16.3 Ionization of Water
Learning Objectives
By the end of this section, you will be able to:
- Describe the autoionization of water
- Use the ion-product constant for water to calculate hydronium and hydroxide ion concentrations
To begin this chapter, Watch How Polarity makes Water Behave Strangely (3min 51s).
Video Source: TED-Ed. (2013, February 4). How polarity makes water behave strangely – Christina Kleinberg [Video]. YouTube.
Earlier in this chapter, the concept of amphoteric species, such as water, was introduced. Because of its highly polar structures, water can function as either an acid or a base, depending on the nature of the solute dissolved in it. In fact, in pure water or in any aqueous solution, water acts both as an acid and a base. A very small fraction of water molecules donate protons to other water molecules to form hydronium ions and hydroxide ions (Figure 16.3a).
This type of reaction, in which a substance ionizes when one molecule of the substance reacts with another molecule of the same substance, is referred to as autoionization.
To visualize this process in a video simulation, watch Autoionization in Liquid Water (3 min 23s).
Video Source: Akumich (2012, February 22). Autoionization of liquid water. [Video]. YouTube.
Pure water undergoes autoionization to a very slight extent. Only about two out of every 109 molecules in a sample of pure water are ionized at 25 °C. The relationship between products and reactants of any reaction at equilibrium can be expressed by its equilibrium constant, K. The equilibrium constant for the ionization of water is called the ion-product constant for water (Kw):
[latex]\text{H}_2\text{O}(l)\;+\;\text{H}_2\text{O}(l)\;{\leftrightharpoons}\;\text{H}_3\text{O}^{+}(aq)\;+\;\text{OH}^{-}(aq)\\[0.7em] K_{\text{w}} = [\text{H}_3\text{O}^{+}][\text{OH}^{-}][/latex]
The slight ionization of pure water is reflected in the small value of the equilibrium constant; at 25 °C, Kw has a value of 1.0 × 10−14. The process is endothermic, and so the extent of ionization and the resulting concentrations of hydronium ion and hydroxide ion increase with temperature. For example, at 100 °C, the value for Kw is about 5.6 × 10−13, roughly 50 times larger than the value at 25 °C.
Example 16.3a
Ion Concentrations in Pure Water
What are the hydronium ion concentration and the hydroxide ion concentration in pure water at 25 °C?
Solution
The autoionization of water yields the same number of hydronium and hydroxide ions. Therefore, in pure water, [H3O+] = [OH−]. At 25 °C:
So:
The hydronium ion concentration and the hydroxide ion concentration are the same, and we find that both equal 1.0 × 10−7M.
Therefore, at 25 °C, Kw has a value of 1.0 × 10−14.
[latex]K_{\text{w}} = [\text{H}_3\text{O}^{+}][\text{OH}^{-}] = 1.0\;\times\;10^{-14}[/latex]
The degree of autoionization of water and hence the value of Kw changes with temperature, so the equation
[latex]K_{\text{w}} = [\text{H}_3\text{O}^{+}][\text{OH}^{-}] = 1.0\;\times\