Question

What is the difference between Arrhenius, bronsted-lowry and Lewis acids and bases?

Answer 1

In the order which you list them, each definition includes and expands on the previous definitions, treating all cases the previous definition could, and broadening out to include more.

Explanation:

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 `OH^-` 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

`HCl + NH_3 rarr NH_4^+ + Cl^-`

(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 `NH_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

`BF_3 + F^-` `rarr BF_4^-`

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