in kaplan they say that the conjugate base of a weak acid is stronger than the conjugate acid.
they later say that the conjugate acid of a weak base is stronger than the conjugate base of a strong acid.
can someone please explain the rationale on this?
Before anything else, let's review the meaning of conjugated base and conjugated acid. Consider the following reaction:
HA<-->H+ + A-
Conjugated base: is the (HA) - (H) = A-
Conjusgated acid: is (A-) + (H) = HA
First statement: The conjugate base of a weak acid is stronger than the conjugate acid. Why?
Consider a weak acid like HF. It dissociates as follows:
HF <--> H+ + F-
Notice that the % dissociation is very small for weak acids. In other words, once in solution, HF is the dominant species. This means that any F- formed will almost immediately react with H+ to produce HF again. Therefore, F- (conjugated base of a weak acid) is a strong reactant. As I mentioned, weak acids like HF hardly dissociates into H+ and F-. In other words, the rate of the forward reaction is very small, which means that HF (conjugated acid) is a weak reactant and has no tendency of dissociating.
Second statement: The conjugate acid of a weak base is stronger than the conjugate base of a strong acid. Why?
Consider a weak base like NH3. In aqueous solutions, this base will react with water via the following reaction:
NH3 + H2O <-->NH4+ + OH-
Since NH3 is a weak base the rate of forward reaction (the reaction that leads to formation of OH-) is very small. In other words, the dominant species in the solution is NH3 rather than NH4+ and OH-. Subsequently, any NH4+ formed will immediately release a H+ and convert back into NH3. This means that NH4+ (Conjugated acid of a weak base) is a strong reactant.
Now let's consider a strong acid like HCl; it dissociates via the following reaction:
HCl<-->H+ + Cl-
HCl is a strong acid so it dissociates 100%. Therefore, the dominant species in solution are H+ and Cl-. Subsequently, any Cl- that is formed will have no tendency of reacting with H+ to produce HCl. This means that Cl- (Conjugated base of a strong acid) is a weak reactant.
Hope this helped.
