Question

How does Bronsted-Lowry define acids and bases?

Answer 1

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^(+)`).

Explanation:

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.