Conjugate Acids and Bases

[Carolina Gura Dis 1B]

Hi, I am just confused on why a strong acid/base has a conjugate weak acid/base. Is this assumption always true?

[Ramdrick Despabiladeras 2E]

Yes, this assumption is always true. Strong acids must be good proton donors. Thus, their conjugate base must not readily reaccept a proton. Otherwise, the conjugate base would reverse the actions of the acid, rendering it a weak acid. Therefore, the conjugate base of a strong acid must be a weak base as it cannot readily reaccept protons. Likewise, a strong base must have a weak conjugate acid as the acid must not readily re-donate its proton.

[Patricia Sourinphoumy Dis1C]

A strong acid is a good proton donor. A weak acid is an acid that does not dissociate so easily. The strength of a weak acid depends on how much it dissociates: the more it dissociates, the stronger the acid.

Contrastingly, a strong base is a base that breaks apart 100% in solution. a weak base does not break easily in solution.

If a compound such as HCl is considered a strong acid, it is a good proton donor. Thus, there is essentially no tendency for the conjugate base Cl- to reaccept a proton. Therefore, Cl- is a weak base.

In the Bronsted-Lowry definition of acids and bases, a conjugate acid-base pair consists of two substances that differ only by the presence of a proton (H+)