Problem 60 - Introductory Chemistry Atoms First

Question

Ammonium ions \(\left(\mathrm{NH}_{4}^{+}\right)\) behave as an acid. (a) Write an equation that shows \(\mathrm{NH}_{4}{\underline{\phantom{xx}}}^{+}(a q)\) behaving as an acid when it reacts with aqueous hydroxide ion. (b) What are the products of neutralizing an aqueous solution of \(\mathrm{NH}_{4} \mathrm{Cl}\) with \(\mathrm{NaOH}\) ?

Step-by-Step Solution

Answer

(a) \(\mathrm{NH}_{4}^{+}(aq) + \mathrm{OH}^{-}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l)\) (b) \(\mathrm{NH}_{4}\mathrm{Cl}(aq) + \mathrm{NaOH}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{NaCl}(aq)\)

Step 1: Write the chemical reaction for the ammonium ion and hydroxide ion.

When an ammonium ion \(\left(\mathrm{NH}_{4}^{+}\right)\) reacts with a hydroxide ion \(\left(\mathrm{OH}^{-}\right)\), it donates a proton (H\(^{+}\)) to the hydroxide ion, forming water (H\(_2\)O) and ammonia (NH\(_3\)). The balanced chemical equation is: \[\mathrm{NH}_{4}^{+}(aq) + \mathrm{OH}^{-}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l)\] (b) Neutralization reaction of \(\mathrm{NH}_{4} \mathrm{Cl}\) with \(\mathrm{NaOH}\)

Step 1: Write the ionic dissociation of \(\mathrm{NH}_{4}\mathrm{Cl}\) and \(\mathrm{NaOH}\) in water.

The ionic dissociation of ammonium chloride (NH\(_4\)Cl) and sodium hydroxide (NaOH) in water can be written as: \[ \mathrm{NH}_{4}\mathrm{Cl}(aq) \rightarrow \mathrm{NH}_{4}^{+}(aq) + \mathrm{Cl}^{-}(aq) \] \[ \mathrm{NaOH}(aq) \rightarrow \mathrm{Na}^{+}(aq) + \mathrm{OH}^{-}(aq) \]

Step 2: Identify which neutralization reaction is going on.

In the neutralization reaction, the ammonium ion \(\left(\mathrm{NH}_{4}^{+}\right)\) reacts with hydroxide ion \(\left(\mathrm{OH}^{-}\right)\) as shown in the equation in part (a). At the same time, sodium ions \(\left(\mathrm{Na}^{+}\right)\) and chloride ions \(\left(\mathrm{Cl}^{-}\right)\) remain in the solution.

Step 3: Write the balanced equation for the neutralization reaction.

When ammonium chloride is neutralized with sodium hydroxide, the resulting products are ammonia, water, sodium ions, and chloride ions. The balanced chemical equation is: \[\mathrm{NH}_{4}^{+}(aq) + \mathrm{OH}^{-}(aq) + \mathrm{Na}^{+}(aq) + \mathrm{Cl}^{-}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{Na}^{+}(aq) + \mathrm{Cl}^{-}(aq)\] Since sodium and chloride ions are unchanged during the reaction, we can rewrite the equation as: \[ \mathrm{NH}_{4}\mathrm{Cl}(aq) + \mathrm{NaOH}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l) + \mathrm{NaCl}(aq) \]

Step 4: Identify the products of the neutralization.

The products of neutralizing an aqueous solution of \(\mathrm{NH}_{4}\mathrm{Cl}\) with \(\mathrm{NaOH}\) are ammonia (NH\(_3\)), water (H\(_2\)O), and sodium chloride (NaCl).

Key Concepts

Chemical Equations

Chemical equations are a shorthand way of describing chemical reactions. They show how molecules interact and rearrange to form new substances. Each part of the equation represents different molecules or ions involved in the reaction. In the context of neutralization reactions, chemical equations help us understand how acids and bases interact to form water and another compound. For example, when ammonium ions \(\mathrm{NH}_{4}^{+}\) react with hydroxide ions \(\mathrm{OH}^{-}\), the chemical equation looks like this:\[\mathrm{NH}_{4}^{+}(aq) + \mathrm{OH}^{-}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l)\]This equation communicates, in a very succinct way, that the ammonium ions donate a proton to the hydroxide ions, resulting in the formation of ammonia \(\mathrm{NH}_{3}\) and water \(\mathrm{H}_{2}\mathrm{O}\). Understanding chemical equations is crucial because it helps predict the products of a reaction and how much of each substance is needed or produced.

Ammonium Ion

An ammonium ion \(\mathrm{NH}_{4}^{+}\) is a positively charged ion that arises when ammonia \(\mathrm{NH}_{3}\) accepts a proton, hence indicating its acidic nature, even though it is derived from a base. This behavior can be understood through the Bronsted-Lowry theory of acids and bases, which states that an acid is a proton donor. The ammonium ion fits this description because it can donate a hydrogen ion to other substances. When it reacts with a hydroxide ion \(\mathrm{OH}^{-}\), it donates this proton, effectively reverting back into neutral ammonia \(\mathrm{NH}_{3}\):

• Ammonium \(\mathrm{NH}_{4}^{+}\) donates a proton (H\(^+\)).
• The product formed is ammonia \(\mathrm{NH}_{3}\).

By understanding how ammonium ions operate in solution, we can predict their reactions with bases and understand their role as a weak acid in aqueous environments.

Hydroxide Ion

A hydroxide ion \(\mathrm{OH}^{-}\) is a basic anion commonly found in many chemical reactions involving bases. It is often associated with alkaline solutions and possesses a strong ability to accept protons, which is central to its role in neutralization reactions. In chemical terms, when a hydroxide ion encounters an acidic species like the ammonium ion \(\mathrm{NH}_{4}^{+}\), it effectively takes a proton to form water \(\mathrm{H}_{2}\mathrm{O}\). The reaction is as follows:

• \(\mathrm{OH}^{-}\) accepts a proton (H\(^+\))
• Forms a water molecule \(\mathrm{H}_{2}\mathrm{O}\)

This ability to form water by neutralizing acids is why hydroxide ions are such a critical component in neutralization reactions, essentially transforming the acidic and basic species into neutral water.

Acid-Base Reactions

Acid-base reactions are essential chemical processes in which an acid reacts with a base to produce water and a salt. This type of reaction is also known as a neutralization reaction.The classic example involves an acid donating a proton to a base, as seen in the reaction between ammonium ion \(\mathrm{NH}_{4}^{+}\) and hydroxide ion \(\mathrm{OH}^{-}\):\[\mathrm{NH}_{4}^{+}(aq) + \mathrm{OH}^{-}(aq) \rightarrow \mathrm{NH}_{3}(aq) + \mathrm{H}_{2}\mathrm{O}(l)\]Key characteristics of acid-base reactions include:

• Proton transfer from the acid to the base.
• Formation of water, which is neutral.
• Often result in the production of a salt, depending on the ions involved.

In the example provided, the neutralization between ammonium chloride \(\mathrm{NH}_{4}\mathrm{Cl}\) and sodium hydroxide \(\mathrm{NaOH}\) gives rise to ammonia \(\mathrm{NH}_{3}\), water \(\mathrm{H}_{2}\mathrm{O}\), and sodium chloride \(\mathrm{NaCl}\). Understanding this process helps explain many everyday occurrences, from antacid action in the stomach to laboratory reactions.