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Conjugate base of strong and weak acids

Posted: Fri Dec 06, 2019 10:37 am
by Michael Du 1E
What is the explanation behind the fact that the conjugate base of a strong acid is neutral, while the conjugate base of a weak acid is a stronger base (basic)? Thank you!

Re: Conjugate base of strong and weak acids

Posted: Sat Dec 07, 2019 1:24 am
by Ryan 1K
If we think of it in terms of a reaction and using a chemical equation, it is easier to understand the logic behind this. The dissociation of an acid or base is a reversible reaction. This means that at equilibrium, both the forward and reverse reactions are occurring simultaneously.

Let's say that the forward reaction is a strong acid donating a H+. This means that the reverse reaction is the conjugate base accepting H+. Strong acids are very likely to donate H+. As a result, the equilibrium heavily favors the forward reaction, meaning that there will be more product than reactant. As such, the reverse reaction is not favorable, and the conjugate base is a weak base. Since the weak base is the conjugate base of a strong acid, the reverse reaction will proceed to a very small extent, and the weak base is considered to not affect the pH.

The reverse logic applies as well. The conjugate acid of a strong base will not affect pH much since the equilibrium will favor the strong base accepting H+, not the conjugate acid donating H+.