When Ammonium Nitrate (\(\text{NH}_4\text{NO}_3\)) is dissolved in water, it forms an aqueous solution. This compound is widely known for its use as a high-nitrogen fertilizer and also serves as a component in industrial explosives. The fundamental question is whether this substance acts as an acid, a base, or something else entirely when mixed with water.
Foundation: Understanding Salts and Aqueous Solutions
A salt is a compound classically formed from the neutralization reaction between an acid and a base. When a salt dissolves in water, it separates completely into its component ions. For example, a salt formed from a strong acid and a strong base, like sodium chloride, creates a neutral solution with a pH of 7.
The acidity or basicity of a salt solution is governed by “parent strength,” meaning the strength of the original acid and base that combined to form the salt. In the Brønsted-Lowry definition, an acid donates a proton, while a base accepts a proton.
If a salt is derived from a strong acid and a weak base, the solution will be acidic (pH less than 7). Conversely, a salt formed from a strong base and a weak acid creates a basic solution (pH greater than 7). The ion originating from the weak parent compound retains the chemical ability to influence the water’s balance, while the ion from the strong parent is chemically inert. The stronger parent dictates the overall character of the dissolved salt solution.
Deconstructing the Chemical Components
Ammonium nitrate (\(\text{NH}_4\text{NO}_3\)) is an ionic compound that dissociates into the ammonium cation (\(\text{NH}_4^+\)) and the nitrate anion (\(\text{NO}_3^-\)) when dissolved.
The nitrate ion (\(\text{NO}_3^-\)) is derived from nitric acid (\(\text{HNO}_3\)), which is classified as a strong acid. The ammonium ion (\(\text{NH}_4^+\)) is the product of ammonia (\(\text{NH}_3\)), which acts as a weak base.
Ammonium nitrate is thus classified as a salt derived from the neutralization of a strong acid (\(\text{HNO}_3\)) and a weak base (\(\text{NH}_3\)). This strong acid/weak base parentage suggests that the final aqueous solution will not be neutral.
The Hydrolysis Reaction and Final Classification
The process by which salt ions interact with water to alter the pH is called hydrolysis. This is a reaction where one or both of the salt’s ions react with water molecules, producing either hydronium ions (\(\text{H}_3\text{O}^+\)) or hydroxide ions (\(\text{OH}^-\)).
The nitrate ion (\(\text{NO}_3^-\)) is the conjugate base of the strong acid, nitric acid. Strong acids have very weak conjugate bases, meaning the nitrate ion is too weak to accept a proton from water. For all practical purposes, the nitrate ion is a spectator ion that has no effect on the solution’s pH.
The ammonium ion (\(\text{NH}_4^+\)), however, is the conjugate acid of the weak base, ammonia. As a result, the ammonium ion is strong enough to act as a weak acid in water, readily donating a proton to a water molecule. This proton donation forms a hydronium ion and neutral ammonia:
\(\text{NH}_4^+ (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{NH}_3 (aq) + \text{H}_3\text{O}^+ (aq)\)Since the ammonium ion acts as an acid, the overall effect when ammonium nitrate is dissolved in water is an increase in the concentration of hydronium ions. This chemical action makes the solution acidic, with a pH value below 7. Therefore, ammonium nitrate is classified as an acidic salt.