Problem 10 - Chemistry The Molecular Science

Question

Write a chemical equation to describe the proton transfer that occurs when each of these acids is added to water. (a) HIO (b) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{COOH}\) (c) HOOCCOOH (d) \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\)

Step-by-Step Solution

Answer

Equations: (a) HIO + H₂O → IO⁻ + H₃O⁺; (b) Pentanoic acid + H₂O → carboxylate + H₃O⁺; (c) Oxalic acid + H₂O → hydrogen oxalate + H₃O⁺; (d) Methylammonium ion + H₂O → methylamine + H₃O⁺.

Step 1: Understand Proton Transfer

Proton transfer involves an acid donating a proton (H⁺) to water or to a base, forming the conjugate base of the acid and hydronium ion (H₃O⁺). This is a crucial step in understanding acid-base reactions.

Step 2: Write Chemical Equation for HIO

For hypoiodous acid (HIO), it donates a proton to water: \[ \text{HIO} + \text{H}_2\text{O} \rightarrow \text{IO}^- + \text{H}_3\text{O}^+ \] Here, HIO acts as the acid, water as the base, resulting in IO⁻ as the conjugate base and H₃O⁺.

Step 3: Write Chemical Equation for Pentanoic Acid

For the carboxylic acid \(\text{CH}_3(\text{CH}_2)_4\text{COOH}\), it donates a proton to water: \[ \text{CH}_3(\text{CH}_2)_4\text{COOH} + \text{H}_2\text{O} \rightarrow \text{CH}_3(\text{CH}_2)_4\text{COO}^- + \text{H}_3\text{O}^+ \] The carboxylic acid donates an H⁺ to form the corresponding carboxylate ion and hydronium.

Step 4: Write Chemical Equation for Oxalic Acid

For oxalic acid (HOOCCOOH), it donates a proton to water: \[ \text{HOOCCOOH} + \text{H}_2\text{O} \rightarrow \text{HOOC-COO}^- + \text{H}_3\text{O}^+ \] Oxalic acid loses a proton to form the hydrogen oxalate ion and the hydronium ion.

Step 5: Write Chemical Equation for Methylammonium Ion

For the methylammonium ion \(\text{CH}_3\text{NH}_3^+\), it donates a proton to water: \[ \text{CH}_3\text{NH}_3^+ + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{NH}_2 + \text{H}_3\text{O}^+ \] The methylammonium ion acts as an acid, transferring a proton to water, resulting in methylamine and hydronium.

Key Concepts

Acid-Base Reactions

Acid-base reactions, also known as proton transfer reactions, are fundamental chemical processes where an acid transfers a proton (H⁺) to a base. In simpler terms, the acid loses a hydrogen ion, while the base gains it. This process often results in the formation of a conjugate base from the acid, and a conjugate acid from the base. Typically, water acts as a base in these reactions, accepting a proton to become the hydronium ion (H₃O⁺).To understand these reactions, consider the role of each participant:

• An **acid** is a molecule or ion that donates a proton.
• A **base** is a molecule or ion that accepts a proton.

The reaction can usually be generalized by the equation:\[ \text{HA} + \text{B} \rightarrow \text{A}^- + \text{HB}^+ \]Here, HA represents an acid, which donates a proton to B, a base. As a result, A⁻ is the conjugate base, and HB⁺ the conjugate acid.

Conjugate Base

When an acid donates a proton during an acid-base reaction, it transforms into a conjugate base. The conjugate base is what remains of the acid after it has released a proton. Understanding conjugate bases is crucial for predicting the outcome of reactions and their corresponding strengths.For instance, in the reaction of hypoiodous acid (HIO) with water:\[ \text{HIO} + \text{H}_2\text{O} \rightarrow \text{IO}^- + \text{H}_3\text{O}^+ \]HIO donates a proton to water, resulting in the conjugate base IO⁻. This conjugate base can be used to measure the strength of the original acid. A strong acid will generally produce a weak conjugate base, and vice versa.Here’s what to remember:

• The **conjugate base** often helps maintain equilibrium in solutions.
• Increased stability of a conjugate base implies lower proton affinity.
• A weak conjugate base suggests a strong parent acid.

Hydronium Ion

In many acid-base reactions, particularly those involving water, the hydronium ion (H₃O⁺) is a key product. It results when water acts as a proton acceptor, gaining an additional hydrogen ion (H⁺). This transformation is central to defining the acidity in aqueous solutions.Consider the reaction of pentanoic acid (\(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{COOH}\)) with water:\[ \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4}\mathrm{COOH} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4}\mathrm{COO}^- + \mathrm{H}_{3}\mathrm{O}^+ \]In this scenario, water receives a proton from the acid, forming the hydronium ion (H₃O⁺). This ion is significant because:

• It indicates the presence of an acidic environment.
• Its concentration is directly associated with the pH of the solution.
• High levels of H₃O⁺ denote a low pH, indicating stronger acidity.

The hydronium ion plays a crucial role in many chemical reactions, acting as a medium that facilitates proton transfers and acidity measurement.