Conjugate acid
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Within the Brønsted-Lowry (protonic) theory of acids and bases, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. The base produced, X−, is called the conjugate base. In aqueous solution, the chemical reaction involved is of the form
- HX + H2O ⇌ X− + H3O+
This principle is discussed in detail in the article on acid-base reaction theories.
Tabulated below are several examples of conjugate acid-base pairs. Acid strength decreases and base strength increases down the table. (The dissociation reaction reaches equilibrium further to the right, with more X− produced.)
| Acid | Base |
|---|---|
| HFSbF5 Fluoroantimonic acid | SbF6− Antimony pentafluoride ion |
| HCl Hydrochloric acid | Cl− Chloride ion |
| H2SO4 Sulfuric acid | HSO4− Hydrogen sulfate ion |
| HNO3 Nitric acid | NO3− Nitrate ion |
| H3O+ Hydronium ion | H2O Water |
| HSO4− Hydrogen sulfate ion | SO42− Sulfate ion |
| H3PO4 Phosphoric acid | H2PO4− Dihydrogen phosphate ion |
| HC2H3O2 Acetic acid | C2H3O2− Acetate ion |
| H2CO3 Carbonic acid | HCO3− Hydrogen carbonate ion |
| H2S Hydrosulfuric acid | HS− Hydrogen sulfide ion |
| H2PO4− Dihydrogen phosphate ion | HPO42− Hydrogen phosphate ion |
| NH4+ Ammonium ion | NH3 Ammonia |
| HCO3− Hydrogencarbonate (bicarbonate) ion | CO32− Carbonate ion |
| HPO42− Hydrogen phosphate ion | PO43− Phosphate ion |
| H2O Water (neutral, pH7) | OH− Hydroxide ion |
