Question

With suitable equations, state the Arrhenius and Bronsted & Lowry definition of an acid and
a base.

Answer 1

In Arrhenius, we are limited to cases in which water is the solvent (this is assumed by Arrhenius). An acid will dissolve in water to produce H

+

 ions, while a base will dissolve in water to produce O

H

−

 ions.

Bronsted and Lowry state that an acid is any substance that will donate a proton (meaning an H

+

 ion). This includes the case where the donation of the proton is made to a water molecule (which therefore includes everything Arrhenius would have considered), but allows for donation of protons to many other substances, opening the door for general acid-base reactions (proton-transfer reactions) such as

$H C l + N H 3 → N H + 4 + C l −$

(Bronsted and Lowry also change our thinking of what makes a substance a base. All that is required is that a particle (atom, molecule or ion) be able to acquire a proton, and that particle is a base. Check out the role of N

H

3

 above)

The Lewis definition goes this one further in stating that an acid is a substance that can receive an electron pair (meaning the lone pair of a particle) and that a base is a substance that can donate a lone pair.

Again, this definition includes all cases that fit into the B-L scheme, because the H

+

 proton Bronsted and Lowry refer to is a proton with an empty orbital. This orbital can bond with the lone pair of a particle such as the ammonia molecule (the N atom has a full orbital not used to bond to the three H atoms, hence a lone pair).

However, Lewis also includes cases in which the H

+

 proton is not the particle being transferred, and so broadens the concept of acids and bases to include many more cases. For example

$B F 3 + F − → B F − 4$

would not be an acid-base reaction according to Bronsted-Lowry, but does qualify in the Lewis sense.