Conjugate acid-base pairs are linked chemicals that differ by exactly one proton, H+. They matter because most acid-base reactions can be understood as a proton moving from one particle to another. In the general reaction HA + B ⇌ A− + HB+, HA donates H+ and becomes A−, while B accepts H+ and becomes HB+.
Recognizing these pairs helps you predict products, compare strengths, and understand pH behavior in solutions.
In a Brønsted-Lowry acid-base reaction, the acid is the proton donor and the base is the proton acceptor. After the proton transfer, the acid forms its conjugate base, and the base forms its conjugate acid. Strong acids tend to have very weak conjugate bases, while weak acids have stronger conjugate bases.
This strength relationship explains why many acid-base reactions favor the side with the weaker acid and weaker base.
Key Facts
- General form: HA + B ⇌ A− + HB+
- Acid donor pair: HA and A− are a conjugate acid-base pair because they differ by one H+.
- Base acceptor pair: B and HB+ are a conjugate acid-base pair because they differ by one H+.
- Brønsted-Lowry acid: acid = H+ donor.
- Brønsted-Lowry base: base = H+ acceptor.
- Strength relationship: Ka × Kb = Kw = 1.0 × 10−14 at 25 °C for a conjugate acid-base pair.
Vocabulary
- Conjugate acid
- A conjugate acid is the particle formed when a base gains a proton.
- Conjugate base
- A conjugate base is the particle formed when an acid loses a proton.
- Proton transfer
- Proton transfer is the movement of an H+ ion from an acid to a base during an acid-base reaction.
- Brønsted-Lowry acid
- A Brønsted-Lowry acid is any substance that donates a proton to another substance.
- Brønsted-Lowry base
- A Brønsted-Lowry base is any substance that accepts a proton from another substance.
Common Mistakes to Avoid
- Calling any two substances on opposite sides conjugate pairs is wrong because conjugate pairs must differ by exactly one H+ and nothing else.
- Forgetting charge changes is wrong because losing H+ decreases the charge by 1, while gaining H+ increases the charge by 1.
- Labeling the strongest acid as having a strong conjugate base is wrong because stronger acids have weaker conjugate bases.
- Treating H2O as always the acid is wrong because water can donate H+ to act as an acid or accept H+ to act as a base depending on the reaction.
Practice Questions
- 1 In the reaction NH3 + H2O ⇌ NH4+ + OH−, identify the two conjugate acid-base pairs.
- 2 For the acid HNO2 with Ka = 4.0 × 10−4 at 25 °C, calculate Kb for its conjugate base NO2− using Ka × Kb = 1.0 × 10−14.
- 3 Explain why the reaction HCl + H2O → H3O+ + Cl− strongly favors products, using the relationship between acid strength and conjugate base strength.