Conjugate acid

Conjugate acid

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. A conjugate acid can also be seen as the chemical substance that releases a proton in the backward chemical reaction. Thus, the term acid. The base produced, X, is called the conjugate base and it absorbs a proton in the backward chemical reaction. In aqueous solution, the chemical reaction involved is of the form

HX + H2O rightleftarrows X + H3O+

This principle is discussed in detail in the article on acid-base reaction theories.

The conjugate base of a weak acid is a strong base, and the conjugate base of a strong acid is a weak base.

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 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, pH 7) OH Hydroxide ion
HFSbF5 Fluoroantimonic acid SbF5 Antimony pentafluoride ion

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