The Bronsted-Lowry theory (BLT) describes acid-base (AB) equilibrium as a transfer of a proton (H+) from one species to another.
In a typical BL AB reactions, the source of the proton (H+) is an acid and the H+ acceptor is a base. Once the acid loses the H+, it becomes a conjugate base. Likewise, once the base accepts the H+, it becomes a conjugate acid. The term “conjugate” refers to the product of the reaction in the acid-base equilibrium.
You can easily identify a BL acid or a base by carefully following the movement of the proton from one species to another. Since the proton donor is an acid, whatever species loses it is an acid. Likewise, whatever gets the H+, is the base!
In the example above, the pyridine (a cyclic compound with the nitrogen) is a base. It accepts the proton from HCl and becomes the conjugate acid on the other side of the equilibrium. And, since the HCl donated the H+ into the system and gave it to our pyridine, it is our acid. Once it loses the proton, it becomes the conjugate base.
Strong acids dissociate completely. Thus, when we say that something is a strong acid, it means that it all has dissociated by donating its protons to the solution. HCl is an example of a strong acid. In a solution there will be virtually no undissociated HCl.
Most organic acids are, however, weak acids. We characterize weak acids with their dissociation equilibrium constants (Ka). For instance, let’s look at the dissociation of a typical organic acid, acetic acid:
By definition, the Ka is a product of the concentrations of the products of the acid dissociation over the concentrations of the leftover acid. For the acetic acid, the dissociation constant Ka = 1.8·10-5. This is a very small number, which means only a very small portion of acetic acid actually dissociates in the solution and the majority of CH3COOH stays as is.
The Ka values are quite inconvenient for a everyday use. Instead of the Ka values, we typically use the pKa values. By definition, the pKa = -logKa, thus:
Typical pKa values of organic substances are usually somewhere between 2 and 18. Of course, there are compounds with drastically larger or lower pKa values. But those are going to be some special molecules that we’ll be using as very strong bases or acids in the future.
For now, remember:
Small Ka = weak acid
Large Ka = Strong acid
Small pKa = strong acid
Large pKa = weak acid
As the Ka and pKa have an inverse relationship, it’s very important to be very careful when reading the question! Your instructor can easily trick you by asking you to arrange molecules according to their pKa’s, Ka’s, or acid strengths. And if you’re not paying attention, you can give an opposite ranking for the answer.