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Reactions of Acids and Bases with Water

As you may have noted already in the acid-base reactions above, we use arrows in both reaction directions to indicate that these are equilibrium processes. Proportions of reagents and products at equilibrium can be described by an equilibrium constant. The equilibrium constant given in is for the reaction of an acid, HA, with water as shown.

Although water is a reactant in the above reaction and belongs in the equilibrium constant, its value of 55.6 M in aqueous solution is so large in comparison with the change in water concentration at equilibrium that we will assume that the value of [H2O] is constant. Using that assumption, we will define the acid dissociation constant, Ka, in to be the following:

Figure %: Definition of the acid dissociation constant

From the form of the above equation we can see that stronger acids, those that dissociate to a greater extent, will have larger values of Ka whereas weaker acids will have smaller values of Ka. A practical range for Ka values runs from 10-12 for very weak acids to 1013 for the strongest acids. Knowing this practical range of acidity constants will aid in judging how reasonable your answers are when you calculate values for Ka in problems.

In an analogous way, we define Kb, the base constant in to be the following:

Figure %: Definition of the base dissociation constant

Stronger bases have larger values of Kb while weaker bases have smaller values of Kb. Kb's of typical bases in inorganic chemistry tend to have a range of values between 10-11 and 103.

As you may have discovered in our above discussion, water can act as both an acid and as a base. For this reason water is said to be amphiprotic. Water is often incorrectly termed amphoteric. An amphiprotic species like water can either donate or accept a proton. Amphoteric species can both donate and accept hydroxide ions, as water cannot. The following reaction in , called the autoionization of water, has the equilibrium constant Kw defined in the manner of Ka and Kb. The dissociation constant Kw for water is 1 x 10-14 at room temperature (298 K), and tends to rise with higher temperatures.

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