Brönsted -Lowry Acid Base Theory

Johannes Brönsted and Thomas M. Lowry gave a generalized definition of acids and bases. They defined them by a common term “proton (H+)”. Acids are those which give proton and bases are those which accept proton.

Brönsted Acid and Base

Let’s see if this theory can explain the basic nature of NH₃.

NH_3(aq)+ H₂O_(l) ⇌ NH₄⁺_(aq) + OH^- _(aq)

Here NH₃accepts proton from H₂O, hence it is called as base.

Conjugate Acid - Base Pair

This theory not only defines acid and base but also clarifies their relation with each other. Acid and base are like the two sides of a coin. As either side of a coin cannot stay alone, acid and base also can’t stay alone. Each of them has its counterpart which is named as conjugate.

In the above equation NH₃ accepts proton and becomes NH₄⁺, here it acts as base.

NH_3(aq) ⟶  NH₄⁺_(aq) 

When you see the reverse reaction, you will see NH₄⁺donates proton and becomes NH₃, thus it acts as acid. NH₃is a base and NH₄⁺is its conjugate acid or vice versa.

NH_3(aq) ⟶ NH₄⁺_(aq) 

Similarly H₂O gives proton to NH₃ and becomes OH^- , so H₂O acts as an acid and OH^- is its conjugate base, which accepts proton from NH₄⁺in reverse reaction. Let’s see one more equation.

HCl_(aq)+ H₂O_(l) ⇌ H₃O⁺_(aq) + Cl^- _(aq)

Here HCl gives proton H⁺ and acts as acid, while H₂O accepts proton and acts as base. I hope now you can find their respective conjugate. HCl has its conjugate base Cl^- and H₂O has its conjugate acid OH^-.

Conjugate Acid-Base Pair

In the above two equations you have seen that H₂O acts as acid when it comes with NH₃ and acts as base when it comes with HCl. That means acid and base are comparative terms.

For example when 2 comes with 1, 2 looks bigger than 1 but if it comes with 3, it looks smaller. Similarly H₂O acts as acid when it comes with NH₃ and acts as base when it comes with stronger acid HCl.

Which factor decides the strength of an acid or base? Readiness to give off the proton decides the strength of any acid. If we compare two acids, the one which readily gives off the proton is the stronger acid. And similarly the one which accepts proton readily is the stronger base.

It is very easy to figure out the corresponding conjugates for acids and bases. If you want to find conjugate acid of any species just add proton (H⁺) to it and if you want to find conjugate base, subtract proton (H⁺) from it. Let’s practice few examples of conjugate acid- base pair:

Species	Conjugate Acid	Conjugate Base
NH3	NH4+	NH2-
H2O	H3O+	HO-
HSO4-	H2SO4	SO42-

Now you must be able to guess the nature of species and also to find the conjugate acid and base of any species. But what happens to those species which lack a Hydrogen? For example, how can we find out whether BF₃ is an acid or base? Arrhenius concept and Brönsted -Lowry acid base theory both are not able to help us in this case. So how do we find the right answer? In the next post we will try to find out its answer.

This work is licensed under the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.