Question

Draw molecular models to represent the following acidbase reactions: (a) \(\mathrm{OH}^{-}+\mathrm{H}_{3} \mathrm{O}^{+} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{NH}_{4}^{+}+\mathrm{NH}_{2}^{-} \longrightarrow 2 \mathrm{NH}_{3}\) Identify the Bronsted acid and base in each case.

Short answer

In (a), \( \mathrm{H}_{3} \mathrm{O}^{+} \) is the acid, \( \mathrm{OH}^{-} \) is the base. In (b), \( \mathrm{NH}_{4}^{+} \) is the acid, \( \mathrm{NH}_{2}^{-} \) is the base.

Step-by-step solution

Identify the Reactants

In part (a) of the reaction, we have \( \mathrm{OH}^{-} \), which is the hydroxide ion, and \( \mathrm{H}_{3} \mathrm{O}^{+} \), which is the hydronium ion. For part (b), the reactants are \( \mathrm{NH}_{4}^{+} \), the ammonium ion, and \( \mathrm{NH}_{2}^{-} \), the amide ion.

Determine Bronsted-Lowry Acid and Base

In the Bronsted-Lowry theory, an acid is a proton donor, and a base is a proton acceptor. In reaction (a), \( \mathrm{H}_{3} \mathrm{O}^{+} \), which donates a proton, is the acid, and \( \mathrm{OH}^{-} \), which accepts a proton, is the base. In reaction (b), \( \mathrm{NH}_{4}^{+} \) donates a proton, making it the acid, while \( \mathrm{NH}_{2}^{-} \) accepts a proton, making it the base.

Represent Acid-Base Reaction with Molecular Models (Part a)

Draw models showing \( \mathrm{OH}^{-} \) accepting a proton from \( \mathrm{H}_{3} \mathrm{O}^{+} \) to form two water molecules. Each water molecule has two hydrogen atoms covalently bonded to an oxygen atom, resulting from the transfer of a hydrogen ion from \( \mathrm{H}_{3} \mathrm{O}^{+} \) to \( \mathrm{OH}^{-} \).

Represent Acid-Base Reaction with Molecular Models (Part b)

Draw models showing \( \mathrm{NH}_{2}^{-} \) accepting a proton from \( \mathrm{NH}_{4}^{+} \) to form two ammonia molecules. Each ammonia molecule consists of a nitrogen atom bonded to three hydrogen atoms, forming from the proton transfer from \( \mathrm{NH}_{4}^{+} \) to \( \mathrm{NH}_{2}^{-} \).

Conclude Bronsted Acid-Base Identification

Reaffirm that in both reactions, the species donating the proton is the Bronsted acid, and the species accepting the proton is the Bronsted base. Thus, for (a), \( \mathrm{H}_{3} \mathrm{O}^{+} \) is the acid and \( \mathrm{OH}^{-} \) is the base; for (b), \( \mathrm{NH}_{4}^{+} \) is the acid and \( \mathrm{NH}_{2}^{-} \) is the base.

Key concepts

Proton Donor

In the Bronsted-Lowry acid-base theory, a proton donor is identified as an acid. Acids have the ability to release a proton (a hydrogen ion, \( ext{H}^+ \)). This process is crucial in many chemical reactions, as it allows for the transfer of protons between molecules. For example, in reaction (a), the hydronium ion \( ( ext{H}_3 ext{O}^+) \) is the proton donor, facilitating the formation of water. In reaction (b), the ammonium ion \( ( ext{NH}_4^+) \) donates a proton to form ammonia.

Key points about proton donors include:

• They often have a high concentration of hydrogen ions.
• They are characterized by their ability to donate a proton during a reaction.
• This donation is essential to the role they play in chemical equilibrium and reaction dynamics.

Proton donors can be found in both organic and inorganic compounds, highlighting their versatility and importance in chemistry.

Proton Acceptor

A proton acceptor is known as a base in the Bronsted-Lowry theory. Bases are defined by their ability to take in a proton from another molecule. In essence, they "grab" a hydrogen ion during interactions with acids. This is a foundational concept in understanding how reactions can affect pH and create new compounds. In reaction (a), the hydroxide ion \( ( ext{OH}^-) \) acts as the proton acceptor, resulting in water formation. Similarly, in reaction (b), the amide ion \( ( ext{NH}_2^-) \) acts as the base by accepting a proton to form ammonia.

Some important aspects of proton acceptors are:

• They usually have lone pairs of electrons which facilitate the acceptance of a proton.
• Proton acceptors are important in balancing chemical equations and sustaining equilibrium.
• They play a vital role in chemical reactions, such as neutralization.

Understanding proton acceptors helps demystify reactions that lead to new molecule formation.

Hydronium Ion

The hydronium ion \( ( ext{H}_3 ext{O}^+) \) plays a pivotal role in acid-base chemistry. It is created when a water molecule binds with an extra hydrogen ion, often making it a critical component in acidic solutions. In the context of the Bronsted-Lowry theory, hydronium ions are exemplary proton donors.

In reaction (a), \( ext{H}_3 ext{O}^+ \) donates a proton to the \( ext{OH}^- \) ion, resulting in the creation of two water molecules:

• Hydronium ions are commonly formed in acidic solutions.
• They are integral to understanding pH levels and acidity.
• Hydronium ions facilitate the proton exchange, crucial for driving acid-base reactions.

This ion encapsulates the essence of acids in aqueous environments, showcasing the dynamic nature of water and acid interactions.

Ammonium Ion

The ammonium ion \( ( ext{NH}_4^+) \) is a fascinating participant in acid-base chemistry, often viewed as a classic example of a proton donor. Formed by the addition of a hydrogen ion to ammonia \( ( ext{NH}_3) \), ammonium ions are frequently explored in reactions involving amines.

In reaction (b), \( ext{NH}_4^+ \) donates a proton to the amide ion \( ( ext{NH}_2^-) \), and as a result, two ammonia molecules are formed:

• Ammonium ions are found in various biological and chemical systems.
• They are essential in nitrogen cycle processes and fertilizer chemistry.
• Understanding \( ext{NH}_4^+ \) ions helps illustrate the behavior of basic and acidic environments.

By studying ammonium ions, one gains clearer insights into proton transfer processes and the formation of nitrogen-based compounds.