Problem 94 - Chemistry Matter and Change

Question

Write the equation for the ionization reaction and the base ionization constant expression for ethylamine \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\right)\) in water.

Step-by-Step Solution

Answer

The ionization reaction for ethylamine in water is: \(C_2H_5NH_2 (aq) + H_2O (l) \rightleftharpoons C_2H_5NH_3^+ (aq) + OH^- (aq)\) The base ionization constant expression (Kb) for ethylamine is: Kb = \(\frac{[C_2H_5NH_3^+][OH^-]}{[C_2H_5NH_2]}\)

Step 1: Write the ionization reaction for ethylamine in water

The ionization reaction occurs when ethylamine accepts a proton from water, forming its conjugate acid and hydroxide ions. The reaction is as follows: \(C_2H_5NH_2 (aq) + H_2O (l) \rightleftharpoons C_2H_5NH_3^+ (aq) + OH^- (aq)\)

Step 2: Write the base ionization constant expression (Kb) for ethylamine

The base ionization constant expression is a ratio of the concentrations of the products to the reactants at equilibrium, excluding pure liquids and solids. Water in this case will not be included in the expression, as it is a pure liquid. Kb = \(\frac{[C_2H_5NH_3^+][OH^-]}{[C_2H_5NH_2]}\) This is the final base ionization constant expression (Kb) for ethylamine ionization in water.

Key Concepts

Ethylamine

Ethylamine, having the chemical formula \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2} \), is a simple amine and a derivative of ammonia. It consists of an ethyl group \( \mathrm{C}_{2} \mathrm{H}_{5} \) attached to an amino group \( \mathrm{NH}_{2} \). Due to its structure, ethylamine can act as a weak base. This is because the nitrogen atom in the amino group has a lone pair of electrons that can accept a proton (H\(^+\)) from water, leading to an ionization reaction. Ethylamines are commonly used in organic synthesis as intermediates, in the production of rubber, and in the manufacture of pharmaceuticals. Understanding its role in ionization helps in predicting how it behaves in aqueous solutions, which is crucial for chemical reactions and formulations. As a base, ethylamine tends to raise the pH of a solution by accepting protons, forming its conjugate acid and hydroxide ions. This characteristic is key to understanding its chemical behavior and applications.

Ionization Reaction

The ionization reaction of a base like ethylamine involves the base accepting a proton from water. This results in the formation of its conjugate acid and hydroxide ions. The chemical equation for the ionization of ethylamine in water is shown as follows:\[ C_2H_5NH_2 \,(aq) + H_2O \,(l) \rightleftharpoons C_2H_5NH_3^+ \,(aq) + OH^- \,(aq) \]

• Formation of Conjugate Acid: Ethylamine accepts a proton to form \( C_2H_5NH_3^+ \), which is its conjugate acid.
• Production of Hydroxide Ions: The hydroxide ion \( OH^- \) is produced, which increases the solution's basicity.

This equilibrium reaction highlights the reversible nature of bases in solution. It's important to note how ethylamine can switch between a neutral molecule and its ionized form based on the presence of water and the need to balance the reaction's forward and reverse pathways.

Equilibrium Expressions

In the context of the ionization of a weak base like ethylamine, equilibrium expressions play a crucial role in understanding the extent of ionization in solution. The equilibrium constant expression, specifically the base ionization constant \( K_b \), is used to determine this.The \( K_b \) expression is derived from the equilibrium concentrations of the species involved. For ethylamine, the expression is:\[ K_b = \frac{[C_2H_5NH_3^+][OH^-]}{[C_2H_5NH_2]} \]This expression involves:

• Numerator: Concentrations of the products \( [C_2H_5NH_3^+] \) and \( [OH^-] \).
• Denominator: Concentration of the reactant \( [C_2H_5NH_2] \).

The \( K_b \) value indicates the strength of the base: the higher the \( K_b \), the stronger the ability of the base to accept protons and produce hydroxide ions. Understanding \( K_b \) is fundamental for predicting how much of the base will ionize in water, and it assists chemists in analyzing the pH and related properties of the solution.