Question

Are carboxylic acids typically strong acids or weak acids? Write an equation showing the acid \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\) ionizing in water.

Short answer

Carboxylic acids are typically weak acids. The ionization reaction of \(CH_3CH_2COOH\) (a carboxylic acid) in water is represented by the following equation: \(CH_3CH_2COOH + H_2O \rightleftharpoons CH_3CH_2COO^- + H_3O^+\) Here, the carboxylic acid donates a hydrogen ion (H+) to a water molecule, forming its conjugate base, the carboxylate ion (\(CH_3CH_2COO^-\)), and a hydronium ion (\(H_3O^+\)). The equilibrium arrows (\(\rightleftharpoons\)) indicate that the ionization reaction is reversible and does not go to completion, which is characteristic of weak acids.

Step-by-step solution

Identify the acidity of carboxylic acids

To determine if carboxylic acids are strong or weak acids, we need to know the extent to which they ionize in water. Carboxylic acids are weak acids because they ionize partially in water, meaning only a small percentage of acid molecules break apart to form ions. Now that we know carboxylic acids are weak acids, let's look at the ionization of the given carboxylic acid in water.

Write the ionization reaction of CH3CH2COOH in water

To write the ionization reaction of the carboxylic acid \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\) in water, we need to show how the acid donates a hydrogen (H+) ion to a water (H2O) molecule, producing its conjugate base (the carboxylate ion) and a hydronium ion (H3O+). Here's the ionization reaction: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-} + \mathrm{H}_{3} \mathrm{O}^{+}\) In this equation, the carboxylic acid \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\) donates a hydrogen ion (H+) to a water molecule, forming its conjugate base, the carboxylate ion \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}\), and a hydronium ion \(\mathrm{H}_{3} \mathrm{O}^{+}\). The double-headed arrow (\(\rightleftharpoons\)) indicates that the ionization reaction is in equilibrium, which is typical of weak acids.

Key concepts

Weak Acids

Carboxylic acids, like many acids, fall under the category of weak acids. To determine the nature of an acid, we evaluate how completely it ionizes in water. Unlike strong acids, which almost fully dissociate into ions, weak acids ionize only partially. This means:

• Only a small fraction of a weak acid's molecules release hydrogen ions (H+).
• The majority of the acid remains in its un-ionized form.

Carboxylic acids, such as ethanoic acid (\( \text{CH}_3\text{CH}_2\text{COOH} \)), demonstrate this partial ionization. When placed in water, only a few of the acid molecules will dissociate to form ions. This key characteristic of carboxylic acids classifies them as weak acids.

Ionization in Water

The ionization process of carboxylic acids in water is a fundamental concept when discussing their acid-base behavior. When a carboxylic acid such as propanoic acid (\( \text{CH}_3\text{CH}_2\text{COOH} \)) is placed in water:

• The acid molecule donates a hydrogen ion (H+), which binds with a water molecule (H2O).
• This forms a hydronium ion (\( \text{H}_3\text{O}^+ \)).
• The remaining part of the acid molecule becomes a conjugate base, forming a carboxylate ion (\( \text{CH}_3\text{CH}_2\text{COO}^- \)).

The ionization reaction can be represented as:\[\text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COO}^- + \text{H}_3\text{O}^+ \]This reaction shows that the equilibrium is dynamic, meaning it shifts back and forth, reflecting the nature of weak acids. Most molecules remain un-ionized, emphasizing the partial ionization characteristic.

Acid-Base Equilibrium

The concept of equilibrium is essential in understanding the behavior of carboxylic acids in water. An equilibrium state is achieved in a reversible chemical reaction when the rate of the forward reaction equals the rate of the backward reaction. In the context of weak acids:

• The forward reaction is the ionization of the acid, leading to the formation of ions.
• The backward reaction involves the recombination of ions back to the acid.

The equation representing the ionization of propanoic acid (\( \text{CH}_3\text{CH}_2\text{COOH} \)) shows:\[\text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COO}^- + \text{H}_3\text{O}^+ \]This double-headed arrow (\( \rightleftharpoons \)) signifies the reaction's equilibrium nature, typical of weak acids. At equilibrium, the concentrations of reactants and products remain constant, allowing us to calculate important values like the acid dissociation constant \(K_a\), which quantifies the strength of a weak acid.