Conjugate acids and conjugate bases appear whenever a proton transfer links two chemical species. Each conjugate pair differs by exactly one H+, so the paired formulas match except for one proton and the corresponding change in charge.
Brønsted–Lowry meaning
A Brønsted–Lowry acid is a proton donor and a Brønsted–Lowry base is a proton acceptor. A proton transfer converts the acid into its conjugate base, and converts the base into its conjugate acid.
Conjugate base: the species remaining after an acid donates one H+.
Conjugate acid: the species formed after a base accepts one H+.
Conjugate pairing patterns
Conjugate partners differ by one proton and a one-unit charge shift. The direction of the shift depends on whether the species donates or accepts H+.
- Formula difference: one additional or missing hydrogen atom (H).
- Charge difference: one unit more positive with one extra H, one unit more negative with one fewer H.
- Pair structure: acid/conjugate base written as HA/A− (or BH+/B for bases in water).
Examples in aqueous reactions
CH3COOH + H2O ⇌ CH3COO− + H3O+
CH3COOH and CH3COO− form a conjugate acid–base pair. H2O and H3O+ form another conjugate pair, showing that water can act as a base in this equilibrium.
NH3 + H2O ⇌ NH4+ + OH−
NH3 and NH4+ form a conjugate base/conjugate acid pair, while H2O and OH− form a conjugate acid/conjugate base pair. Water behaves as an acid in this equilibrium.
Visualization of proton transfer and conjugate pairs
Strength relationship within a conjugate pair
Conjugate strength is inverse: a stronger acid has a weaker conjugate base, and a stronger base has a weaker conjugate acid. This inverse relationship follows from equilibrium constants in water.
Ka, Kb, and Kw connection
For a conjugate acid–base pair HA/A− in water at 25 °C, the product of the acid dissociation constant and the base hydrolysis constant equals the ionic product of water:
\[ K_a(\mathrm{HA}) \cdot K_b(\mathrm{A^-}) = K_w \]With \(K_w = 1.0 \times 10^{-14}\) at 25 °C, the logarithmic form becomes:
\[ pK_a + pK_b = pK_w = 14.00 \]Common conjugate pairs and amphiprotic behavior
Several important species in general chemistry participate in conjugate pairing in more than one way. Water and hydrogen carbonate are standard examples of amphiprotic behavior (acidic in one context, basic in another).
| Conjugate acid | Conjugate base | One-proton link | Typical role in water |
|---|---|---|---|
| H3O+ | H2O | H3O+ ⇌ H2O + H+ | H3O+ as acid; H2O as base |
| H2O | OH− | H2O ⇌ OH− + H+ | H2O as acid; OH− as base |
| NH4+ | NH3 | NH4+ ⇌ NH3 + H+ | NH4+ as weak acid; NH3 as weak base |
| CH3COOH | CH3COO− | CH3COOH ⇌ CH3COO− + H+ | Weak acid / weak conjugate base |
| H2CO3 | HCO3− | H2CO3 ⇌ HCO3− + H+ | Acid/conjugate base pair; buffer relevance |
| HCO3− | CO32− | HCO3− ⇌ CO32− + H+ | Amphiprotic intermediate |
Frequent misconceptions
- Charge accounting: a missing H corresponds to a one-unit decrease in charge, and an added H corresponds to a one-unit increase in charge.
- “Conjugate” scope: conjugate pairing refers to a one-proton relationship, not simply “related formulas” or “similar names.”
- Strong acids: very strong acids have conjugate bases that are negligibly basic in water; strong dissociation implies a weak partner base.
Summary
Conjugate acids and conjugate bases are paired species connected by a single proton transfer. The conjugate base forms when an acid donates H+, and the conjugate acid forms when a base accepts H+. Within a conjugate pair in water, the inverse strength relationship is quantified by \(K_a \cdot K_b = K_w\) and \(pK_a + pK_b = 14.00\) at 25 °C.