Conjugate Acid-Base Pairs

Definition

Conjugate acid-base pairs are related chemical species that differ by the presence or absence of a single proton (H+). When an acid donates a proton, it becomes a conjugate base, and when a base accepts a proton, it becomes a conjugate acid. These pairs are fundamental to understanding the Brønsted-Lowry theory of acids and bases, as well as the concepts of pH, relative acid-base strengths, hydrolysis, polyprotic acids, and acid-base titrations.

5 Must Know Facts For Your Next Test

1. Conjugate acid-base pairs are formed when an acid donates a proton to a base, and the acid becomes a conjugate base, while the base becomes a conjugate acid.
2. The strength of a conjugate acid-base pair is determined by their relative ability to donate or accept protons, which is measured by their acid dissociation constant (Ka) or base dissociation constant (Kb).
3. Hydrolysis of salts involves the reaction of the salt with water, resulting in the formation of conjugate acid-base pairs that determine the pH of the solution.
4. Polyprotic acids can donate multiple protons, forming multiple sets of conjugate acid-base pairs that influence the pH of the solution at different stages of the titration process.
5. The endpoint of an acid-base titration is determined by the equivalence point, where the concentration of the conjugate acid-base pair is at its maximum.

Review Questions

• Explain how conjugate acid-base pairs are formed and their relationship to the Brønsted-Lowry theory of acids and bases.
  • Conjugate acid-base pairs are formed when an acid donates a proton (H+) to a base, and the acid becomes a conjugate base, while the base becomes a conjugate acid. This process is the foundation of the Brønsted-Lowry theory, which defines acids as proton donors and bases as proton acceptors. The strength of a conjugate acid-base pair is determined by their relative ability to donate or accept protons, as measured by their acid dissociation constant (Ka) or base dissociation constant (Kb).
• Describe the role of conjugate acid-base pairs in the determination of pH and the hydrolysis of salts.
  • The pH of a solution is determined by the concentration of hydrogen ions (H+), which is influenced by the presence of conjugate acid-base pairs. When a salt is dissolved in water, the resulting solution undergoes hydrolysis, where the salt reacts with water to form conjugate acid-base pairs. The pH of the solution is then determined by the relative strengths of these conjugate acid-base pairs, with stronger acids and bases having a greater impact on the pH.
• Explain how conjugate acid-base pairs are involved in the titration process and the determination of the endpoint in acid-base titrations.
  • In an acid-base titration, the endpoint is determined by the equivalence point, where the concentration of the conjugate acid-base pair is at its maximum. As the titration progresses, the added titrant reacts with the analyte, forming conjugate acid-base pairs. The pH of the solution changes throughout the titration, and the endpoint is reached when the moles of the titrant added are equal to the moles of the analyte. At the equivalence point, the concentration of the conjugate acid-base pair is at its highest, providing a clear indication of the completion of the titration.

Related terms

Brønsted-Lowry Acid: A Brønsted-Lowry acid is a substance that can donate a proton (H+) to another substance.

Brønsted-Lowry Base: A Brønsted-Lowry base is a substance that can accept a proton (H+) from another substance.

pH:

pH is a measure of the concentration of hydrogen ions (H+) in a solution, which determines the acidity or basicity of the solution.