Methylammonium chloride (\(\text{CH}_3\text{NH}_3\text{Cl}\)) forms an acidic solution when dissolved in water. This compound is classified as a salt, created from the neutralization reaction between an acid and a base. The acidity of the resulting aqueous solution, which means it will have a pH below 7, is determined by the specific chemical behavior of the ions released. The compound’s overall behavior is a direct result of the chemical nature of its two component ions.
Dissociation of the Salt in Water
When solid methylammonium chloride is introduced into water, it behaves as a strong electrolyte and undergoes complete dissociation. The highly polar nature of water molecules causes the compound to break apart into its individual charged ions. The ionic compound separates into the positively charged methylammonium cation (\(\text{CH}_3\text{NH}_3^+\)) and the negatively charged chloride anion (\(\text{Cl}^-\)).
The strong attractive forces between the polar water molecules and the ions overcome the ionic bonds holding the solid salt together. This dissociation is a one-way reaction, meaning virtually all of the solid salt separates into its ions in the aqueous solution. The chemical equation for this initial step is \(\text{CH}_3\text{NH}_3\text{Cl} (s) \rightarrow \text{CH}_3\text{NH}_3^+ (aq) + \text{Cl}^- (aq)\). The subsequent acid-base behavior is entirely dependent on how these liberated ions interact with the surrounding water.
Hydrolysis and the Acidic Ion
The dissolved ions must be examined individually to determine how they influence the water’s pH through hydrolysis. The chloride ion (\(\text{Cl}^-\)) is the conjugate base of hydrochloric acid (\(\text{HCl}\)), a strong acid. Since strong acids completely donate their protons, their conjugate bases are extremely weak and have no tendency to accept a proton from water. Therefore, the chloride ion is considered a spectator ion that does not react with water or affect the overall pH of the solution.
The methylammonium ion (\(\text{CH}_3\text{NH}_3^+\)) is the conjugate acid of methylamine (\(\text{CH}_3\text{NH}_2\)), a weak base. Weak bases only partially accept protons, meaning their conjugate acids retain a significant ability to donate a proton. The methylammonium ion is chemically active and reacts with water by donating a proton (\(\text{H}^+\)) to a water molecule.
This proton donation process generates two new species: the neutral weak base methylamine (\(\text{CH}_3\text{NH}_2\)) and the hydronium ion (\(\text{H}_3\text{O}^+\)). The chemical equation for this equilibrium reaction is \(\text{CH}_3\text{NH}_3^+ (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{CH}_3\text{NH}_2 (aq) + \text{H}_3\text{O}^+ (aq)\). The production of hydronium ions is the defining characteristic of an acidic solution, confirming that the \(\text{CH}_3\text{NH}_3\text{Cl}\) salt makes the water acidic.
Determining the Final pH
The acidic nature of the final solution is a direct consequence of the chemical strengths of the acid and base used to form the salt. Methylammonium chloride is the product of a reaction between a strong acid (\(\text{HCl}\)) and a weak base (\(\text{CH}_3\text{NH}_2\)). The resulting solution will always be acidic because the ion derived from the strong component (\(\text{Cl}^-\)) is inert, while the ion derived from the weak component (\(\text{CH}_3\text{NH}_3^+\)) is reactive.
The extent of the solution’s acidity is quantified by the acid dissociation constant (\(K_a\)) for the methylammonium ion. This value is related to the base dissociation constant (\(K_b\)) of its conjugate base, methylamine, through the ion product of water (\(K_w\)). The relationship is \(K_a \times K_b = K_w\), where \(K_w\) is \(1.0 \times 10^{-14}\) at \(25^\circ\text{C}\).
Given that the \(K_b\) for methylamine is approximately \(4.4 \times 10^{-4}\), the calculated \(K_a\) for the methylammonium ion is about \(2.3 \times 10^{-11}\). Although this \(K_a\) value indicates the methylammonium ion is a weak acid, the hydronium ions it produces are enough to lower the pH below 7. Since the \(\text{Cl}^-\) ion has no competing effect, the net result is an increase in the concentration of \(\text{H}_3\text{O}^+\) relative to pure water, confirming an acidic solution.