Question

In each of the following chemical equations, identify the conjugate acid-base pairs. a. \(\mathrm{NH}_{4}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NH}_{3}+\mathrm{H}_{3} \mathrm{O}^{+}\) b. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}\) c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3} \mathrm{NH}_{3}^{+}+\mathrm{OH}\)

Short answer

In the given chemical equations, the conjugate acid-base pairs are: a. \(\mathrm{NH}_{4}^{+}\) (acid) and \(\mathrm{NH}_{3}\) (base); \(\mathrm{H}_{2} \mathrm{O}\) (base) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (acid). b. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) (acid) and \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\) (base); \(\mathrm{H}_{2} \mathrm{O}\) (base) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (acid). c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (base) and \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) (acid); \(\mathrm{H}_{2} \mathrm{O}\) (acid) and \(\mathrm{OH}^{-}\) (base).

Step-by-step solution

Identify the species in the equation

In this equation, we have the following species: \(\mathrm{NH}_{4}^{+}\), \(\mathrm{H}_{2} \mathrm{O}\), \(\mathrm{NH}_{3}\), and \(\mathrm{H}_{3} \mathrm{O}^{+}\).

Identify the conjugate acid-base pairs

Comparing the species, we notice that \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{NH}_{3}\) differ only by a hydrogen ion. Thus, \(\mathrm{NH}_{4}^{+}\) (acid) and \(\mathrm{NH}_{3}\) (base) form a conjugate acid-base pair. Similarly, \(\mathrm{H}_{2} \mathrm{O}\) (base) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (acid) form another conjugate acid-base pair. b. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}\)

Identify the species in the equation

In this equation, we have the following species: \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\), \(\mathrm{H}_{2} \mathrm{O}\), \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\), and \(\mathrm{H}_{3} \mathrm{O}^{+}\).

Identify the conjugate acid-base pairs

Comparing the species, we notice that \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) and \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\) differ only by a hydrogen ion. Thus, \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) (acid) and \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\) (base) form a conjugate acid-base pair. Similarly, \(\mathrm{H}_{2} \mathrm{O}\) (base) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) (acid) form another conjugate acid-base pair. c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CH}_{3} \mathrm{NH}_{3}^{+}+\mathrm{OH}^{-}\)

Identify the species in the equation

In this equation, we have the following species: \(\mathrm{CH}_{3} \mathrm{NH}_{2}\), \(\mathrm{H}_{2} \mathrm{O}\), \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\), and \(\mathrm{OH}^{-}\).

Identify the conjugate acid-base pairs

Comparing the species, we notice that \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) and \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) differ only by a hydrogen ion. Thus, \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (base) and \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) (acid) form a conjugate acid-base pair. Similarly, \(\mathrm{H}_{2} \mathrm{O}\) (acid) and \(\mathrm{OH}^{-}\) (base) form another conjugate acid-base pair.

Key concepts

Acid-Base Reactions

Acid-base reactions are chemical processes where an acid donates a proton (H\(^+\)) to a base. This transfer of protons is central to the reaction's dynamics. These reactions often lead to the formation of a conjugate acid-base pair, which consists of two species that differ only by one proton.

This type of reaction can be represented by a general equation:

• Acid + Base \(\rightleftharpoons\) Conjugate Base + Conjugate Acid

For example, consider the reaction involving ammonium ion (NH\(_4^+\)) and water (H\(_2\)O).
In this reaction, NH\(_4^+\), the acid, donates a proton to water, forming ammonia (NH\(_3\)) and hydronium ion (H\(_3\)O\(^+\)). This results in two conjugate acid-base pairs:

• NH\(_4^+\) (acid) and NH\(_3\) (conjugate base)
• H\(_2\)O (base) and H\(_3\)O\(^+\) (conjugate acid)

Understanding these reactions is crucial in many areas of chemistry, including biochemical processes and industrial applications.

Bronsted-Lowry Theory

The Bronsted-Lowry theory is a fundamental concept in acid-base chemistry. It defines acids and bases based on their ability to donate or accept protons (H\(^+\)) rather than their composition or state of matter.

According to this theory:

• An acid is a substance that donates a proton.
• A base is a substance that accepts a proton.

This perspective expands the traditional definitions of acids and bases. It allows for more substances to be categorized under these terms. In the Bronsted-Lowry framework, water is an exemplary amphoteric substance. This means that it can act as both an acid and a base depending on the reaction.

For instance, in the reaction between acetic acid (HC\(_2\)H\(_3\)O\(_2\)) and water, acetic acid acts as an acid by donating a proton to water, which becomes the base. The resulting ion, acetate (C\(_2\)H\(_3\)O\(_2^-\)), is the conjugate base, and hydronium ion (H\(_3\)O\(^+\)) is the conjugate acid.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They show how reactants transform into products.

In acid-base chemistry, these equations are used to illustrate the process of proton transfer, forming conjugate acid-base pairs.

A chemical equation must be balanced, meaning the number of atoms for each element is equal on both sides of the equation. This conservation of mass principle is vital in accurately predicting the outcomes of reactions.

Consider the equation for the reaction of methylamine (CH\(_3\)NH\(_2\)) with water. It can be written as:

• CH\(_3\)NH\(_2\) + H\(_2\)O \(\rightleftharpoons\) CH\(_3\)NH\(_3^+\) + OH\(^-\)

In this reaction, methylamine acts as a base, accepting a proton to form its conjugate acid, CH\(_3\)NH\(_3^+\). Water donates the proton, becoming the hydroxide ion (OH\(^-\)).
Understanding how these equations work is crucial for predicting the behavior of acidic and basic substances in various chemical and biological systems.