Through examples found in the part on acids and also bases proton-transfer procedures are damaged into two hypothetical steps: (1) donation that a proton by an 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 example ). The theoretical steps space useful because they make it basic to watch what varieties is left after an acid donated a proton and what types is formed when a base accepted a proton. We shall use theoretical steps or half-equations in this section, however you should bear in mental that free protons never ever actually exist in aqueous solution.

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

\< \textNH_4^+ \rightarrow \textH^+ +\textNH_3\>

The submicroscopic representations listed below show the donation the the proton the ammonium. The remove of this proton results in NH3, which is easily seen in ~ the submicroscopic level.

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

\<\textF^- + \textH^+ \rightarrow \textHF\>

The submicroscopic representation over shows exactly how the addition of a proton come fluoride converts a weak basic (F- in green) into a weak mountain (HF).

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


The usage of conjugate acid-base pairs permits us to make a very straightforward statement about relative toughness of acids and bases. The stronger an acid, the weaker that conjugate base, and, conversely, the more powerful a base, the weaker that is conjugate acid.

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

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