Introduction to Aqueous Equilibria

The first step in working equilibrium problems in aqueous solution is to identify the species that are present, and determine if any reaction will take place. After this step, then we can determine what species can take part in the equilibrium.

Memorizing the spectator ions will help you identify the different types of species that can react in solution. Knowing them allows you to quickly identify all neutral salts, strong acids, and strong bases. Hydrogen ion (H⁺) and a spectator anion will dissociate completely, and is therefore a strong acid. Hydroxide ion (OH^-) with a spectator cation dissociates completely, and therefore is a strong base.

Weak acids are compounds that partially dissociate to produce an equilibrium concentration of H⁺. An example equilibrium is acetic acid in water (vinegar is about 5% acetic acid):

CH₃COOH_(aq) H⁺_(aq) + CH₃COO^-_(aq)

Weak bases partially react with water to produce an equilibrium concentration of OH^-. An example equilibrium is ammonia in water:

NH_3(aq) + H₂O NH⁺_(aq) + OH^-_(aq)

For more information read the documents on acid-base definitions, acid-base reactions, and the general solution of equilibria problems.

Examples

The concept that weak acids and weak bases only partially dissociate is often confusing at first, so two illustrative examples are discussed here.

What is present in solution when a weak acid is dissolved in water? As an example, consider pouring 5.0 mL of pure acetic acid and 100.0 mL of water into a beaker. What is in the beaker?

There is mostly water in the beaker. The next most abundant specie is acetic acid, CH₃COOH_(aq) [remember that the (aq) subscript means that the substance is dissolved in water]. The acetic acid partially dissociates, so there is also a small amount of H⁺_(aq) and CH₃COO^-_(aq).

This behavior is very different from a strong acid such as nitric acid, HNO₃. If 5.0 mL of nitric acid was mixed with 100.0 mL of water, you would have an aqueous solution containing only H⁺_(aq) and NO₃^-_(aq).

What happens when a weak base is dissolved in water? As an example consider pouring a quart of ammonia cleaner into a bucket of water.

There is mostly water in the bucket. The next most abundant specie is ammonia, NH_{3 (aq)}. A small amount of the ammonia reacts with water to produce a small amount of NH₄⁺_(aq) and OH^-_(aq).