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

The aqueous solution of ammonia consists of (a) \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{OH}^{-}\).

Step 1: Understanding Aqueous Ammonia

Aqueous ammonia refers to ammonia (NH₃) dissolved in water. In solution, NH₃ can accept a proton from water, leading to a reaction that produces ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). It is useful to remember that ammonia is a weak base, and in water, it partially ionizes to produce these ions.

Step 2: Identifying Reaction Products

The chemical reaction of ammonia in water can be described by the equation: \(\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-\). This equation shows that the products of dissolved ammonia in water are ammonium ions (NH₄⁺) and hydroxide ions (OH⁻).

Step 3: Determining Solution Components

From the reaction, we know that the components in the aqueous solution of ammonia include both \(\mathrm{NH}_4^+\) and \(\mathrm{OH}^-\). Each of these ions signifies that both options (a) and (b) are present in the solution.

Step 4: Selecting the Correct Answer

Among the given options, the one that correctly specifies the constituents of the aqueous ammonia solution includes both \(\mathrm{NH}_4^+\) and \(\mathrm{OH}^-\). Therefore, option (a) \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{OH}^{-}\) is the correct choice.

Key Concepts

Ammonium Ion (NH₄⁺)

In an aqueous ammonia solution, the ammonium ion ( NH₄⁺) plays a crucial role. Ammonium ions are generated when ammonia ( NH₃) accepts a proton (H⁺) from water molecules. This proton transfer forms NH₄⁺ and hydroxide ions ( OH⁻). This process reflects ammonia's ability as a weak base to interact with water and produce ions.

The presence of the ammonium ion indicates that ammonia's interaction with water produces a balance between different species in the solution. Ammonium ions are essential in maintaining this equilibrium, containing extra hydrogen, thus behaving like a conjugate acid. Here are some important points about NH₄⁺:

• Has one more hydrogen than ammonia (NH₃).
• Is positively charged, indicating a gain of a proton.
• Acts as a conjugate acid in the acid-base balance.

Understanding the structure and role of NH₄⁺ helps us grasp how weak bases like ammonia behave in aqueous solutions.

Hydroxide Ion (OH⁻)

The hydroxide ion ( OH⁻) is another product of ammonia's reaction with water. In this reaction, ammonia acts as a weak base and partly converts into NH₄⁺ and OH⁻. So, the presence of OH⁻ in solution points to the basic nature of the water.

OH⁻ is a key participant in determining the pH level of the solution. It is a fundamental unit in chemistry, often responsible for alkaline properties due to its negative charge. Here's what you should know:

• Acts as the conjugate base in water.
• Responsible for increasing the pH of the solution.
• Indicates the basic nature when released in solution.

Hydroxide ions are essential in understanding ammonia's equilibrium in water, as they represent the shift from neutral to more basic conditions.

Weak Base Behavior

Ammonia is classified as a weak base. This means it does not completely dissociate in water to form its ions. Instead, the equilibrium between the ammonia, water, NH₄⁺, and OH⁻, remains intact.

In weak bases, the ionization is partial, this contrasts with strong bases which fully dissociate. With ammonia:

• Only some molecules pick up hydrogen ions to form NH₄⁺.
• It's an equilibrium process, depicted by a reversible reaction arrow (↔).
• This partial reaction leads to a moderate pH increase.

Understanding weak base behavior is key, as it impacts the solution's pH and chemical balance. This behavior of ammonia explains why both the NH₄⁺ and OH⁻ ions exist in solution, creating a unique equilibrium in an aqueous environment.