Question

How do the components of a conjugate acid-base pair differ from one another? Give an example of a conjugate acid-base pair to lllustrate your answer.

Short answer

In a conjugate acid-base pair, the conjugate acid is the species formed when a base accepts a proton, while the conjugate base is the species left behind after an acid donates a proton. They differ by the presence or absence of one proton (H+). For instance, in the reaction between ammonia (NH3) and water (H2O): NH3 + H2O ⇌ NH4+ + OH-, NH4+ is the conjugate acid of NH3, and OH- is the conjugate base of H2O.

Step-by-step solution

Understand Brønsted-Lowry Acid-Base Theory

The Brønsted-Lowry acid-base theory defines acids as proton (H+) donors and bases as proton acceptors. A conjugate acid-base pair consists of two species that differ from each other by the presence or absence of a proton (H+).

Define Conjugate Acid and Conjugate Base

A conjugate acid is the species formed when a base accepts a proton. Similarly, a conjugate base is the species left behind after an acid donates a proton. The conjugate acid and base differ by one proton.

Provide an example of a conjugate acid-base pair

Consider the reaction between ammonia (NH3) and water (H2O): NH3 + H2O ⇌ NH4+ + OH- NH3 acts as a base since it accepts a proton from H2O, which then act as an acid. In this process, NH3 forms its conjugate acid NH4+ while water (H2O) forms its conjugate base OH-. The conjugate acid-base pair in this example is: - Conjugate Acid: NH4+ (ammonium ion) - Conjugate Base: NH3 (ammonia) This reaction shows that the major difference between the components of a conjugate acid-base pair is the presence or absence of one proton (H+).

Key concepts

Brønsted-Lowry Acid-Base Theory

The Brønsted-Lowry Acid-Base Theory is a fundamental concept in chemistry. It focuses on the role of protons in acid-base reactions. According to this theory, an acid is a substance that can donate a proton (H+), while a base is a substance that can accept a proton. This definition extends beyond traditional Arrhenius acids and bases, which are limited to aqueous solutions, allowing for a broader understanding of acid-base chemistry.
In the Brønsted-Lowry model, the process is reversible. Thus, when an acid donates a proton, it becomes a base, and when a base accepts a proton, it becomes an acid. This reversibility explains the formation of conjugate acid-base pairs. A conjugate acid-base pair consists of two species that differ by one proton, emphasizing the transient nature of protons in chemical reactions.
Understanding this theory is crucial for analyzing chemical reactions, especially those in solutions where acids and bases frequently interact.

Proton Donor and Acceptor

In the Brønsted-Lowry theory, the terms 'proton donor' and 'proton acceptor' are essential for identifying acids and bases.
A proton donor is a species that releases a proton into the solution. When it loses this proton, it forms its corresponding conjugate base. Conversely, a proton acceptor gains a proton from its surroundings, forming its conjugate acid as a result.

• Proton Donor: Helps identify substances as acids.
• Proton Acceptor: Helps identify substances as bases.

For example, when hydrochloric acid ( ext{HCl}) is dissolved in water, it donates a proton to the water and forms the chloride ion ( ext{Cl}^-), making water the proton acceptor. This concept is pivotal in explaining many natural and industrial chemical reactions, where identifying the movement of protons helps predict reaction outcomes and products.

Ammonia and Ammonium Ion Example

A classic example of a conjugate acid-base pair is the reaction involving ammonia (NH3) and water (H2O). In this reaction, ammonia acts as a Brønsted-Lowry base because it accepts a proton from water. As a result, the water molecule donates a proton, acting as a Brønsted-Lowry acid.
The reaction can be represented as follows:
\[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]
This equation shows how ammonia (NH3) becomes the ammonium ion (NH4+), which is its conjugate acid. Meanwhile, water (H2O), after donating a proton, becomes the hydroxide ion (OH^-), its conjugate base.

• Conjugate Acid: NH4+ (ammonium ion)
• Conjugate Base: NH3 (ammonia)

This example illustrates the key difference between components of a conjugate acid-base pair: the presence or absence of an additional proton. It also demonstrates the dynamic balance in reversible reactions involving acids and bases.