How does Bronsted-Lowry define acids and bases?

A Brønsted-Lowry acid is any substance (molecule or ion) that can donate a hydrogen ion (##H^(+)##).

A Brønsted-Lowry base is any species that can accept a hydrogen ion (##H^(+)##).

Basically, acids are proton donors and bases are proton acceptors.

Take the reaction below for example:

##HI + NH_3 rightleftharpoons NH_4^(+) + I^(-)##

Hydroiodic acid (##HI##) is the Brønsted-Lowry acid because it donates a hydrogen ion. Ammonia (##NH_3##) is the Brønsted-Lowry base because it accepts the hydrogen ion.

The Brønsted-Lowry theory also introduces the concept of conjugate acid-base pairs. A conjugate acid-base pair are two species that differ by a (##H^(+)##) ion.

Based on the reaction above, the ammonium ion (##NH_4^(+)##) is the conjugate acid of the base ammonia and the iodide ion ##(I^(-)##) is the conjugate base of hydroiodic acid.

Here's a tip to let you know which substance is the conjugate acid and which substance is the conjugate base:

• A conjugate base has one less (##H^(+)##) proton than the acid you started with.

• A conjugate acid has one more (##H^(+)##) proton than the base you started with.

Here's a general depiction of conjugate acid-base pairs:

##HA## represents an acid

##B## represents a base

##A^(-)## represents the conjugate base since this substance has one less proton than the acid, HA.

##HB^(+)## represents the conjugate acid since this substance has one more proton than the base, B.