Is there any easy trick to know if a substance is a Brønsted–Lowry acid or base?

Question

I know that Brønsted–Lowry acids can donate a proton and a Brønsted–Lowry base can receive a proton. I have also memorized the strong acids and bases, which makes it easier to figure out. However, if I was given a substance that I didn't memorize, I would struggle to tell if it is an acid or base. Are there any quick methods to know if a substance is an acid or base? Or do I have to memorize as many as I can?

Answer 1

Not perfect, but here are a couple rules of thumb to start with for the most common cases. The key with both rules is to look at the molecular structure.

1. A Bronsted acid typically has hydrogen attached to oxygen, sulfur or a halogen, all these being relatively electronegative elements. The hydrogen could also be attached to another nonmetal, especially nitrogen or phosphorous, if the atom is a positive charge center like the nitrogen in ammonium ion. If the atom is both a positive charge center and oxygen, sulfur or a halogen, you likely have a strong acid.

2. A Bronsted-Lowry base typically has a nonbonding electron pair that can combine with a proton, thus resembling a Lewis base. In stable compounds the nonbonding pair us usually on the same kind of atom as one of the elements mentioned above for acids. This is one reason Bronsted-Lowry acid-base reactions are typically fast and reversible.