Ionization of Water and the pH Scale
المؤلف:
Jerome L. Rosenberg and Lawrence M. Epstein
المصدر:
College Chemistry
الجزء والصفحة:
p 115
17-7-2017
2191
Ionization of Water and the pH Scale
Every aqueous solution contains hydronium (H3O+) and hydroxide (OH-) ions as a result of the water autoionization process,
(1.1)
[H2O] does not appear in the equilibrium constant expression since, in dilute solutions, H2O is very close to the standard state condition of pure liquid. In pure water at 25°C, [H3O+] = [OH-] = Kw1/2 = 1.00 × 10-7 M.
Because the water ionization is endothermic, Kw increases with increasing T. In 0.10 M HCl solution, HCl is completely ionized,
and [H3O+] = 0.10 M, [OH-] = 1.0 × 10-13 M. Similarly 0.10 M NaOH is completely ionized,
and [OH-] = 0.10 M, [H3O+] = 1.0 × 10-13 M. Because of the enormous range in the concentrations of H3O+ and OH- ions, we usually use a logarithmic scale to express these concentrations:
(1. 2)
(1. 3)
Since the H3O+ and OH- concentrations are related by Eq. (1.1),
(1.4)
(In general, pX = -log10[X].) Thus, for 0.10 M HCl, pH = 1.00, pOH = 13.00, and for 0.10 M NaOH, pH = 13.00, pOH = 1.00.
Water is not unique in undergoing auto-ionization. Several other solvents are capable of acting both as acids and bases, e.g., the following equilibrium occurs in liquid ammonia (bp -33°C):
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