Water (H 2 O) is an interesting compound in many respects. Here, we will consider its ability to behave as an acid or a base.
In some circumstances, a water molecule will accept a proton and thus act as a Brønsted-Lowry base. We saw an example in the dissolving of HCl in H2O:
In other circumstances, a water molecule can donate a proton and thus act as a Brønsted-Lowry acid. For example, in the presence of the amide ion (see Example 4 in Section 10.2 “Brønsted-Lowry Definition of Acids and Bases”), a water molecule donates a proton, making ammonia as a product:
In this case, NH2 − is a Brønsted-Lowry base (the proton acceptor).
So, depending on the circumstances, H2O can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base. Water is not the only substance that can react as an acid in some cases or a base in others, but it is certainly the most common example—and the most important one. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic compound.
A water molecule can act as an acid or a base even in a sample of pure water. About 6 in every 100 million (6 in 10 8 ) water molecules undergo the following reaction:
This process is called the autoionization of water (Figure 10.2 “Autoionization”) and occurs in every sample of water, whether it is pure or part of a solution. Autoionization occurs to some extent in any amphiprotic liquid. (For comparison, liquid ammonia undergoes autoionization as well, but only about 1 molecule in a million billion (1 in 10 15 ) reacts with another ammonia molecule.)
Figure 10.2 Autoionization
A small fraction of water molecules—approximately 6 in 100 million—ionize spontaneously into hydronium ions and hydroxide ions. This picture necessarily overrepresents the amount of autoionization that really occurs in pure water.
Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.
