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Conjugate Pairs
Conjugate Pairs
The reason that some reactions achieve an equilibrium instead of going to completion is because there is a reverse reaction. At equilibrium, both the forward and reverse reactions occur, but they occur at equal rates. That is to say, products are produced by the forward reaction as quickly as they are consumed by the reverse reaction. Similarly, reactants are consumed at the same rate as they are produced. Taking another look at the dissociation reaction for acetic acid, this means that acetate (Ac– or CH3COO–) can act as a weak base. In fact, acetate can react with water to form hydroxide and undissociated acetic acid.
While some of the acetic acid produced will eventually dissociate to form H3O+ ions, overall, more OH– ions are produced in the process. So acetate ions (from sodium acetate, for example) can be used as a weak base.
In this case, acetate is referred to as the conjugate base of acetic acid. A conjugate base is the molecule or ion remaining after an acid loses an H+ ion. Similarly, a conjugate acid is the molecule or ion created when a base gains an H+ ion. Because the relationship between a conjugate acid and conjugate base is defined by the transfer of an H+ ion, they always come in pairs. Some ions or molecules can even undergo both the loss or gain of an H+ ion to become a conjugate acid or conjugate base. This is common for anions derived from acids that have multiple acidic H+ ions. For these cases, it is important to look at whether an H+ ion is being lost or gained and what the resulting product is. That process will determine the conjugate acid/base relationship. In other words, it is not always possible to tell whether a chemical is a conjugate acid or base just by looking at its identity, and that it might be necessary to look at the reactions in which it is involved. For a few more examples of conjugate pairs, see the table below.
Examples of conjugate pairs.
There is also a relationship between the Ka and Kb values for a conjugate acid and base pair. If one value is known, the other can be determined through the auto-ionization constant of water, Kw, which is another equilibrium constant.
Kw is usually a known value. At 25 °C it is 1.0 x 10–14. While this value does vary with temperature, usually the corresponding value is given or can be found in a reference.
Buffers
Buffers
When relatively equal amounts of a weak acid and its conjugate base are combined in water, they form a solution known as a buffer. A buffer is a solution that reduces the change in the concentration of H3O+ and OH– ions when another acid or base is added. This is particularly relevant for biological systems which are often sensitive to changes in the concentration of these ions. One useful equation that can be derived for a buffer is the Henderson-Hasselbalch equation, which relates the concentration of H3O+ ions in solution to the concentrations of the weak acid, its conjugate base, and the Ka value of the acid.
This equation also takes another form,
where pH = –log[H3O+]
and pKa = –log Ka,
common ways of expressing the concentration of H3O+ ions and values for Ka, respectively. Also note that [Conj. base] is moved to the numerator and [Weak acid] is moved to the denominator as part of the formula change.
Combining a weak base and its conjugate acid can also create a buffer, in which case the relevant equations would be,
When using these equations, the Ka value of the conjugate acid must be used. A common mistake is using the Kb value for the weak base.
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