Ion Product of Water
Definition and meaning of Ion Product of Water in chemistry.
The ion product of water (Kw) is the equilibrium constant for the autoionization of water, equal to the product of the molar concentrations of hydrogen ions and hydroxide ions: Kw = [H+][OH-].
In more detail
Water undergoes a slight self-ionization, 2 H2O ⇌ H3O+ + OH-, and Kw is the equilibrium constant for this process (with [H2O] omitted since it is a pure liquid). At 25 degrees C, Kw = 1.0 x 10^-14, so in pure water [H+] = [OH-] = 1.0 x 10^-7 M, giving a neutral pH of 7.00. Because Kw links [H+] and [OH-] in any aqueous solution, knowing one concentration lets you calculate the other, which is essential for pH and pOH calculations. Since autoionization is endothermic, Kw increases with temperature, so neutral pH is not always exactly 7.
Key facts
| Field | General Chemistry |
|---|---|
| Formula | Kw = [H+][OH-] |
| Value at 25 C | 1.0 x 10^-14 |
| pKw at 25 C | 14.00 |
In an acidic solution at 25 degrees C where [H+] = 1.0 x 10^-3 M, the hydroxide concentration is [OH-] = Kw / [H+] = (1.0 x 10^-14) / (1.0 x 10^-3) = 1.0 x 10^-11 M.
Frequently asked questions
Is Kw always exactly 1.0 x 10^-14?
No, that value holds only at 25 degrees C. Because water autoionization is endothermic, Kw increases as temperature rises (for example, it is about 2.9 x 10^-14 at 40 degrees C), so neutral pH is slightly below 7 at higher temperatures.
Why does Kw let you find [OH-] from [H+]?
Because Kw = [H+][OH-] holds in any aqueous solution at a given temperature, rearranging gives [OH-] = Kw/[H+], allowing either concentration to be calculated once the other is known.