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.
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.
Reaction Between Hydrogen Cyanide and Water
$$
\text{HCN} + \text{H}_2\text{O} \rightleftharpoons \text{CN}^- + \text{H}_3\text{O}^+
$$
- Acid 1: $ \text{HCN} $ (donates $ H^+ $)
- Conjugate Base 1: $ \text{CN}^- $
- Base 2: $ \text{H}_2\text{O} $ (accepts $ H^+ $)
- Conjugate Acid 2: $ \text{H}_3\text{O}^+ $
Thus, the two conjugate acid–base pairs in this reaction are:
- $ \text{HCN} / \text{CN}^- $
- $ \text{H}_2\text{O} / \text{H}_3\text{O}^+ $
Can you identify the conjugate acid–base pairs in the following reaction?
$$
\text{H}_2\text{CO}_3 + \text{H}_2\text{O} \rightleftharpoons \text{HCO}_3^- + \text{H}_3\text{O}^+
$$
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.
Water Acting as an Acid
$$
\text{H}_2\text{O} \rightleftharpoons \text{OH}^- + H^+
$$
Here, water donates a proton, acting as an acid.
Water Acting as a Base
$$
\text{H}_2\text{O} + H^+ \rightleftharpoons \text{H}_3\text{O}^+
$$
Here, water accepts a proton, acting as a base.
To identify amphiprotic species, look for molecules or ions that have both a proton to donate and the ability to accept a proton.
- In IB Chemistry (First Examination 2025), there is no need to know about amphiprotic species by heart.
- However, it is important to understand the duality of proton transfer, as we will also explore in the next article.
Stepwise Proton Transfer: Polyprotic Acids
- Polyprotic acids, such as sulfuric acid ($ \text{H}_2\text{SO}_4 $) and carbonic acid ($ \text{H}_2\text{CO}_3 $), can donate more than one proton.
- Each step of proton donation forms a new conjugate acid–base pair.
Dissociation of Carbonic Acid
- First step:
$$
\text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + H^+
$$- Acid: $ \text{H}_2\text{CO}_3 $
- Conjugate Base: $ \text{HCO}_3^- $
- Second step:
$$
\text{HCO}_3^- \rightleftharpoons \text{CO}_3^{2-} + H^+
$$- Acid: $ \text{HCO}_3^- $
- Conjugate Base: $ \text{CO}_3^{2-} $
- Notice how $ \text{HCO}_3^- $ acts as both an acid (in the second step) and a base (in the first step).
- This makes $ \text{HCO}_3^- $ amphiprotic.
- 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.
- For the reaction below, identify the conjugate acid–base pairs:$$
\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-
$$ - Write the stepwise dissociation reactions for phosphoric acid ($ \text{H}_3\text{PO}_4 $) and identify all conjugate acid–base pairs.
