Problem 9 - 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) \(\mathrm{HCO}_{3}^{-}\) (b) HCl (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) HCN

Step-by-Step Solution

Answer

These are the equations: (a) \( ext{HCO}_{3}^{-} + ext{H}_2 ext{O} ightarrow ext{H}_2 ext{CO}_3 + ext{OH}^{-} \) (b) \( ext{HCl} + ext{H}_2 ext{O} ightarrow ext{H}_3 ext{O}^+ + ext{Cl}^- \) (c) \( ext{CH}_3 ext{COOH} + ext{H}_2 ext{O} ightarrow ext{CH}_3 ext{COO}^- + ext{H}_3 ext{O}^+ \) (d) \( ext{HCN} + ext{H}_2 ext{O} ightarrow ext{CN}^- + ext{H}_3 ext{O}^+ \)

Step 1: Understanding a Proton Transfer

In a proton transfer reaction, an acid donates a proton (\( ext{H}^+ \)) to water. This results in the formation of the corresponding conjugate base of the acid and a hydronium ion (\( ext{H}_3 ext{O}^+ \)).

Step 2: Writing the Equation for \( ext{HCO}_{3}^{-} \)

When bicarbonate, \( ext{HCO}_{3}^{-} \), acts as an acid, it donates a proton to water, forming carbonic acid and hydroxide ion: \( ext{HCO}_{3}^{-} + ext{H}_2 ext{O} ightarrow ext{H}_2 ext{CO}_3 + ext{OH}^{-} \).

Step 3: Writing the Equation for HCl

Hydrochloric acid (HCl) donates a proton to water to form hydronium ion and chloride ion: \( ext{HCl} + ext{H}_2 ext{O} ightarrow ext{H}_3 ext{O}^+ + ext{Cl}^- \).

Step 4: Writing the Equation for \( ext{CH}_3 ext{COOH} \)

Acetic acid, \( ext{CH}_3 ext{COOH} \), donates a proton to water to give acetate ion and hydronium ion: \( ext{CH}_3 ext{COOH} + ext{H}_2 ext{O} ightarrow ext{CH}_3 ext{COO}^- + ext{H}_3 ext{O}^+ \).

Step 5: Writing the Equation for HCN

Hydrogen cyanide (HCN) donates a proton to water, forming cyanide ion and hydronium ion: \( ext{HCN} + ext{H}_2 ext{O} ightarrow ext{CN}^- + ext{H}_3 ext{O}^+ \).

Key Concepts

Acid-Base Chemistry

Acid-base chemistry is a fascinating area of chemistry that focuses on the reactions between acids and bases. These reactions always involve the transfer of a proton (\(\text{H}^+\)), significantly impacting the chemical behavior of substances.
In this context:

• Acids are substances that can donate a proton to another substance. They have a characteristic sour taste and are commonly found in vinegar and citrus juices.
• Bases are substances that can accept a proton. They are often found in cleaning products and have a slippery feel.

Understanding the behavior of acids and bases helps chemists predict and manipulate chemical reactions, which is crucial in fields ranging from pharmaceuticals to material science.

Chemical Equations

Chemical equations are used to represent chemical reactions in a concise format. They show which substances are reactants and which are products, along with their physical states and proportions.
To write a chemical equation for a proton transfer reaction:

• Identify the acid donating the proton and the base accepting it. Water often acts as the base in these reactions.
• Show the products: the conjugate base of the acid and the hydronium ion (\(\text{H}_3\text{O}^+\)).
• Ensure the equation is balanced with equal numbers of atoms on both sides.

For example, when \(\text{HCl}\) is added to water, the chemical equation \(\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-\) is balanced, clearly depicting the proton transfer.

Conjugate Acid-Base Pairs

In acid-base chemistry, a conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton.
Here's how it works:

• A conjugate base is formed when an acid loses a proton.
• A conjugate acid is formed when a base gains a proton.

These pairs are crucial for understanding the direction and extent of acid-base reactions. For instance:

• In the reaction \(\text{CH}_3\text{COOH} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COO}^- + \text{H}_3\text{O}^+\), \(\text{CH}_3\text{COOH}\) and \(\text{CH}_3\text{COO}^-\) are a conjugate acid-base pair.
• Similarly, \(\text{H}_2\text{O}\) and \(\text{H}_3\text{O}^+\) form another conjugate acid-base pair.

Recognizing these pairs helps in predicting whether a reaction will proceed in a forward or reverse direction, aiding in the understanding of the reaction's equilibrium.