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Conjugates

Let's look at that generic acid/base reaction again.

Notice that when you label all the species as an acid (proton donor) or a base (proton acceptor) you get a couple of acid/base pairs that only differ in their formula by one proton. One is the HA/A^– and the other is the B/BH⁺. This type of relationship is called conjugate pairs. A matching pair will be called an acid/base conjugate pair. The acid is always the one WITH the proton and the base is the one WITHOUT the proton. These pairs have special chemical relationships and it is important to be able to recognize them and know their behaviors.

Now consider the acetic acid ionization:

Notice the matching of the conjugate pairs. One pair is acetic acid and acetate ion or CH₃COOH/CH₃COO^– and the other pair is water and hydronium ion or H₂O/H₃O⁺.

NOTE! The two species of a conjugate pair always differ by ONE and only ONE proton (H⁺).

Generic Weak Acids (HA) in Water

The reaction above for acetic acid can be shown in a much more generic (aka simpler) way that works for all acids:

HA(aq) ⇌ H⁺(aq) + A^–(aq)

This leads to the generic equilibrium expression for all weak acids:

K_a  =   [H⁺][A^– ]
[HA]

Remember that HA and A^– are conjugate pairs and have a special relationship. One can easily turn into the other by a simple removal or addition of a single proton (H⁺). And just to nail down the concept yet again... What follows is just the conjugate base, A^–, in water doing its thing as a base.

Generic Weak Bases (B) in Water

(lets do the same exact thing for weak bases)

The reaction for any generic base (B) works like this:

B(aq) + H₂O(𝓁) ⇌ BH⁺(aq) + OH^–(aq)

This leads to the generic equilibrium expression for all weak bases:

K_b  =   [OH^–][BH⁺ ]
[B]

Remember that B and BH⁺ are conjugate pairs and have a special relationship. One can easily turn into the other by a simple removal or addition of a single proton (H⁺). And just to nail down the concept yet again and for another "weak base"... What follows is the conjugate base of HA, A^–, in water doing its thing as a base.

Generic Conjugate Bases of Weak Acids (A^– ) in Water

Do know that the way you get an ion like A^– into water, is to dissolve the sodium salt of HA into water. The sodium salt is NaA(s). Note that is it the same as HA except the proton has been swapped out for a Na⁺ ion.

A^–(aq) + H₂O(l) ⇌ OH^–(aq) + HA(aq)

And here is the equilibrium expression for that base reaction:

K_b  =   [OH^– ][HA]
[A^– ]

Any set of acid/base conjugate pairs in water will have a special relationship of their ionization constants K_a and K_b.

K_w = K_a·K_b

This is the same inverse relationship that H⁺ and OH^– have in water. We say that conjugates of each other have opposite strengths as acid and base. The stronger one is (bigger K value) the weaker the other is (smaller K value).

This means when you neutralize a typical weak acid with a strong base, you will end up with the even weaker conjugate acid (read about "opposite" strengths in the next section). This is shown below:

HA + OH^– → H₂O + A^–

The HA (weak acid) is completely removed and replaced by its conjugate, A^–. So in general this means

weak acid   +   strong base   →   water   +   weak base

Of course there is the opposite of this situation as well:

weak base   +   strong acid   →   water   +   weak acid

It is only when you mix both strong acid and strong base that you truly get a neutral salt solution - neither acidic nor basic.

strong acid   +   strong base   →   water   +   salt