Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

Aqueous solution of ammonia consists of (a) \(\mathrm{NH}_{4}^{+}\)and \(\mathrm{OH}^{-}\) (b) \(\mathrm{OH}^{-}\) (c) \(\mathrm{NH}_{4}^{+}\) (d) \(\mathrm{H}^{+}\)

Step-by-Step Solution

Answer

(a) NH_{4}^{+} and OH^{-}

Step 1: Understanding the Problem

The exercise asks which ions are present in an aqueous solution of ammonia. Ammonia is dissolved in water, so we need to understand the chemical reactions occurring.

Step 2: Chemical Reaction

In water, ammonia (NH_{3}) reacts with water to form ammonium ions (NH_{4}^{+}) and hydroxide ions (OH^{-}) as follows: NH_{3} (aq) + H_{2}O (l) ightarrow NH_{4}^{+} (aq) + OH^{-} (aq).

Step 3: Identifying the Ions

From the chemical reaction, it is clear that the ions produced in the solution are NH_{4}^{+} and OH^{-}.

Step 4: Evaluating Options

Given the options, (a) NH_{4}^{+} and OH^{-} matches the ions identified from the reaction.

Key Concepts

Ammonium Ion

Ammonium ions, denoted as \( NH_{4}^{+} \), are formed when ammonia \( (NH_{3}) \) reacts with water. This reaction involves the transfer of a proton from the water to the ammonia molecule.

• Ammonia acts as a base, receiving a proton (\( H^{+} \)) from the water, resulting in the formation of the ammonium ion.
• Ammonium ions are positively charged due to the additional proton.

The presence of ammonium ions in a solution indicates that a reaction has occurred where ammonia has actively participated in a proton exchange mechanism. This process is crucial in the concept of acid-base reactions, often called Bronsted-Lowry acid-base reactions. By understanding these reactions, we can better predict the behavior of ammonia in various environments.

Hydroxide Ion

Hydroxide ions, symbolized as \( OH^{-} \), are negatively charged ions. They are a critical result of ammonia's reaction with water. When ammonia reacts with water, hydroxide ions are produced alongside ammonium ions.

• The generation of \( OH^{-} \) ions explains why an aqueous solution of ammonia is slightly basic.
• The presence of hydroxide ions increases the pH of the solution, making it more alkaline.

The concept of hydroxide ions is fundamental in understanding the basicity of solutions. Recognizing the occurrence of \( OH^{-} \) production helps in understanding why ammonia is used as a cleaning agent, as they react with dirt and grease, breaking them down.

Chemical Reaction of Ammonia in Water

The chemical reaction of ammonia in water is a fundamental aspect of chemistry, illustrating how molecules interact in solution. When ammonia \( (NH_{3}) \) is introduced to water \( (H_{2}O) \), it leads to the production of ammonium ions and hydroxide ions. The reaction can be represented by:\[NH_{3}(aq) + H_{2}O(l) \rightarrow NH_{4}^{+}(aq) + OH^{-}(aq)\]

• This reaction demonstrates ammonia's behavior as a weak base.
• It highlights the concept of dynamic equilibrium, as the reaction is reversible, allowing for the continuous formation and dissociation of these ions.

Understanding this reaction helps predict the chemical behavior of solutions containing ammonia. It also provides insight into why ammonia solutions can have varied pH levels depending on concentration and equilibrium dynamics. This foundational reaction underscores many applications in both industrial and laboratory settings, showcasing the practical importance of basic chemistry principles.