Ammonia (\(\text{NH}_3\)) is a weak base. It is a common household chemical, often recognized by its distinct odor in cleaning solutions or fertilizers. Solutions of ammonia are widely used for their ability to break down grease and oil, a function directly related to their basic nature. While its classification as a base is straightforward, its designation as a weak base defines how it behaves when mixed with water.
Defining Chemical Bases
The definition of a chemical base has evolved to describe substances like ammonia, which do not contain the hydroxide ion (\(\text{OH}^-\)) in their initial structure. The older Arrhenius definition classified a base as a substance that produces hydroxide ions when dissolved in water. Because ammonia produces hydroxide ions indirectly through a reaction with water, the Brønsted-Lowry definition provides a more complete explanation.
The Brønsted-Lowry definition states that a base is any species that can accept a proton (\(\text{H}^+\)). Ammonia’s structure includes a lone pair of electrons on the nitrogen atom, making it suited to attract and bond with an \(\text{H}^+\) ion. This proton-accepting ability is the core reason for its classification as a base. By accepting a proton, ammonia raises the concentration of hydroxide ions in the solution, making the solution basic.
Understanding Weakness and Equilibrium
In chemistry, the term “weak” describes the extent to which a substance reacts or ionizes when dissolved in water. A weak base, such as ammonia, undergoes only partial ionization in an aqueous solution. This means that only a small percentage of the ammonia molecules react with water at any given time.
The reaction is reversible and establishes chemical equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction. Strong bases, like sodium hydroxide (\(\text{NaOH}\)), dissociate almost completely in water. The partial reaction of a weak base results in a significantly lower concentration of hydroxide ions compared to a strong base of the same concentration.
Chemists quantify this weakness using the base-dissociation constant, \(K_b\). This constant is the equilibrium expression for the reaction of the base with water. A very small \(K_b\) value indicates a weak base because the concentration of the original unreacted base is much greater than the concentration of the product ions at equilibrium. The \(K_b\) value for ammonia is approximately \(1.8 \times 10^{-5}\), confirming its classification as a weak base.
How Ammonia Behaves in Water
When ammonia (\(\text{NH}_3\)) is dissolved in water (\(\text{H}_2\text{O}\)), it acts as a Brønsted-Lowry base by accepting a proton from a water molecule. The water molecule acts as an acid, donating a proton. This reaction forms two new ions: the ammonium ion (\(\text{NH}_4^+\)) and the hydroxide ion (\(\text{OH}^-\)). The production of the hydroxide ion makes the solution basic.
The chemical equation representing this reversible process is:
$\(\text{NH}_3 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{NH}_4^+ (aq) + \text{OH}^- (aq)\)$
Because ammonia is a weak base, the double arrow indicates that the reaction heavily favors the reactants. Only a small fraction of ammonia molecules, often less than 5%, react to form ions; the majority remains as unreacted \(\text{NH}_3\). This limited formation of hydroxide ions means the resulting solution is basic, but its \(\text{pH}\) is not as high as it would be if a strong base were used.