Problem 13 - Introductory Chemistry Atoms First

Question

In Reaction \(15.7\), water is shown acting as a base. How is water behaving in the following reaction? $$ \mathrm{NH}_{2}^{-}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NH}_{3}+\mathrm{OH}^{-} $$ Explain your answer.

Step-by-Step Solution

Answer

In the reaction \(\mathrm{NH}_{2}^{-}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NH}_{3}+\mathrm{OH}^{-}\), water is behaving as an acid because it donates a hydrogen ion (H+) to the \(\mathrm{NH}_{2}^{-}\) ion, resulting in the formation of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\).

Step 1: Understand the role of acids and bases

Acids are substances that donate a hydrogen ion (H+), while bases are substances that accept a hydrogen ion. In a reaction, when a molecule accepts a hydrogen ion, it is functioning as a base; while if it donates a hydrogen ion, it is functioning as an acid.

Step 2: Identify the reactants and products of the reaction

In the given reaction: $$ \mathrm{NH}_{2}^{-}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NH}_{3}+\mathrm{OH}^{-} $$ The reactants are \(\mathrm{NH}_{2}^-\) and \(\mathrm{H}_{2} \mathrm{O}\), while the products are \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\).

Step 3: Study the water molecule in the reaction

In the reaction: $$ \mathrm{NH}_{2}^{-}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{NH}_{3}+\mathrm{OH}^{-} $$ Water is one of the reactants and participates by giving away one of its hydrogen ions (H+) to the \(\mathrm{NH}_{2}^-\) ion.

Step 4: Determine how water is behaving in the reaction

When \(\mathrm{H}_{2}\mathrm{O}\) donates a hydrogen ion to \(\mathrm{NH}_{2}^{-}\) to form \(\mathrm{NH}_{3}\), it leaves behind an \(\mathrm{OH}^{-}\) ion. Since water is donating a hydrogen ion (H+) in the reaction, it is functioning as an acid.

Step 5: Provide the explanation

In the given reaction, water is behaving as an acid because it donates a hydrogen ion (H+) to the \(\mathrm{NH}_{2}^{-}\) ion, resulting in the formation of \(\mathrm{NH}_{3}\) and \(\mathrm{OH}^{-}\).

Key Concepts

Bronsted-Lowry Acids and Bases

When discussing the Bronsted-Lowry theory, it's all about the transfer of protons, specifically hydrogen ions (H+). According to this theory, a substance that donates a proton is termed as an acid, while the one that accepts a proton is known as a base. It's essential to think of protons as lightweight particles that are often traded in chemical reactions.

Take hydronium ion \(\mathrm{H}_3\mathrm{O}^+\)as an example. It acts as a Bronsted-Lowry acid because it can donate a proton to another substance, leaving behind a water molecule. Similarly, ammonia \(\mathrm{NH}_3\) can accept a proton from water, making it a Bronsted-Lowry base, as it forms ammonium \(\mathrm{NH}_4^+\).

• Acids: Proton donors
• Bases: Proton acceptors

This way, the Bronsted-Lowry model broadens our understanding. It allows us to identify various substances, like water, that can switch roles between acid and base depending on the reaction.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds. In the context of the acid-base reaction detailed here, the focus is on how molecules interact to transfer hydrogen ions.

In the reaction \( \mathrm{NH}_2^{-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_3 + \mathrm{OH}^{-} \), the key players are hydroxide \(\mathrm{OH}^{-}\)and water \(\mathrm{H}_2\mathrm{O}\). Water starts by donating a proton. This process decreases water's hydrogen count and converts it into hydroxide.

• Reactants interact in a specific pathway
• Sometimes substances can have dual roles, acting differently depending on conditions

Such reactions can completely change the nature of substances involved, a fascinating aspect of chemistry that underscores how dynamic chemical processes can be.

Proton Transfer

Proton transfer is central in many reactions, particularly in acid-base chemistry. It's the movement of a proton from one molecule to another, reshaping their identities in the process.

In the given reaction \( \mathrm{NH}_2^{-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_3 + \mathrm{OH}^{-} \), \(\mathrm{H}_2\mathrm{O}\)donates a proton to \(\mathrm{NH}_2^{-}\), converting it into \(\mathrm{NH}_3\). Water becomes a donor here, as it transfers a proton and consequently becomes hydroxide \(\mathrm{OH}^{-}\).

• Proton transfer shifts chemical balance
• Transforms reactants to products
• Essential in defining roles of acids and bases

Proton transfer is a core mechanism that reveals much about the reactivity and properties of molecules involved, showcasing the elegance of chemistry in transforming matter at a fundamental level.