Question

How can you tell the difference between an acid and a base given this chemical equation: `C_5H_5N + H_2O -> C_5H_5NH^+ + OH^-`?

Answer 1

Yes. Use the Lewis-Bronsted definition of acids and bases.

Explanation:

The Lewis-Bronsted definition of acids and bases expanded on the Arrhenius definition by making any proton donor an acid, and any proton acceptor a base.

Further, we can use the concept of "conjugate" acids and bases to identify each in this equation. A "conjugate" is the result of the balance between acids and bases. The compound that is an acid on the left-hand side of an equation must produce a "conjugate" that is a base on the right-hand side.

This gives us two ways to identify them in this equation. The first is to identify the proton donors and acceptors. The second is to identify an easily-recognized acid or base and use the conjugate concept to find the others.

In this case it is easiest to recognize the `OH^-` as a base (check: yes, it would accept a proton). That means the other compound on the right must be the acid. BOTH are "conjugates" of the original acid-base pair on the reactant side, so the original "acid" is `H_2O` (because the conjugate base is the `OH^-`) and the original base is the `C_5H_5N`. The conjugate acid is the `C_5H_5NH^+` (yes, it could donate that proton).