Problem 47 - Chemistry The Science in Context

Question

When methylamine, \(\mathrm{CH}_{3} \mathrm{NH}_{2},\) dissolves in water, the resulting solution is slightly basic. Which compound is the Bronsted-Lowry acid and which is the base?

Step-by-Step Solution

Answer

Question: Identify the Bronsted-Lowry acid and base in the reaction of methylamine dissolving in water. Answer: In the reaction of methylamine dissolving in water, the Bronsted-Lowry acid is water, and the Bronsted-Lowry base is methylamine.

Step 1: Write the chemical equation

To begin, write down the chemical equation for the reaction of methylamine dissolving in water. The equation is: \(\mathrm{CH}_{3} \mathrm{NH}_{2} + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3} \mathrm{NH}_{3}^{+} + \mathrm{OH}^{-}\) This is an equilibrium reaction, so the double arrow indicates that the reaction can proceed in both forward and reverse directions.

Step 2: Identify the Bronsted-Lowry acid and base

Recall that according to the Bronsted-Lowry definition, an acid is a substance that donates a proton (\(\mathrm{H}^{+}\)) and a base is a substance that accepts a proton. In our reaction, we can see that: 1. Methylamine (\(\mathrm{CH}_{3} \mathrm{NH}_{2}\)) gains a proton from water (\(\mathrm{H}_{2} \mathrm{O}\)) and is converted to its conjugate acid, methylammonium ion (\(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\)). Therefore, methylamine is the Bronsted-Lowry base in this reaction. 2. Water (\(\mathrm{H}_{2} \mathrm{O}\)) donates a proton to methylamine and is converted to its conjugate base, hydroxide ion (\(\mathrm{OH}^{-}\)). Therefore, water is the Bronsted-Lowry acid in this reaction. So, in the reaction of methylamine dissolving in water, the Bronsted-Lowry acid is water, and the Bronsted-Lowry base is methylamine.

Key Concepts

Methylamine Reaction

When methylamine, or \(\text{CH}_3\text{NH}_2\), dissolves in water, a fascinating chemical reaction occurs. Methylamine acts as a basic substance, which means it tends to accept protons. In this reaction, methylamine comes into contact with water molecules, which behave as an acidic substance, meaning they can donate protons.

Here's the balanced chemical equation for this process:

• \(\text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{NH}_3^+ + \text{OH}^-\)

In this reaction, methylamine \((\text{CH}_3\text{NH}_2)\) accepts a proton from water \((\text{H}_2\text{O})\) and becomes the methylammonium ion \((\text{CH}_3\text{NH}_3^+)\), its conjugate acid. The water, having donated a proton, turns into hydroxide \((\text{OH}^-)\), its conjugate base. This exchange leads to the formation of a basic solution, as the hydroxide ions increase the pH.

By understanding this reaction on a molecular level, we can see how substances like methylamine can alter the environment they dissolve in.

Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry, seen in the reaction between methylamine and water. In this context, equilibrium means that the forward and reverse reactions occur at the same rate. Without any further disturbances, the concentrations of reactants and products remain constant over time.

In our equation, the double arrows \(\rightleftharpoons\) represent this state of balance:

• \(\text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{NH}_3^+ + \text{OH}^-\)

Even in equilibrium, the reaction is dynamic, meaning that molecules constantly react and form products without changing the overall concentration. Some conditions, like temperature or concentration changes, can shift equilibrium, altering the ratio of methylamine to hydroxide. Understanding chemical equilibrium helps predict how changes impact the system and whether the solution becomes more acidic or basic.

This concept is vital for chemists as it describes how reactions stabilize, providing insights into reaction dynamics and behavior.

Conjugate Acid-Base Pairs

Within the framework of Bronsted-Lowry acid-base theory, conjugate acid-base pairs are tightly linked. They consist of two species that transform into each other by the gain or loss of a proton \((\text{H}^+)\). In the methylamine-water reaction, we can clearly see these pairs in action.

Consider the reaction equation:

• \(\text{CH}_3\text{NH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{NH}_3^+ + \text{OH}^-\)

Here, \(\text{CH}_3\text{NH}_2\) (methylamine) and \(\text{CH}_3\text{NH}_3^+\) (methylammonium ion) form one conjugate pair. Methylamine accepts a proton to become its conjugate acid, \(\text{CH}_3\text{NH}_3^+\).

Similarly, \(\text{H}_2\text{O}\) and \(\text{OH}^-\) (hydroxide ion) are another conjugate pair. Water donates a proton to become \(\text{OH}^-\), its conjugate base.

Recognizing these pairs allows us to understand the acid-base behavior of substances and how they interact in reactions, reflecting the dynamic shifts in proton transfer at the molecular level.