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 |
|---|---|
| HCl Hydrochloric acid | Cl- Chloride ion |
| H2SO4 Sulphuric acid | HSO4- Hydrogen sulphate ion |
| HNO3 Nitric acid | NO3- Nitrate ion |
| H3O+ Hydronium ion | H2O Water |
| HSO4- Hydrogen sulphate 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 Hydrosulphuric acid | HS- Hydrogen sulphide ion |
| H2PO4- Dihydrogen phosphate ion | HPO4+ Hydrogen phosphate ion |
| NH4+ Ammonium ion | NH3 Ammonia |
| HCO3- Hydrogen carbonate ion | CO32- Carbonate ion |
| HPO42- Hydrogen phosphate ion | PO43- Phosphate ion |
| H2O Water (neutral, pH7) | OH- Hydroxide ion |