IB DP Chemistry -R3.1.2 Conjugate acid-base pairs- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R3.1.2 Conjugate acid-base pairs – Study Notes – New Syllabus
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Reactivity 3.1.2 – Conjugate Acid–Base Pairs
Reactivity 3.1.2 – Conjugate Acid–Base Pairs
- A conjugate acid–base pair consists of two species that differ by exactly one proton (H+).
- When a Brønsted–Lowry acid donates a proton, it becomes its conjugate base.
- When a Brønsted–Lowry base accepts a proton, it becomes its conjugate acid.
General Pattern
- \( \text{Acid} \rightleftharpoons \text{Conjugate base} + \text{H}^+ \)
- \( \text{Base} + \text{H}^+ \rightleftharpoons \text{Conjugate acid} \)
Important Features
- Conjugate acid–base pairs are always found on opposite sides of a reversible reaction arrow.
- They play a critical role in maintaining chemical equilibrium and in buffer systems.
- A single substance can participate in multiple conjugate pairs (e.g., \( \text{H}_2\text{O} \) can be both an acid and a base).
How to Deduce the Conjugate Acid or Base of a Given Species
Conjugate Acid of a Base:
- Add one proton (H+) to the base.
- This increases the positive charge of the species by one unit.
Conjugate Base of an Acid:
- Remove one proton (H+) from the acid.
- This decreases the positive charge (or increases the negative charge) by one unit.
Charge and Formula Adjustments
- When determining conjugates, adjust both the chemical formula and the ionic charge appropriately.
- Make sure mass and charge are conserved.
Example
In the reaction between ethanoic acid and water:
\( \text{CH}_3\text{COOH}(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{CH}_3\text{COO}^-(aq) + \text{H}_3\text{O}^+(aq) \)
Identify the conjugate acid and base.
▶️Answer/Explanation
- \( \text{CH}_3\text{COOH}/\text{CH}_3\text{COO}^- \) form a conjugate acid–base pair. They differ by one proton.
- \( \text{H}_2\text{O}/\text{H}_3\text{O}^+ \) form another conjugate acid–base pair.
- In each case, the acid donates H+ to form the conjugate base, and the base gains H+ to form the conjugate acid.
Example
In the reaction:
\( \text{HCO}_3^-(aq) + \text{OH}^-(aq) \rightleftharpoons \text{CO}_3^{2-}(aq) + \text{H}_2\text{O}(l) \)
Identify the conjugate acid and base.
▶️Answer/Explanation
- \( \text{HCO}_3^-/\text{CO}_3^{2-} \) form a conjugate acid–base pair.
- \( \text{OH}^-/\text{H}_2\text{O} \) form another conjugate acid–base pair.
- HCO₃⁻ donates a proton (acts as acid), becoming CO₃²⁻.
- OH⁻ accepts the proton (acts as base), becoming H₂O.
Example
What is the conjugate acid of the base \( \text{NH}_3 \)?
▶️Answer/Explanation
- Add a proton (H+) to NH₃ → \( \text{NH}_4^+ \)
- Conjugate acid of NH₃ is NH₄⁺.
Example
What is the conjugate base of the acid \( \text{H}_2\text{SO}_4 \)?
▶️Answer/Explanation
- Remove a proton (H+) from H₂SO₄ → \( \text{HSO}_4^- \)
- Conjugate base of H₂SO₄ is HSO₄⁻.