Problem 107 - Introductory Chemistry Atoms First

Question

Write an equilibrium to show how a primary amine, \(\mathrm{H}_{2} \mathrm{NR}\), makes water basic. To which side does the equilibrium lie? What does this imply?

Step-by-Step Solution

Answer

The equilibrium equation for a primary amine (H2NR) reacting with water (H2O) is: \[ \mathrm{H}_{2}\mathrm{NR} + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3}\mathrm{N^{+}R} + \mathrm{OH}^{-} \] Amines are weak bases, so the equilibrium mainly lies to the left side. The presence of hydroxide ions (OH⁻) produced in the reaction makes the water basic, implying that primary amines act as weak bases in aqueous solutions.

Step 1: Write the equilibrium equation for the reaction

To write the equilibrium equation, we need to represent the reaction between a primary amine (H2NR) and water (H2O), and the products formed. The general reaction is given as follows: \[ \mathrm{H}_{2}\mathrm{NR} + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3}\mathrm{N^{+}R} + \mathrm{OH}^{-} \]

Step 2: Determine the equilibrium position

To determine which side the equilibrium lies, we will compare the acidity constants of the reactants and products. The stronger the reactant, the more it will favor the formation of the products and vice versa. Amines are known to be weak bases, which means they only partially ionize in water. Therefore, this equilibrium will lie mainly to the left side.

Step 3: Analyze the implications

As the equilibrium primarily lies to the left, it means that only a small amount of hydroxide ions (OH⁻) are produced. However, the presence of these hydroxide ions makes the water basic, which means that primary amines act as weak bases in aqueous solutions.

Key Concepts

Equilibrium Reaction

Understanding equilibrium reactions is crucial for grasping acid-base interactions in chemistry. An equilibrium reaction occurs when a chemical process reaches a state where the rate of the forward reaction equals the rate of the reverse reaction, resulting in a balance of reactants and products.

This balance does not mean the amounts of reactants and products are equal but that their concentrations no longer change with time. The position of equilibrium describes where this balance lies; for instance, it can favor the reactants, the products, or be in the middle. In the case of amine basicity, the reaction between a primary amine and water primarily lies to the left, indicating the insufficient ionization of the amine to form large quantities of hydroxide ions.

Acid-Base Chemistry

Within the context of acid-base chemistry, substances are characterized by their ability to donate or accept protons (H+ ions). Acids are proton donors, and bases are proton acceptors. The acid-base behavior of a substance is pivotal when it reacts in water which itself can act both as an acid and a base.

When a primary amine reacts with water, it accepts a proton from water, resulting in the production of hydroxide ions (OH⁻) and the amine converting into its conjugate acid. This act of accepting a proton is central to understanding how amines can increase the pH of a solution, hence rendering it basic. Due to this property, measuring the pH of a solution can provide insights into the equilibrium state and base strength.

Weak Bases

Amines fall under the category of weak bases, which is essential to know for anyone studying acid-base chemistry. What distinguishes weak bases from strong ones is their partial ionization in water. Unlike strong bases which dissociate completely in aqueous solutions, weak bases like amines do not release a significant number of hydroxide ions.

This partial dissociation means that when amines are in equilibrium in a water solution, only a small portion of the amine molecules convert into their ionized form, creating a relatively low concentration of hydroxide ions. This is reflected in the equilibrium position leaning towards the reactants, indicating that the base does not significantly deplete the number of unreacted amine molecules.

Hydroxide Ions

The presence and concentration of hydroxide ions (OH⁻) is an indicator of the basicity of a solution. These negatively charged ions are what make a solution basic. In the reaction between a primary amine and water, the creation of hydroxide ions is what tips the scale towards basicity.

Even though a primary amine is a weak base, it still contributes to the generation of hydroxide ions. However, since the amine is a weak base, the equilibrium reaction does not produce a large quantity of OH⁻. Still, the small increase in hydroxide ion concentration is sufficient to make the water basic, which is central to the concept of amine basicity in water. This illustrates the nuanced but impactful role weak bases play in influencing the pH of solutions.