Problem 141 - Introductory Chemistry Atoms First

Question

Ammonia, \(\mathrm{NH}_{3}\), is a gas, but it is very soluble in water, so when you bubble ammonia gas into water you produce an aqueous solution of ammonia. Interestingly, this solution is basic (hydroxide ions are formed). Postulate a reaction that occurs between ammonia and water to make the solution basic, and classify the reaction type.

Step-by-Step Solution

Answer

The balanced chemical equation for the reaction between ammonia (NH3) and water (H2O) is: \[NH_{3(g)} + H_{2}O_{(l)} \rightleftharpoons NH_{4^{+}}_{(aq)} + OH^{-}_{(aq)}\] This reaction is classified as a Brønsted-Lowry acid-base reaction, with ammonia acting as a base gaining a proton (H+) and water acting as an acid donating a proton (H+) to form hydroxide ions (OH-) and ammonium ions (NH4+).

Step 1: Identify the reactants

The given reactants are ammonia (NH3) and water (H2O).

Step 2: Determine the possible products

Ammonia is a weak base, and water can act as both an acid and a base. When ammonia and water react, they form hydroxide ions (OH-) and ammonium ions (NH4+).

Step 3: Write the balanced chemical equation

By combining the reactants and products, we have the balanced chemical equation for the reaction: \[NH_{3(g)} + H_{2}O_{(l)} \rightleftharpoons NH_{4^{+}}_{(aq)} + OH^{-}_{(aq)}\]

Step 4: Classify the reaction type

The reaction between ammonia and water is classified as a Brønsted-Lowry acid-base reaction. Specifically, ammonia works as a Brønsted-Lowry base that can gain a proton (H+), and water acts as a Brønsted-Lowry acid which donates a proton (H+) to form hydroxide ions (OH-) and ammonium ions (NH4+).

Key Concepts

Ammonia Solubility in Water

Ammonia, noted by its chemical formula \(\mathrm{NH}_{3}\), is known for being incredibly soluble in water. This high solubility is due to the polar nature of water molecules, which interact effectively with ammonia molecules.
When ammonia gas comes into contact with water, it dissolves easily, resulting in a solution that contains ammonia in its aqueous form. Ammonia acts as a base in this solution, which means it has the ability to accept protons from water molecules.
To picture this process, imagine ammonia molecules surrounded by water molecules. Water, with its partial positive and negative charges, is able to penetrate into the gaseous ammonia, pulling it apart and integrating it into a solution.

Hydroxide Ion Formation

The formation of hydroxide ions is a key reaction when ammonia is dissolved in water. It arises from the interaction of ammonia with water molecules, where water serves as a proton donor.
In this reaction, water donates a hydrogen ion \(\text{H}^+\) to ammonia, turning it into ammonium \(\text{NH}_4^+\) and leaving behind hydroxide ions \(\text{OH}^-\). The equation reflecting this process is:
\[NH_{3} + H_{2}O \rightleftharpoons NH_{4^+} + OH^-\]

• Ammonia (\(\text{NH}_3\)) acts as a base,
• Water (\(H_{2}O\)) acts as an acid,
• The result is hydroxide ions \(\text{OH}^-\) being formed.

The presence of hydroxide ions explains why the solution becomes basic. A basic solution contains an excess of \(\text{OH}^-\) ions, which is a direct outcome of this reaction.

Acid-Base Reaction Classification

Brønsted-Lowry theory provides a framework to classify ammonia's reaction with water. Here, we identify the roles of each substance in this reaction through the lens of acid and base behavior.
A Brønsted-Lowry acid, like water in this case, donates a proton. Meanwhile, a Brønsted-Lowry base, like ammonia, accepts a proton.

• Ammonia accepts a proton, behaving as a base.
• Water donates a proton, acting as an acid.

This classification emphasizes the ability of substances to act as proton donors or acceptors, providing a clear understanding of the interactions taking place. In essence, ammonia's transformation into \(\text{NH}_4^+\) and water's conversion to \(\text{OH}^-\) fit nicely into this acid-base scheme.
The exchange of protons is the hallmark of a Brønsted-Lowry acid-base reaction, making this a classic example of such a chemical interaction.