Through examples found in the sections on acids and also bases proton-transfer procedures are damaged into two hypothetical steps: (1) donation the a proton by one acid, and also (2) accept of a proton by a base. (Water served as the basic in the acid example and also as the mountain in the base instance ). The hypothetical steps room useful because they make it basic to view what types is left after ~ an acid donated a proton and what types is created when a base accepted a proton. Us shall use hypothetical steps or half-equations in this section, yet you must bear in psychic that cost-free protons never actually exist in aqueous solution.

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Suppose we first consider a weak acid, the ammonium ion. When it donates a proton to any other species, we can write the half-equation:

< extNH_4^+ ightarrow extH^+ + extNH_3>

The submicroscopic representations listed below show the donation of the proton that ammonium. The remove of this proton outcomes in NH3, which is quickly seen at the submicroscopic level.

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But NH3 is one of the link we know as a weak base. In various other words, as soon as it donates a proton, the weak acid NH4+ is transformed right into a weak basic NH3. One more example, this time beginning with a weak base, is provided by fluoride ion:

< extF^- + extH^+ ightarrow extHF>

The submicroscopic representation over shows just how the addition of a proton to fluoride counter a weak base (F- in green) into a weak acid (HF).

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The instance just described for NH4+ and also NH3 or for F– and also HF uses to every acids and also bases. Anytime an mountain donates a proton, the acid transforms into a base, and whenever a basic accepts a proton, an mountain is formed. An acid and also a basic which differ only by the presence or lack of a proton are called a conjugate acid-base pair. Hence NH3 is dubbed the conjugate basic of NH4+, and NH4+ is the conjugate mountain of NH3. Similarly, HF is the conjugate mountain of F–, and also F– the conjugate base of HF.


The usage of conjugate acid-base pairs enables us to make a very an easy statement around relative staminas of acids and bases. The stronger an acid, the weaker that is conjugate base, and, whereas the stronger a base, the weaker that is conjugate acid.

TABLE (PageIndex1):Important Conjugate Acid-Base Pairs.

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