The Bronsted-Lowry and Lewis Definition of Acids and Bases

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Amy Meyers

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Dawn Mills

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Learn the Bronsted-Lowry and Lewis definitions of an acid and base. Discover how these theories differ from each other and from the Arrhenius theory of an acid and base. Learn how to identify an acid in terms of proton donation and a base as a proton acceptor, and explain what a conjugate acid or base is.

Bronsted-Lowry Acid and Base

Johannes Bronsted and Thomas Lowry
Bronsted Lowry Photos

You have previously learned about an Arrhenius acid, which was proposed by Swedish scientist Svante Arrhenius in 1887. His theory is good and valid but has certain problems or drawbacks. The Arrhenius theory proposes that an acid is a substance that can give away a hydrogen atom in the form of an ion (H+) when dissolved in water, producing an aqueous acid. This limits acids to being only aqueous solutions and does not account for an acid, like hydrochloric acid, in gaseous form. His definition also does not account for substances that can sometimes act as an acid and sometimes as a base.

In 1923, a Danish chemist, named Johannes Bronsted, and British scientist, Thomas Lowry, both independently proposed a different definition of an acid. They said that an acid should be any substance that can donate a proton. In this case, a proton is just a hydrogen atom that has lost its electron to become a positively charged ion of H+. This definition of an acid is now called a Bronsted-Lowry acid.

An example of this is when hydrogen chloride dissolves in water. The hydrogen on the hydrogen chloride donates its hydrogen proton to the water, resulting in a cation of H3O+ and an anion of Cl-. HCl + H2O = H3O+ + Cl-.

In contrast to the acid definition, a Bronsted-Lowry base is a substance that accepts protons. An example of a proton acceptor is ammonia (NH3). The ammonia is happy to accept a proton from the hydrogen of water (H2O) to become NH4. NH3 + H2O = NH4+ + OH-.

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  • 0:07 Bronsted-Lowry Acid and Base
  • 2:08 Conjugate Acids and Bases
  • 3:37 Lewis Acid and Base
  • 5:09 Lesson Summary
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The proton acceptor ammonia takes on a proton from H20.
Proton Acceptor Ammonia

Conjugate Acids and Bases

The Bronsted-Lowry definition of acids and bases is pretty simple. An acid is a proton donor, and a base is a proton acceptor. In the example of ammonia dissolving in water (NH3 + H2O = NH4+ + OH-), the water donates a proton, so it is the acid. The ammonia accepts the proton, so it is the base. This reaction is reversible. The hydrogen on the ammonium ion (NH4+) can go back to the hydroxide ion (OH-) to form NH3 and H2O (ammonia and water) again. In this case, because the ammonium ion is donating a proton, it is called a conjugate acid. The hydroxide ion is accepting the proton and is called a conjugate base. Every Bronsted-Lowry acid has a conjugate base, and every Bronsted-Lowry base has a conjugate acid.

  • Conjugate acid: an acid that forms when a base gains a proton.
  • Conjugate base: a base that forms when an acid loses a proton.

All Arrhenius acids and bases are also Bronsted-Lowry acids and bases, but the opposite isn't true. An Arrhenius acid/base must be a substance dissolved in water. A Bronsted-Lowry acid/base can be dissolved in water, like an Arrhenius acid/base, but it does not have to be.

G.N. Lewis created the electron dot structure illustrating valence electrons.
Lewis Dot Structure Example

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Additional Activities

Bronsted-Lowry and Lewis Acids and Bases

These exercises will test your understanding of the Bronsted-Lowry and Lewis definitions of acids and bases. Solutions are provided below to help you check your work.


Consider the following reaction:

NH3 + H2 O → NH4+ + OH-

1. Which reactant is acting as the Bronsted-Lowry acid?

2. Which reactant is acting as the Bronsted-Lowry base?

3. If you consider the reverse reaction, which compound is acting as the Bronsted-Lowry acid?

4. If you consider the reverse reaction, which compound is acting as the Bronsted-Lowry base?

5. What is an amphoteric compound?

Consider the following equation:

HNO2 + PO4 3- → NO2 - + HPO42-

6. What are the conjugate acid and base pairs?

Consider the following reaction:

Al3+ + 6H2 O → [Al(H2 O)6 ]3+

7. Which compound is acting as the Lewis acid? Why?

8. Which compound is acting as the Lewis base? Why?


1.H2 O

2. NH3

3. NH4 +

4. OH-

5. A compound capable of acting as an acid or base

6. HNO2 , NO2 - and PO4 3- , HPO4 2-

7. Al3+ . Its valance shell is not filled and is a cation (positive charge).

8. H2 O. The lone pairs of electrons on the water allow it to act as an anion and a Lewis base.

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