Ammonia (\(\text{NH}_3\)) is a weak base. When dissolved in water, it demonstrates its basic nature by increasing the concentration of hydroxide ions (\(\text{OH}^-\)) in the solution. The designation of “weak” means only a small fraction of ammonia molecules participate in the chemical change. Understanding ammonia’s role requires examining how it interacts with water at a molecular level.
Defining Bases The Brønsted-Lowry Perspective
To classify ammonia’s behavior, chemists use the Brønsted-Lowry theory. This framework defines a base as any substance capable of accepting a proton, which is a positively charged hydrogen ion (\(\text{H}^+\)). This definition is particularly helpful for understanding \(\text{NH}_3\) because it focuses on the transfer of the proton itself.
The older Arrhenius definition classified a base as a substance that produces hydroxide ions (\(\text{OH}^-\)) when dissolved in water. The Brønsted-Lowry concept, however, explains that ammonia acts as a base by chemically reacting with the water solvent to generate the hydroxide ions. The focus on proton transfer makes the Brønsted-Lowry definition a more accurate descriptor for compounds like ammonia.
The Mechanism of Ammonia’s Basicity
Ammonia’s molecular structure provides the specific chemical feature that allows it to function as a base. The central nitrogen atom in the \(\text{NH}_3\) molecule possesses a lone pair of non-bonding electrons. This pair is available to interact with other molecules.
When ammonia dissolves in water, the lone pair attracts a proton (\(\text{H}^+\)) from a nearby water molecule (\(\text{H}_2\text{O}\)). The water molecule acts as an acid, donating its proton to the ammonia molecule. This process forms two new species: the ammonium ion (\(\text{NH}_4^+\)) and the hydroxide ion (\(\text{OH}^-\)).
The reaction is represented by the equation: \(\text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq})\). The formation of the hydroxide ion makes the solution basic, as a higher concentration of \(\text{OH}^-\) ions corresponds to a pH greater than 7.
The Crucial Difference Weak Versus Strong
The defining characteristic of ammonia as a weak base is its behavior in chemical equilibrium. When ammonia reacts with water, the reaction establishes a balance between the reactants and the products. This is known as partial dissociation, where the forward reaction (forming ions) and the reverse reaction (re-forming ammonia and water) occur simultaneously at equal rates.
In a typical ammonia solution, only a very small percentage of the original \(\text{NH}_3\) molecules, often less than one percent, react to produce \(\text{OH}^-\) ions. This limited reaction contrasts sharply with a strong base, such as sodium hydroxide (\(\text{NaOH}\)), which dissociates completely in water.
Chemists quantify this weakness using the Base Dissociation Constant (\(\text{K}_b\)). The \(\text{K}_b\) value for ammonia is small, approximately \(1.8 \times 10^{-5}\) at \(25^\circ \text{C}\). This small number confirms that the equilibrium lies heavily toward the reactants, indicating a low concentration of \(\text{OH}^-\) ions. Strong bases have \(\text{K}_b\) values that are large, indicating the reaction goes to completion.