Conjugate acid–base pairs Knowledge-Based Study

Study notes

1. Proton Transfer in Acid–Base Reactions

   In the Brønsted–Lowry acid–base theory, an acid is defined as a proton ($\\text{H}^+$) donor, while a base is defined as a proton ($\\text{H}^+$) acceptor.

   Unlike the Arrhenius definition, which requires aqueous hydroxide or hydronium ions to be formed directly from dissolution, the Brønsted–Lowry model emphasizes the dynamic transfer of a proton from one chemical species to another in a reaction equilibrium.

   • Acid: Any molecule or ion that can donate a proton ($\\text{H}^+$).
   • Base: Any molecule or ion that can accept a proton ($\\text{H}^+$).

   What is a Conjugate Pair?

   A conjugate acid–base pair consists of two chemical species that differ by exactly one hydrogen ion (proton, $\\text{H}^+$).

   When an acid loses a proton, the resulting entity can act as a base in the reverse reaction. This species is called the conjugate base of the original acid. Conversely, when a base accepts a proton, it forms its conjugate acid.

   • Conjugate Base: The particle formed when an acid donates a proton. It has one fewer hydrogen atom and a charge that is one unit lower (more negative).
   • Conjugate Acid: The particle formed when a base gains a proton. It has one additional hydrogen atom and a charge that is one unit higher (more positive).

   General relationship:

   $$\\text{Acid} \rightleftharpoons \\text{Conjugate Base} + \\text{H}^+$$

   $$\\text{Base} + \\text{H}^+ \rightleftharpoons \\text{Conjugate Acid}$$

   Tracking Proton Movement

   Consider the reaction of acetic acid with water:

   $$\\text{CH}_3\\text{COOH}(aq) + \\text{H}_2\\text{O}(l) \rightleftharpoons \\text{CH}_3\\text{COO}^-(aq) + \\text{H}_3\\text{O}^+(aq)$$

   In the forward reaction:

   • $\\text{CH}_3\\text{COOH}$ loses a proton to become $\\text{CH}_3\\text{COO}^-$. Therefore, $\\text{CH}_3\\text{COOH}$ is an acid, and $\\text{CH}_3\\text{COO}^-$ is its conjugate base.
   • $\\text{H}_2\\text{O}$ gains a proton to become $\\text{H}_3\\text{O}^+$. Therefore, $\\text{H}_2\\text{O}$ acts as a base, and $\\text{H}_3\\text{O}^+$ is its conjugate acid.

   This reaction contains two conjugate acid–base pairs:

   1. $\\text{CH}_3\\text{COOH}$ / $\\text{CH}_3\\text{COO}^-$
   2. $\\text{H}_3\\text{O}^+$ / $\\text{H}_2\\text{O}$

   Versatility and Relative Strength

   Some species can either donate or accept a proton depending on the chemical environment. A substance capable of behaving as both a Brønsted–Lowry acid and base is called amphiprotic.

   For example, water ($\\text{H}_2\\text{O}$) acts as a base when reacting with acids like $\\text{HCl}$, forming its conjugate acid ($\\text{H}_3\\text{O}^+$). However, in the reaction with ammonia ($\\text{NH}_3$):

   $$\\text{NH}_3(aq) + \\text{H}_2\\text{O}(l) \rightleftharpoons \\text{NH}_4^+(aq) + \\text{OH}^-(aq)$$

   Water donates a proton to $\\text{NH}_3$, acting as an acid and generating its conjugate base, $\\text{OH}^-$.

   Inverse Relationship of Strength

   An important principle governing conjugate pairs is the inverse relationship between the strengths of an acid and its conjugate base:

   • A strong acid has a very weak, negligible conjugate base (e.g., $\\text{HCl}$ forms $\\text{Cl}^-$, which has virtually no affinity for protons in aqueous solution).
   • A weak acid possesses a relatively stronger conjugate base that competes for protons in equilibrium.