Self-ionization of water

Acids and Bases
Acid dissociation constant
Acid-base extraction
Acid–base reaction
Acid–base titration
Dissociation constant
Acidity function
Buffer solutions
pH
Proton affinity
Self-ionization of water
Acid types
Brønsted · Lewis · Mineral
Organic · Strong
Superacids · Weak
Base types
Brønsted · Lewis · Organic
Strong · Superbases
Non-nucleophilic · Weak

The self-ionization of water (also autoionization of water, and autodissociation of water) is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−. It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

Contents

Concentrations

Chemically pure water has an electrical conductivity of 0.055 µS·cm−1. According to the theories of Svante Arrhenius, this must be due to the presence of ions. The ions are produced by the self-ionization reaction

H2O + H2O H3O+ + OH−

This equilibrium applies to pure water and any aqueous solution. The chemical equilibrium constant, Keq, for this reaction is given by

K_{eq} = \frac{[H_3O^%2B] [OH^-]}{[H_2O]^2}

If the concentration of dissolved solutes is not very high, the concentration [H2O] can be taken as being constant at ca. 55.5M.[1] The ionization constant, dissociation constant, self-ionization constant, or ionic product of water, symbolized by Kw is given by

K_w=[H_3O^%2B][OH^-] = K_{eq} \times [H_2O]^2

where [H3O+] is the concentration of hydrogen or hydronium ion, and [OH−] is the concentration of hydroxide ion. At 25 °C Kw is approximately equal to 1.0×10−14. Water molecules dissociate into equal amounts of H3O+ and OH−, so their concentrations are equal to ca. 1.0 × 10−7 mol dm−3. A solution in which the H3O+ and OH− concentrations equal each other is considered a neutral solution. Pure water is neutral, but most water samples contain impurities. If an impurity is an acid or base this will affect the concentrations of hydronium ion and hydoxide ion. Water samples which are exposed to air will absorb the acid carbon dioxide and the concentration of H3O+ will increase. The concentration of OH- will decrease in such a way that the product [H3O+][OH-] remains constant.

Dependence on temperature, pressure and ionic strength

The dependence of the water ionization on temperature and pressure has been investigated thoroughly.[2] The value of pKw decreases as temperature increases from the melting point of ice to a minimum at ca. 250 °C, after which it increases up to the critical point of ca. 374 °C. It decreases with increasing pressure.

With electrolyte solutions, the value of pKw is dependent on ionic strength of the electrolyte. Values for sodium chloride are typical for a 1:1 electrolyte. With 1:2 electrolytes, MX2, pKw decreases with increasing ionic strength.[3]

Isotope effects

Heavy water, D2O, self-ionizes less than normal water, H2O; oxygen forms a slightly stronger bond to deuterium because the larger mass of deuterium difference results in a lower zero-point energy, a quantum mechanical effect. The following table compares the values of pKw for H2O and D2O.[4]

pKw values for pure water
T/°C 10 20 25 30 40 50
H2O 14.535 14.167 13.997 13.830 13.535 13.262
D2O 15.439 15.049 14.869 14.699 14.385 14.103

Mechanism

The rate of reaction for the dissociation

H2O → H+ + OH-

depends on the activation energy, ΔE‡. According to the Boltzmann distribution the proportion of water molecules that have sufficient energy, due to thermal population, is given by

\frac{N}{N_0} = e^{-\frac{\Delta E^\ddagger}{kT}}

where k is the Boltzmann constant. Thus some dissociation can occur because sufficient thermal energy is available. The following sequence of events has been proposed on the basis of electric field fluctuations in liquid water.[5] Random fluctuations in molecular motions occasionally (about once every 10 hours per water molecule[6]) produce an electric field strong enough to break an oxygen-hydrogen bond, resulting in a hydroxide (OH−) and hydronium ion (H3O+); the proton of the hydronium ion travels along water molecules by the Grotthuss mechanism and a change in the hydrogen bond network in the solvent isolates the two ions, which are stabilized by solvation. Within 1 picosecond, 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 reorientate themselves in water.[7][8][9]

See also

References

  1. ^ McMurry, John. (2004) Organic Chemistry, pg 44
  2. ^ International Association for the Properties of Water and Steam (IAPWS)
  3. ^ Harned, H.S.; Owen,, B.B. (1958). The Physical Chemistry of Electrolytic Solutions (£rd. ed.). New York: Reinhold Publishing Corp.,. pp. 634–649, 752–754. 
  4. ^ Lide, D. R. (Ed.) (1990). CRC Handbook of Chemistry and Physics (70th Edn.). Boca Raton (FL):CRC Press. 
  5. ^ Geissler, P. L.; Dellago, C.; Chandler, D.; Hutter, J.; Parrinello, M. (2001). "Autoionization in liquid water". Science 291 (5511): 2121–2124. doi:10.1126/science.1056991. PMID 11251111. 
  6. ^ Eigen, M.; de Maeyer, L. (1955). "Untersuchungen über die Kinetik der Neutralisation I". Z. Elektrochem. 59: 986. 
  7. ^ Stillinger, F. H. (1975). "Theory and Molecular Models for Water". Adv. Chem. Phys. 31: 1. doi:10.1002/9780470143834.ch1. 
  8. ^ Rapaport, D. C. (1983). "Hydrogen bonds in water". Mol. Phys. 50 (5): 1151. doi:10.1080/00268978300102931. 
  9. ^ Chen, S.-H. & Teixeira, J. (1986). "Structure and Dynamics of Low-Temperature Water as Studied by Scattering Techniques". Adv. Chem. Phys 64: 1. doi:10.1002/9780470142882.ch1. 

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