Conjugate Acid-Base Pairs (Leaving Cert Chemistry): Revision Notes
Conjugate Acid-Base Pairs
Understanding conjugate acid-base pairs
When we study acids and bases using Brønsted-Lowry theory, we discover an important relationship called conjugate acid-base pairs. This concept helps us understand how acids and bases are connected through proton transfer reactions.
A conjugate acid-base pair is any pair of substances that differ by exactly one proton (H⁺). This fundamental relationship is central to understanding acid-base chemistry.
When an acid donates a proton, it becomes its conjugate base. When a base accepts a proton, it becomes its conjugate acid.
How conjugate pairs work
Let's examine how this works using ethanoic acid (the acid found in vinegar) as our example:
Worked Example: Ethanoic Acid Proton Transfer
When an acid donates a proton:
- Ethanoic acid (CH₃COOH) donates a proton to water
- CH₃COOH → CH₃COO⁻ + H⁺
- The CH₃COOH becomes CH₃COO⁻ (its conjugate base)
When a base accepts a proton:
- The acetate ion (CH₃COO⁻) can accept a proton
- CH₃COO⁻ + H⁺ → CH₃COOH
- The CH₃COO⁻ becomes CH₃COOH (its conjugate acid)
The complete equilibrium reaction shows both conjugate pairs:
In this reaction:
- CH₃COOH and CH₃COO⁻ form one conjugate acid-base pair
- H₂O and H₃O⁺ form another conjugate acid-base pair
Key principles
Every acid has a corresponding conjugate base, and every base has a corresponding conjugate acid. The crucial point to remember is that conjugate pairs always differ by exactly one proton.
We can represent this relationship generally using:
- HA to represent any acid
- A⁻ to represent its conjugate base
- HA ⇌ A⁻ + H⁺
Identifying conjugate pairs
To identify conjugate acid-base pairs in reactions, follow these key steps:
- Look for species that differ by one proton only
- The acid is the species with the extra H⁺
- The base is the species missing the H⁺
Worked Example: HCO₃⁻ (hydrogen carbonate ion)
Finding the conjugate acid:
- HCO₃⁻ accepts a proton: HCO₃⁻ + H⁺ → H₂CO₃
- Conjugate acid = H₂CO₃
Finding the conjugate base:
- HCO₃⁻ donates a proton: HCO₃⁻ → CO₃²⁻ + H⁺
- Conjugate base = CO₃²⁻
Worked Example: Complex reaction
In the reaction: HNO₃ + H₂F₂ ⇌ H₂NO₃⁺ + HF₂⁻
The conjugate pairs are:
- HNO₃ and H₂NO₃⁺ (differ by one proton)
- H₂F₂ and HF₂⁻ (differ by one proton)
Important exam considerations
Key exam tip: When identifying conjugate bases of polyprotic acids (acids that can donate multiple protons), remember that conjugate pairs differ by only one proton.
For example:
- The conjugate base of H₂SO₄ is HSO₄⁻ (not SO₄²⁻)
- The conjugate base of HSO₄⁻ is SO₄²⁻
This is because each conjugate relationship involves the transfer of exactly one proton.
Practical applications
Understanding conjugate acid-base pairs helps you:
- Predict reaction directions - stronger acids form weaker conjugate bases
- Write equilibrium expressions correctly
- Identify all species in acid-base reactions
- Explain buffer systems and their behaviour
When working with equilibrium reactions, remember that the reaction can proceed in both directions. The forwards reaction shows the acid donating a proton, while the reverse reaction shows the conjugate base accepting a proton.
Remember!
Key Points to Remember:
- Conjugate acid-base pairs always differ by exactly one proton (H⁺)
- When an acid donates a proton, it becomes its conjugate base
- When a base accepts a proton, it becomes its conjugate acid
- Every acid-base reaction involves two conjugate pairs
- For polyprotic acids, each proton loss creates a different conjugate pair