14.1 Brønsted-lowry acids and bases  (Page 2/14)

We call the product that remains after an acid donates a proton the conjugate base    of the acid. This species is a base because it can accept a proton (to re-form the acid):

We call the product that results when a base accepts a proton the base’s conjugate acid    . This species is an acid because it can give up a proton (and thus re-form the base):

In these two sets of equations, the behaviors of acids as proton donors and bases as proton acceptors are represented in isolation. In reality, all acid-base reactions involve the transfer of protons between acids and bases. For example, consider the acid-base reaction that takes place when ammonia is dissolved in water. A water molecule (functioning as an acid) transfers a proton to an ammonia molecule (functioning as a base), yielding the conjugate base of water, OH ⁻ , and the conjugate acid of ammonia, ${\text{NH}}_4{}^{\text{+}}$ :

The reaction between a Brønsted-Lowry acid and water is called acid ionization    . For example, when hydrogen fluoride dissolves in water and ionizes, protons are transferred from hydrogen fluoride molecules to water molecules, yielding hydronium ions and fluoride ions:

When we add a base to water, a base ionization    reaction occurs in which protons are transferred from water molecules to base molecules. For example, adding pyridine to water yields hydroxide ions and pyridinium ions:

Notice that both these ionization reactions are represented as equilibrium processes. The relative extent to which these acid and base ionization reactions proceed is an important topic treated in a later section of this chapter. In the preceding paragraphs we saw that 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:

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    .

Pure water undergoes autoionization to a very slight extent. Only about two out of every 10 ⁹ molecules in a sample of pure water are ionized at 25 °C. The equilibrium constant for the ionization of water is called the ion-product constant for water ( K _w )    :

The slight ionization of pure water is reflected in the small value of the equilibrium constant; at 25 °C, K _w has a value of 1.0 $\times$ 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 K _w is about 5.6 $\times$ 10 ⁻¹³ , roughly 50 times larger than the value at 25 °C.