Aniline, \(\mathrm{C}_{6} \mathrm{H}_{7} \mathrm{~N}\), is a molecular compound that is a weak base. The molecule has a lone pair of electrons on the \(\mathrm{N}\) atom. (a) Why can aniline act as a base? (b) Write the equilibrium expression that shows how aniline in water makes the solution basic. (Be careful about the relative lengths of the two arrows you show in the expression.)

Aniline can act as a base because it has a lone pair of electrons on the nitrogen (N) atom. The presence of this lone pair allows it to accept a proton (H+) from other molecules, according to the Bronsted-Lowry theory of acids and bases. When aniline accepts a proton, it forms a bond with the hydrogen atom, resulting in the formation of a new molecule.

To write the equilibrium expression, we need to show the reaction of aniline with water and how it makes the solution basic: Aniline + Water \(\rightleftharpoons\) Anilinium Ion + Hydroxide Ion Using the chemical formulas for each species, we can represent the reaction as: \(\mathrm{C_6H_7N}\) + \(\mathrm{H_2O}\) \(\rightleftharpoons\) \(\mathrm{C_6H_8N^+}\) + \(\mathrm{OH^-}\) Now we can write the equilibrium expression, which is the product of the concentrations of the products divided by the product of the concentrations of the reactants (ignoring the concentration of water since it's a pure liquid and its concentration is constant): \(K_b\) = \(\dfrac{[\mathrm{C_6H_8N^+}][\mathrm{OH^-}]}{[\mathrm{C_6H_7N}]}\) This equilibrium expression shows how aniline in water makes the solution basic, as the formation of hydroxide ions increases the pH of the solution, making it more alkaline.