Comparing the Concepts

These three conceptual definitions can be compared with one another by looking at how each one views what happens when an acid dissociates in water. Let's use hydrochloric acid (HCl) as an example. (These equations are also shown in example 18 in your workbook.)

The first equation shows that hydrochloric acid dissociates to form hydrogen ion and chloride ion. This is the Arrhenius view.

HCl H⁺ + Cl¯

The second equation shows a closer approximation to what is really going on. The HCl doesn't just dissociate, but reacts with water to form hydronium ion, H₃O⁺. The idea here is that a proton is transferred from one chemical to another in an acid-base reaction. This is the Brønsted-Lowry point of view.

HCl + H₂O H₃O⁺ + Cl¯

The third equation shows the Lewis way to look at the formation of the hydronium ion. A hydrogen ion from HCl combines with a water molecule to form H₃O⁺. The reaction occurs because H₂O provides an electron pair for the hydrogen ion to accept. The electron dots emphasize the Lewis point of view.

H⁺ +

··
H : O : H
··

·· +
H : O : H
··
H

We will deal primarily with acids and bases from the Brønsted and Lowry point of view. At times we will also use the Arrhenius definition of acids and bases. Actually, what we will do is use whichever definition is most appropriate to the particular aspect of acids and bases that we are trying to study at the time.

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E-mail instructor: Eden Francis