Through examples discovered in the part on acids and bases proton-transfer processes are broken into two hypothetical steps: (1) donation of a proton by an acid, and (2) accept of a proton by a base. (Water offered as the basic in the mountain example and also as the acid in the base instance ). The theoretical steps room useful since they do it straightforward to check out what varieties is left after an mountain donated a proton and what types is formed when a base accepted a proton. Us shall use hypothetical steps or half-equations in this section, yet you need to bear in mental that free protons never ever actually exist in aqueous solution.

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Suppose we an initial consider a weak acid, the ammonium ion. When it donates a proton to any type of other species, we deserve to write the half-equation:

< extNH_4^+ ightarrow extH^+ + extNH_3>

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

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

< extF^- + extH^+ ightarrow extHF>

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

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The situation just explained for NH4+ and also NH3 or because that F– and also HF applies to all acids and bases. Whenever an mountain donates a proton, the acid alters into a base, and whenever a basic accepts a proton, an acid is formed. One acid and also a base which differ only by the presence or absence of a proton are called a conjugate acid-base pair. Hence NH3 is called the conjugate base of NH4+, and also NH4+ is the conjugate mountain of NH3. Similarly, HF is the conjugate acid of F–, and also F– the conjugate base of HF.


The use of conjugate acid-base pairs enables us to make a very an easy statement about relative strengths of acids and also bases. The more powerful an acid, the weaker the conjugate base, and, conversely, the stronger a base, the weaker the conjugate acid.

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

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altwhich of the following is a conjugate acid/base pair?