ABC - Models of Acids and Bases (Lesson)

Models of Acids and Bases

Solutions of acids and bases have been widely studied for many years due to their widespread use and utility. In order to better understand these categories of solutions, chemists have developed several models to describe the way that acids and bases behave. In this lesson, two of the most commonly referred models are described. The simplest model is discussed first followed by a second model which is much more inclusive and still widely utilized today.

The Arrhenius Acid/Base Model

The oldest model used to describe properties of acids and bases is the Arrhenius model developed by Svante Arrhenius in 1884. According to this model, acids derive their chemical properties from their possession of an H⁺ ion in their chemical structure (often referred to as a proton).  Therefore, when an Arrhenius acid is added to water, it increases the concentration of H⁺ ions in the solution thereby making it more acidic.

In a similar fashion, compounds categorized as bases are those that contain the hydroxide (OH-) ion. As can be seen in the image above, when bases dissolve in water, they increase the concentration of hydroxide ions in solution, making the solution more basic.

The Brønsted-Lowry Acid/Base Model

While the Arrhenius model works well for substances that fit the criteria, it fails to take into account a broader range of compounds that contain neither H⁺ or OH^- ions in the chemical structure yet still alter the acidity or basicity of a solution. To address this issue, two chemists in the early 20th century (Johannes Nicolaus Brønsted and Thomas Martin Lowry) developed a model to expand upon Arrhenius's model. Instead of focusing on the chemical structure of compound, the Brønsted-Lowry model instead focuses on a compound's ability to accept or donate protons. Substances that have the ability to accept a proton (H⁺ ion) are considered Brønsted-Lowry bases while those that have the ability to donate protons are considered Brønsted-Lowry acids.

Because this model of acids and bases is more inclusive, it is typically the most utilized model to describe this chemical property. One example of a substance that does not contain an OH^- ion yet still behaves as base under the Brønsted-Lowry model is NH₃. The image shown below helps illustrate this concept:

Notice first that NH₃ does not contain OH^- ions thereby not allowing it to be considered an Arrhenius base. However, as shown in the equilibrium above, when NH₃ is placed in water it has the ability to accept a proton from water. As a result, hydroxide ions are produced and the solution becomes more basic.  

Notice also that the terms "conjugate base" and "conjugate acid" are used in this equilibrium. When a substance loses a proton, as water does in this example, the conjugate base of this substance is formed. This forms what is referred to as a conjugate acid/base pair. The other pair is formed when NH₃ gains a proton, thereby forming NH₄⁺. The ammonium ion (NH₄⁺) is therefore the conjugate acid of NH₃.

 

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