Conjugate Acid–Base Pairs: Understanding Proton Transfer in Chemistry
- In the Brønsted–Lowry theory, an acid is defined as a proton donor, while a base is a proton acceptor.
- When an acid donates a proton, it transforms into its conjugate base.
- Similarly, when a base accepts a proton, it transforms into its conjugate acid.
- Together, these two species, one acid and one base, form a conjugate acid–base pair.
Definition
Conjugate acid-base pair
A conjugate acid–base pair consists of two species that differ by exactly one proton ($H^+$).
Generic Representation:
$$
\text{Acid} \rightleftharpoons \text{Base} + H^+
$$
Hydrochloric Acid ($ \text{HCl} $)
- When hydrogen chloride ($ \text{HCl} $) dissolves in water, it donates a proton to the water molecule:
$$
\text{HCl} \rightarrow \text{Cl}^- + H^+
$$- Acid: $ \text{HCl} $ (proton donor)
- Conjugate Base: $ \text{Cl}^- $ (formed after $ \text{HCl} $ loses a proton)
- This is one conjugate acid–base pair: $ \text{HCl} $ and $ \text{Cl}^- $.
Let’s take another look at a familiar reaction:$$
\text{NH}_3 + H^+ \rightleftharpoons \text{NH}_4^+
$$
- Base: $ \text{NH}_3 $ (proton acceptor)
- Conjugate Acid: $ \text{NH}_4^+ $ (formed after $ \text{NH}_3 $ gains a proton)Here, $ \text{NH}_3 $ and $ \text{NH}_4^+ $ form a conjugate acid–base pair.
Identifying Conjugate Acid–Base Pairs in Reactions
To identify conjugate acid–base pairs in a chemical reaction, follow these steps:
- Locate the species that donates a proton: this is the acid.
- Identify the species that forms after the proton is donated: this is the conjugate base of the acid.
- Locate the species that accepts a proton: this is the base.
- Identify the species that forms after the proton is accepted: this is the conjugate acid of the base.
