Conjugate means "partner", hence an acid may have a "partner" base, formed by the loss of a hydrogen ion.
Syllabus reference R3.1.2Reactivity 3.1.2 - A pair of species differing by a single proton is called a conjugate acid–base pair.
- Deduce the formula of the conjugate acid or base of any Brønsted–Lowry base or acid.
Guidance
Tools and links
- Structure 3.1 - What is the periodic trend in the acid–base properties of metal and non-metal oxides?
- Structure 3.1 - Why does the release of oxides of nitrogen and sulfur into the atmosphere cause acid rain?
Conjugate acid - base pairs
The idea of conjugate acid - base pairs comes from the idea that all reactions are fundamentally reversible (if not in practice, at least in theory). When a proton is transferred to another species, that product is then capable, in turn, of transferring the proton back - in other words it is itself capable of behaving as an acid.
As this acid was created by accepting the proton in the first place it is called the conjugate (paired) acid of the original base. We say that the two species form an acid - conjugate base pair.
- An acid on the left hand side always has a conjugate base on the right hand side
- A base on the left hand side always has a conjugate acid on the right hand side
- These pairs of species are called conjugate acid - base pairs
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Example: Finding the conjugate partner in a reaction involves considering the reverse reaction or considering where the proton (hydrogen ion) goes to. In the following example consider the hydrogen ion and remember that the hydrogen ion donator is the acid. CH3COOH + H2SO4 → CH3COOH2+ + HSO4- On the left hand side it is the sulfuric acid that donates the proton. It is behaving as an acid. Its conjugate base is the species on the other side of the equation that would accept the proton to go in the reverse direction. On the right hand side it would be the hydrogen sulfate ion (HSO4- ) that accepts the proton and therefore it is the conjugate base of the sulfuric acid. On the left hand side of the equation the ethanoic acid accepts a proton and is therefore the Brønsted-Lowry base. It has a conjugate acid on the right hand side that can release a proton - this is the CH3COOH2+ion. |
Conjugate acid - base pair examples
Look at the following equations as examples of conjugate acid - base pairs. Remember that the conjugate partners are found by considering the reverse reaction.
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acid
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base
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conjugate base
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conjugate acid
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H2SO4
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+
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CH3COOH
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→
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HSO4-
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+
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CH3COOH2+
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CH3COOH
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+
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NH3
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→
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CH3COO-
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+
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NH4+
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H2O
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+
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H2O
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→
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H3O+
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+
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OH-
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HCl
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+
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H2O
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→
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Cl-
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+
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H3O+
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proton donor
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proton acceptor
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⇋
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proton acceptor
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proton donor
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Logically, if the reactions involve the transfer of a proton (hydrogen ion) from the acid to a base the structures of the acid and its conjugate base must differ in structure by that hydrogen ion.
Look at the acid and conjugate base in the above equations, and ou can see that the acid-conjugate base pairs differ by one hydrogen ion, as do the base-conjugate acid pairs.
The Syllabus states that the position of the hydrogen ion should be indicated. The hydrogen ion being transferrred to the base is accepted by the base. It must be accepted in a location where there is a lone pair of electrons to hold it.
In the first equation above, it is the lone pair on the oxygen atom of the ethanoic acid that accepts the proton. It is therefore correct to write the formula of the conjugate acid of the ethanoic acid (base) as CH3COOH2+, as this indicates that the proton is accepted by the oxygen (or one of them) of the carboxyl group. It would be incorrect to write the formula as CH4COOH+ as this would suggest that the hydrogen has been accepted by the wrong part of the structure.