-base reaction theories
An acid-base reaction is a chemical reaction that occurs between an acid and a base. Several concepts that provide alternative definitions for the reaction mechanisms involved and their application in solving related problems exist...
External links