Problem 41 - Chemistry and Chemical Reactivity

Question

Write two chemical equations, one that shows \(\mathrm{H}_{2} \mathrm{O}\) reacting (with HBr) as a Bronsted base and a second that shows \(\mathrm{H}_{2} \mathrm{O}\) reacting (with \(\mathrm{NH}_{3}\) ) as a Bronsted acid.

Step-by-Step Solution

Answer

\(\text{H}_2\text{O} + \text{HBr} \rightarrow \text{H}_3\text{O}^+ + \text{Br}^-\); \(\text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{OH}^-\).

Step 1: Understanding Bronsted Acid-Base Theory

According to the Bronsted-Lowry theory, an acid is a species that donates a proton (H+), while a base is a species that accepts a proton. This theory will guide us in writing the reactions involving water as both an acid and a base.

Step 2: Writing the First Equation (Water as a Bronsted Base)

When water reacts with hydrobromic acid (HBr), water acts as a Bronsted base. The reaction is: \[ \text{H}_2\text{O} + \text{HBr} \rightarrow \text{H}_3\text{O}^+ + \text{Br}^- \]. Water accepts a proton from HBr, forming hydronium ion \(\text{H}_3\text{O}^+\) and bromide ion \(\text{Br}^-\).

Step 3: Writing the Second Equation (Water as a Bronsted Acid)

When water reacts with ammonia (NH3), water acts as a Bronsted acid. The reaction is: \[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{OH}^- \]. Water donates a proton to ammonia, forming ammonium ion \(\text{NH}_4^+\) and hydroxide ion \(\text{OH}^-\).

Key Concepts

HBr reaction with water

When hydrobromic acid (HBr) interacts with water (\(\text{H}_2\text{O}\)), it's a fascinating example of a Bronsted-Lowry acid-base reaction. In this reaction, water plays the role of a base. This means water is acting as a proton acceptor. Let's break it down further.

HBr is a strong acid, which means it has a high tendency to donate protons. In this process, it breaks apart into its component ions: hydrogen ion (H+) and bromide ion (Br-). As per the Bronsted-Lowry theory, the hydrogen ion (H+) is the proton getting transferred. Water steps in and accepts this proton, transforming into a hydronium ion (\(\text{H}_3\text{O}^+\)).

Here's the full reaction: \[ \text{H}_2\text{O} + \text{HBr} \rightarrow \text{H}_3\text{O}^+ + \text{Br}^- \]

• Water, acting as a base, accepts the proton.
• Resulting in hydronium ion and bromide ion as products.

This reaction not only highlights the acid-base behavior but also underscores the importance of understanding proton transfer in chemical reactions.

NH3 reaction with water

In the reaction between ammonia (NH3) and water, the roles reverse, and water acts as a Bronsted-Lowry acid. Here, water donates a proton to ammonia, showcasing another aspect of proton transfer reactions.

Ammonia (\(\text{NH}_3\)) is a weak base and has the ability to accept protons. When it encounters water, it accepts a proton from water. This proton transfer results in the formation of an ammonium ion (\(\text{NH}_4^+\)) and a hydroxide ion (\(\text{OH}^-\)). This change is invaluable for understanding how different substances can interchangeably act as acids or bases.

The equation expressed is:\[ \text{H}_2\text{O} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{OH}^- \]

• Water acts as an acid, donating a proton.
• The conversion results in ammonium and hydroxide ions.

This reaction is pivotal in illustrating how water can function flexibly as both an acid or a base, depending on the reactants involved.

Proton transfer reactions

Proton transfer reactions are a hallmark of Bronsted-Lowry acid-base theory and are fundamental in chemistry. These reactions revolve around the exchange of protons between different substances.

In the context of water reacting with HBr and NH3, we've seen water acting both ways – as an acid in one scenario and as a base in another. This dual nature is essential for numerous chemical reactions and allows for the dynamic equilibrium we observe in solutions.

• When a substance donates a proton, it's acting as a Bronsted acid.
• When a substance accepts a proton, it's acting as a Bronsted base.

The key is knowing that the environment or the reactants play a crucial role in determining the behavior of the substances involved. Understanding these principles helps us predict and manipulate chemical reactions, which is crucial in industries from pharmaceuticals to environmental science. Thus, mastering these concepts can significantly broaden one's grasp of chemical processes.