Problem 2 - Chemical Principles in the Laboratory

Question

An aqueous solution of \(\mathrm{NH}_{3}\) has a pH of \(11.6 .\) The ammonia (NH, \(_{3}\) ) molecule is the conjugate base of the NH_t (called "ammonium") ion. Write the net ionic equation for the reaction that makes an aqueous solution of \(\mathrm{NH}_{3}\) basic. (Eq. 5.)

Step-by-Step Solution

Answer

\( \mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_4^+ + \mathrm{OH}^- \) is the net ionic equation.

Step 1: Understanding the Molecules Involved

Ammonia, or \( \mathrm{NH}_{3} \), is known to act as a base in water. This is because \( \mathrm{NH}_{3} \) can accept a proton (H+) from water to form \( \mathrm{NH}_4^+ \) (ammonium ions). Water, in this case, donates a proton and becomes hydroxide ions \( \mathrm{OH}^- \).

Step 2: Writing the Chemical Equation

Based on the above understanding, the chemical equation for the reaction of ammonia in water is: \( \mathrm{NH}_{3} + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^- \).

Step 3: Determining the Net Ionic Equation

Since we are dealing with an aqueous solution, we write the net ionic equation by focusing on the ions that participate in the reaction. The \( \mathrm{NH}_3 \) is reacting with \( \mathrm{H}_2\mathrm{O} \) to form \( \mathrm{NH}_4^+ \) and \( \mathrm{OH}^- \). Water is a liquid and does not appear in the net ionic equation as an ion. The net ionic equation is: \( \mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_4^+ + \mathrm{OH}^- \).

Step 4: Verification of Basicity

The formation of \( \mathrm{OH}^- \) ions is what makes the solution basic, as hydroxide ions are responsible for increasing the pH of the solution. The presence of these ions indicates that the solution is basic, consistent with a pH of 11.6.

Key Concepts

Conjugate Acid-Base Pairs

In chemistry, a conjugate acid-base pair consists of two species that differ by a single proton (H\(^+\)). When a base gains a proton, it transforms into its conjugate acid. Conversely, an acid loses a proton to become its conjugate base. This concept is vital when understanding reactions in aqueous solutions, such as the behavior of ammonia (NH\(_3\)).Ammonia acts as a base by accepting a proton from water, transforming into ammonium (NH\(_4^+\)). Water, donating this proton, becomes hydroxide (OH\(^-\)). Hence, NH\(_3\) and NH\(_4^+\) are a conjugate acid-base pair. This unique relationship helps maintain the acid-base balance in a solution. By using conjugate pairs, chemists can predict and understand the direction of many chemical reactions.

Aqueous Solutions

Aqueous solutions are simply solutions where water is the solvent. In these solutions, various substances dissolve and dissociate into ions, allowing different kinds of chemical reactions to occur. Water is a universal solvent and facilitates the ionization of compounds. This is why water is involved in numerous chemical processes, including those involving acids and bases.In an aqueous solution of ammonia (NH\(_3\)), ammonia reacts with water molecules. As a result, some ammonia molecules accept protons from water, creating ammonium ions (NH\(_4^+\)) and hydroxide ions (OH\(^-\)). This transformation highlights the unique capability of water to allow compounds like ammonia to exhibit their basic properties effectively. Understanding how substances behave in aqueous solutions is essential for predicting the outcomes of many chemical reactions, including net ionic equations and their effects on pH.

Solution pH

The pH of a solution indicates how acidic or basic (alkaline) the solution is. pH is a scale of 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is basic. The pH level is determined by the concentration of hydrogen ions (H\(^+\)) in a solution.When ammonia (NH\(_3\)) is dissolved in water, it increases the solution's pH by forming hydroxide ions (OH\(^-\)). A solution becomes basic due to the increased presence of hydroxide ions, which occur when ammonia accepts a proton from water molecules. In our exercise, the pH of 11.6 clearly shows that the aqueous NH\(_3\) solution is quite basic. This high pH is consistent with the generation of OH\(^-\) ions. By manipulating the components in a solution, chemists can control the pH, which is crucial in both industrial and laboratory settings, influencing reaction rates and behaviors.