R3.1.2 Conjugate acid-base pairs

Conjugate Acid–Base Pairs: Understanding Proton Transfer in Chemistry

What Are Conjugate Acid–Base Pairs?

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 aconjugate 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:

$Acid ⇌ Base + H^{+}$

Example

Hydrochloric Acid ( $HCl$ )

When hydrogen chloride ( $HCl$ ) dissolves in water, it donates a proton to the water molecule:
$HCl \to {Cl}^{-} + H^{+}$

Acid: $HCl$ (proton donor)
Conjugate Base: ${Cl}^{-}$ (formed after $HCl$ loses a proton)

This is one conjugate acid–base pair: $HCl$ and ${Cl}^{-}$ .

Example

Let’s take another look at a familiar reaction: ${NH}_{3} + H^{+} ⇌ {NH}_{4}^{+}$

Base: ${NH}_{3}$ (proton acceptor)
Conjugate Acid: ${NH}_{4}^{+}$ (formed after ${NH}_{3}$ gains a proton)Here, ${NH}_{3}$ and ${NH}_{4}^{+}$ 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.

Example

Reaction Between Hydrogen Cyanide and Water

$HCN + H_{2} O ⇌ {CN}^{-} + H_{3} O^{+}$

Acid 1: $HCN$ (donates $H^{+}$ )
Conjugate Base 1: ${CN}^{-}$
Base 2: $H_{2} O$ (accepts $H^{+}$ )
Conjugate Acid 2: $H_{3} O^{+}$

Thus, the two conjugate acid–base pairs in this reaction are:

$HCN / {CN}^{-}$
$H_{2} O / H_{3} O^{+}$

Self review

Can you identify the conjugate acid–base pairs in the following reaction? $H_{2} {CO}_{3} + H_{2} O ⇌ {HCO}_{3}^{-} + H_{3} O^{+}$

Amphiprotic Species: Dual Roles in Proton Transfer

Some species can act as both a Brønsted–Lowry acid and a base, depending on the reaction.
These are called amphiprotic species. Water is the most common example.

Example

Water Acting as an Acid

$H_{2} O ⇌ {OH}^{-} + H^{+}$
Here, water donates a proton, acting as an acid.

Water Acting as a Base

$H_{2} O + H^{+} ⇌ H_{3} O^{+}$
Here, water accepts a proton, acting as a base.

Tip

To identify amphiprotic species, look for molecules or ions that have both a proton to donate and the ability to accept a proton.

Stepwise Proton Transfer: Polyprotic Acids

Polyprotic acids, such as sulfuric acid ( $H_{2} {SO}_{4}$ ) and carbonic acid ( $H_{2} {CO}_{3}$ ), can donate more than one proton.
Each step of proton donation forms a new conjugate acid–base pair.

Example

Dissociation of Carbonic Acid

First step:
$H_{2} {CO}_{3} ⇌ {HCO}_{3}^{-} + H^{+}$

Acid: $H_{2} {CO}_{3}$
Conjugate Base: ${HCO}_{3}^{-}$

Second step:
${HCO}_{3}^{-} ⇌ {CO}_{3}^{2 -} + H^{+}$

Acid: ${HCO}_{3}^{-}$
Conjugate Base: ${CO}_{3}^{2 -}$

Notice how ${HCO}_{3}^{-}$ acts as both an acid (in the second step) and a base (in the first step). This makes ${HCO}_{3}^{-}$ amphiprotic.

Common Mistake

Students often confuse the number of protons donated with the number of conjugate pairs formed. Remember, each proton transfer corresponds to a new conjugate acid–base pair.

Reflection

Self review

Identify Conjugate Pairs: For the reaction below, identify the conjugate acid–base pairs: ${NH}_{3} + H_{2} O ⇌ {NH}_{4}^{+} + {OH}^{-}$
Amphiprotic Species: Explain why ${HCO}_{3}^{-}$ is amphiprotic.
Polyprotic Acid Dissociation: Write the stepwise dissociation reactions for phosphoric acid ( $H_{3} {PO}_{4}$ ) and identify all conjugate acid–base pairs.

Theory of Knowledge

The concept of conjugate acid–base pairs highlights how scientific models evolve.

How does the Brønsted–Lowry theory expand upon earlier models, such as the Arrhenius definition of acids and bases?

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R3.1.2 Conjugate acid-base pairs

Conjugate Acid–Base Pairs: Understanding Proton Transfer in Chemistry

What Are Conjugate Acid–Base Pairs?

Generic Representation:

Hydrochloric Acid (HCl)

Identifying Conjugate Acid–Base Pairs in Reactions

Reaction Between Hydrogen Cyanide and Water

Amphiprotic Species: Dual Roles in Proton Transfer

Water Acting as an Acid

Water Acting as a Base

Stepwise Proton Transfer: Polyprotic Acids

Dissociation of Carbonic Acid

Reflection

You've read 2/2 free chapters this week.

Introduction to Conjugate Acid-Base Pairs

Hydrochloric Acid ( $HCl$ )