Self-ionization of water

The self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium (H₃O⁺) and a hydroxide ion (OH^-):

<math>H_2O_{(l)}+H_2O_{(l)} \Leftrightarrow H_3O^+_{(aq)} + OH^-_{(aq)}<math>

The reaction is also known as the autoionization or autodissociation of water. It is an example of autoprotolysis, and relies on the amphoteric nature of water.

Water, however pure, is not a simple collection of H₂O molecules. Even in "pure" water, sensitive equipment can detect a very slight electrical conductivity. According to the theories of Svante Arrhenius, this must be due to the presence of ions.

Contents

1 Concentration and Frequency 2 Acidity 3 Mechanism 4 See Also 5 References

Concentration and Frequency

The preceding reaction has a chemical equilibrium constant. For reactions in water (or any aqueous solutions), the molarity (a unit of concentration) of water, [H₂O], is practically constant and is omitted from the equilibrium 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. After omitting [H₂O], the equilibrium expression is:

K_w = [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 (298K), K_w = [H₃O⁺][OH^-] = 1.0 × 10^-14. Pure water ionizes or dissociates into equal amounts of H₃O⁺ and OH^-, so their molarities are equal:

[H₃O⁺] = [OH^-].

At SATP, the concentrations of hydroxide and hydronium are both 1.0 × 10^-7. Since the concentration of water molecules in water is largely unaffected by dissociation and [H₂O] equals approximately 56 mol.l^-1, it follows that for every 5.6 × 10⁸ water molecules, one pair will exist as ions. Any solution in which the H₃O⁺ 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. K_w is sensitive to both pressure and temperature; it increases when either increases. As temperature decreases, K_w also decreases.

By definition, pK_w = -log₁₀ K_w. At SATP, pK_w = -log₁₀ (1.0 × 10^-14) = 14. pK_w also varies with temperature. As temperature increases, pK_w decreases; and as temperature decreases, pK_w increases.

Hydroxide and hydronium ions have low concentrations in water, and they are rarely produced: a randomly selected water molecule will dissociate within approximately 10 hours (1).

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 distillation or some other water purification method. Removal of all ions from water is next to impossible, since water self-ionizes quickly to reach equilibrium.

Acidity

pH is a logarithmic measure of the acidity (or alkalinity) of an aqueous solution. By definition, pH = -log₁₀ [H₃O⁺]. Since [H₃O⁺] = [OH^-] in a neutral solution, by mathematics, for a neutral aqueous solution pH = 7 at SATP.

Self-ionization is the process that determines the pH of water. Since the concentration of hydronium at SATP (~25°C) is 1.0 × 10^-7mol.l^-1, the pH of pure liquid water at this temperature is 7. Since K_w increases as temperature increases, hot water has a higher concentration of hydronium than cold water, and is more acidic.

Mechanism

Geissler et al. have determined that electric field fluctuations in liquid water cause molecular dissociation (2). They propose the following sequence of events that takes place in about 150 fs: the system begins in a neutral state; the solvent's electric field breaks a hydrogen bond between two water molecules, creating a hydroxide and hydroxonium ion; the proton of the hydroxonium 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 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 (3,4,5).

See Also

• Standard Ambient Temperature and Pressure
• Chemical equilibrium

References

1. Eigen, M. & de Maeyer, L. (1955) Untersuchungen über die Kinetik der Neutralisation I. Z. Elektrochem. 59 986.
2. Geissler, P. L.; Dellago, C.; Chandler, D.; Hutter, J. & Parrinello, M. (2001) Autoionization in liquid water. Science 291 2121-2124.
3. Stillinger, F. H. (1975) Adv. Chem. Phys. 31 1.
4. Rapaport, D. C. (1983) Mol. Phys. 50 1151.
5. Chen, S.-H. & Teixeira, J. (1986) Adv. Chem. Phys 64 1.