Bronsted-Lowry Acid: Definition & Examples

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  • 0:01 What is a Bronsted-Lowry Acid?
  • 3:20 Bronsted-Lowry…
  • 5:23 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Two scientists came to a conclusion: an acidic solution behaves as proton donors, which is in the form of a hydrogen ion. This is what we call the Bronsted-Lowry definition of an acid, which is the main focus of this lesson.

What is a Bronsted-Lowry Acid?

What makes a solution an acid? There have been different definitions proposed by different scientists. However, there were two scientists who independently proposed essentially the same theory about the definition of acids and bases.

In 1923, Johannes Nicolaus Bronsted and Thomas Martin Lowry, from Denmark and England respectively, published the same theory on how acidic and basic solutions behaved. According to Bronsted and Lowry, acids are solutions that donate protons and bases are solutions that accept protons.

These scientists came up with this theory independently and relatively at the same time, so the theory has both of their names attached to it: Bronsted-Lowry Theory of Acids and Bases.

A Bronsted-Lowry acid is a solution that donates protons, and is known as a proton donor. It donates protons in the form of a hydrogen ion (H+). This is reinforced by the definition of an acid, which is a solution that has an excess of hydrogen ions (H+). The Bronsted-Lowry base, on the other hand, is a solution that accepts protons, and is known as a proton acceptor.

Going back to a Bronsted-Lowry acid, in general it must contain a hydrogen ion that it can give up. When the acid gives up the hydrogen ion, which has a charge equal to positive one (+1), the charge of the acid decreases by positive one (+1). A Bronsted-Lowry acid-base reaction generally looks like this:

Bronsted-Lowry Acid Base Reaction

Here the Bronsted-Lowry acid, HA, gives up or donates its hydrogen ion, and the Bronsted-Lowry base, B, accepts the hydrogen ion. As a result, when the hydrogen ion attaches itself to the base, the base gains a charge of positive one, as shown by the positive charge at the products side, HB+. In the products side, you also see that after losing the hydrogen ion, A- is all by itself and has a charge of negative one because it donated its hydrogen ion. The double arrow between the reactants and the products is what we call an equilibrium sign, which means that it is possible for the reaction to occur forwards and backwards.

In this reaction, we see that the products of the acid-base reaction are a conjugate acid and a conjugate base. A conjugate acid is the substance that is produced after the base accepts the proton. When this occurs, its charge increases by +1. A conjugate base, on the other hand, is produced after the acid donates its proton. When this occurs, its charge decreases by +1. A conjugate acid-base pair is a set of two substances, where one substance differs from the other substance by one proton or hydrogen atom. In this reaction, the conjugate acid base pairs are: HA/A- and B/HB+.

How do we know again which substance is the Bronsted-Lowry acid? We do this by counting the hydrogen ion for each substance in the chemical reaction. If a substance lost a hydrogen ion, then it is a Bronsted-Lowry acid.

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