Question

What are some examples of the properties of bases?

Answer 1

• They might want protons (Bronsted-Lowry definition)
• They might want to donate electrons (Lewis definition)
• They might donate `"OH"^(-)` to solution (Arrhenius definition)
• The conjugate base of a weak acid is a strong base
• The conjugate base of a strong acid is a weak base

A nice example of something that has most of the following properties is `"HSO"_4^(-)`.

This base wants a proton according to the Bronsted-Lowry definition, and it will get that proton by donating electrons according to the Lewis definition, using the lone pairs on the `"O"^(-)`.

It is the conjugate base of `"H"_2"SO"_4`, a strong acid; thus, it is a weak base.

(Since the `"pKa"` of `"H"_2"SO"_4` is about `1000`, it is reasonable to say that most `"H"_2"SO"_4` is actually deprotonated.)

Furthermore, although it might be difficult, it can be deprotonated to get `"SO"_4^(2-)` (the `"pKa"` of `"HSO"_4^(-)` is about `2`). That makes it also a (relatively strong weak) acid by the Bronsted-Lowry definition, because it can donate a proton, AND it makes it a (relatively strong weak) acid by the Lewis definition, because it accepts electrons in order to donate that proton.

(This makes the conjugate base, `"SO"_4^(2-)`, a strong base.)

It is not, however, a base by the Arrhenius definition, but an acid. It donates protons (`"H"^(+)`) to solution upon dissociation, while donating `"OH"^(-)` would require disrupting its resonance structures, which is unfavorable.