Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: AussieKenDoll on April 11, 2019, 07:02:05 AM
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The conjugate base of a weak acid is a weak base according to Daniel C.Harris Quantitative Chemical Analysis, Is this correct?
I thought The conjugate base of a weak acid is a strong base?
From Daniel C.Harris Quantitative Chemical Analysis,
"The conjugate base of a weak acid is a weak base. The conjugate acid of a weak base is a weak acid. Consider a weak acid, HA, with Ka=10^-4. The conjugate base, A-, has Kb= Kw/Ka=10^-10. That is, if HA is a weak acid, A- is a weak base. If Ka were 10^-5, then Kb would be 10^-9. As HA becomes a weaker acid, A- becomes a stronger base (but never a strong base). Conversely, the greater the acid strength of HA, the less the base strength of A-. However, if either A- or HA is weak, so is its conjugate. If HA is strong (such as HCl), its conjugate base (Cl-) is so weak that it is not a base at all in water."
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Harris is correct, though that is not how it is always expressed.
I thought The conjugate base of a weak acid is a strong base?
As Harris says, the stronger the (weak) acid, the weaker the (weak) base, and vice versa. However, e.g. acetic acid is a weak acid and acetate is a weak base, in the sense that a solution of either will be partly ionised - will contain some HOAc and some AcO-. The conjugate base of a strong acid (stronger than H3O+) is a base so weak it is essentially not protonated at all in water, and hence not really considered a base at all, rather than as a weak base. The conjugate acid of a strong base (stronger than OH-) is an acid so weak that it is essentially undissociated in water, so not really considered an acid, rather than as a weak acid. (It may be possible to deprotonate it in other media, e.g. with a very strong base like BuLi in an aprotic solvent, but here we're considering aqueous solutions.)