In your previous chemistry courses, you learned about the modified Arrhenius theory. This theory sought to describe, explain, and predict acid-base reactions by proposing that acids dissolve in water to produce aqueous hydronium ions and bases dissolve in water to form aqueous hydroxide ions.

\( \mathrm { HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq) } \)

\( \mathrm { NH_3(aq) + H_2O(l) \rightarrow NH_4^{+}(aq) + OH^-(aq) } \)

However, this theory could not explain the acid-base reactions that did not occur in an aqueous solution. Neither did it propose a mechanism to explain why some substances appear to have both acidic and basic properties. A new theory in 1923, known as the Bronsted-Lowry Theory, explains these anomalies by focusing on the role of an acid and a base in a reaction.

The Proton Transfer Concept

According to the Bronsted-Lowry concept, a substance is categorized as an acid or a base based on how it behaves and not on its properties. An acid is a proton donor and a base is a proton acceptor. Remember that a proton is a hydrogen atom that has lost its electron.

HCl donates a proton to a water molecule and, therefore, is an acid.

The Bronsted-Lowry concept does not require that water be one of the reactants. Also,  a substance can sometimes act as both an acid and a base, as shown.

Consider the following example:

 

In the first reaction shown above, HSO₃^- acts as an acid because it donates a proton to the hydroxide ion.

In the second reaction shown above, HSO₃^- acts as a base because it accepts a proton from the hydronium ion.

HSO₃^- is referred to as an amphiprotic substance because it can either donate or accept a proton. Later in this module, you will learn how to predict how these substances behave in various circumstances.

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Read pages 722 to 724 in the textbook to enhance your understanding of the Bronsted-Lowry concept.

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Check Your Understanding

Complete "Practice" questions 1, 2, and 3 on page 724 of the textbook. Click the link below to check your answers.