Question

Organic acids contain the carboxyl group Using acetic acid, \(\mathrm{CH}_{3}-\mathrm{COOH}\), and propionic acid, \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOH},\) write equations showing how the carboxyl group enables these substances to behave as weak acids when dissolved in water.

Short answer

When acetic acid and propionic acid dissolve in water, their carboxyl groups (COOH) donate a hydrogen ion (H+) to water, forming corresponding anions and hydronium ions (H₃O⁺). This can be represented by the following equations: For acetic acid: \( CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq) \) For propionic acid: \( CH₃CH₂COOH(aq) + H₂O(l) ⇌ CH₃CH₂COO⁻(aq) + H₃O⁺(aq) \) These equations demonstrate how the carboxyl group enables these substances to behave as weak acids in water.

Step-by-step solution

Write the general equation for weak acids in water

When a weak acid (HA) dissolves in water, it donates a hydrogen ion (H+) to the water (H₂O), which forms hydronium ions (H₃O+) and the corresponding anions (A⁻). This can be represented by the following general equation: HA(aq) + H₂O(l) ⇌ H₃O⁺ + A⁻

Write the equation for acetic acid in water

Acetic acid (CH₃COOH) has a carboxyl group (COOH) that donates a hydrogen ion (H+) to water when dissolved. This results in the formation of acetate ions (CH₃COO⁻) and hydronium ions (H₃O⁺). The equation can be written as: CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)

Write the equation for propionic acid in water

Propionic acid (CH₃CH₂COOH) also has a carboxyl group (COOH) that donates a hydrogen ion (H+) to water when dissolved. This results in the formation of propionate ions (CH₃CH₂COO⁻) and hydronium ions (H₃O⁺). The equation can be written as: CH₃CH₂COOH(aq) + H₂O(l) ⇌ CH₃CH₂COO⁻(aq) + H₃O⁺(aq) By examining these equations, we can see how the carboxyl group in acetic acid and propionic acid allows them to behave as weak acids when dissolved in water. They both donate a hydrogen ion (H+) to water, forming their corresponding anions and hydronium ions.

Key concepts

Weak Acids in Water

Understanding how weak acids like acetic acid and propionic acid interact with water is crucial in chemistry. When dissolved, these acids partially dissociate to yield hydrogen ions, which interact with the water molecules to form hydronium ions (H₃O⁺).

The dissociation of a weak acid in water can be represented by the general chemical equation:
HA(aq) + H₂O(l) ⇌ A⁻(aq) + H₃O⁺(aq)

This reaction is reversible, indicating that the formation of ions is not complete and both the acid (HA) and its conjugate base (A⁻) remain in the solution. As these reactions reach a state of dynamic equilibrium, the strength of an acid is often given by its acid dissociation constant (Ka), which quantifies the extent of dissociation in water. The smaller the Ka value, the weaker the acid.

Acetic Acid Chemical Equation

The chemical equation for the dissociation of acetic acid (CH₃COOH) in water can be a specific demonstration of the behavior of weak acids. Acetic acid, which is found in vinegar, partially ionizes in water to form acetate ions (CH₃COO⁻) and hydronium ions.

The balanced reaction is as follows:
CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)

This reversible reaction demonstrates the weak acid nature of acetic acid. In an aqueous solution, only a small fraction of acetic acid molecules donate their hydrogen atom to form hydronium ions, hence the equilibrium lies to the left, favoring the reactants.

Propionic Acid Reaction

The behavior of propionic acid in water is similar to that of acetic acid. Propionic acid (CH₃CH₂COOH) also contains a carboxyl group that can donate a proton to water. The resulting reaction, much like acetic acid, leads to the formation of propionate ions and hydronium ions.

The chemical equation for this reaction is:
CH₃CH₂COOH(aq) + H₂O(l) ⇌ CH₃CH₂COO⁻(aq) + H₃O⁺(aq)

This equation shows the reversible process where propionic acid acts as a weak acid. A portion of the propionic acid molecules dissociate in water, but since it is a weak acid, the conjugate base, propionate ions, exists concurrently in a state of equilibrium with the undissociated propionic acid and hydronium ions.

Hydronium Ion Formation

The formation of hydronium ions (H₃O⁺) is a key aspect in the behavior of acids in water. When a weak acid donates a hydrogen ion to water, this proton (H⁺) does not exist freely in solution; rather, it immediately associates with a water molecule to form a hydronium ion.

The general reaction for hydronium ion formation from a proton and water is:
H⁺ + H₂O(l) → H₃O⁺(aq)

The presence and concentration of these hydronium ions are central to the concept of pH, which measures the acidity or basicity of a solution. Weak acids contribute fewer hydronium ions to the solution compared to strong acids, which fully dissociate, hence they have a higher pH (less acidic). The equilibrium established in the dissociation of weak acids demonstrates that not all of the protons form hydronium ions, which is an important consideration in order to understand acid strength and pH levels.