Arrhenius Acid Base Theory Explained


The Arrhenius acid base theory was introduced in the late 19th century. Created by Svante Arrhenius, the idea was that acids were a substance that would disassociate in water to yield ions that were electrically charged. One would be a hydrogen ion. Bases, on the other hand, would yield hydroxide ions.

Then, if hydrogen ions and hydroxide ions were to react, then they would be able to produce water.

Arrhenius used this theory to propose that certain compounds be classified as an “acid” or a “base” based on the types of ions that formed when it was added to water.

This has created a list of compounds that are referred to as Arrhenius acids and Arrhenius bases.

What Are Arrhenius Acids and Bases?

In the Arrhenius acid base theory, an acid is anything that increases its concentration of hydrogen ions or protons within an aqueous solution. A classic example that is used in textbooks to describe this process utilizes hydrochloric acid.

Hydrochloric acid is abbreviated as HCl because of its construction. When it is added to an aqueous solution (water), the substance dissociates into H ions and CL ions, which are positive and negative respectively. This causes a higher concentration of hydrogen ions, which meets the definition of an Arrhenius acid.

An Arrhenius base increases the concentration of hydroxide ions within an aqueous solution. The classic example that is used for this description is sodium hydroxide, or NaOH.

When it is added to water, the substance dissociates into Na and OH, which are positive and negative respectively. This results in a higher concentration of hydroxide ions, which classifies the substance as an Arrhenius base.

Not every base is classified as an Arrhenius base because the definition has narrowed over time. Sometimes, this type of base must also contain at least one unit of OH within its chemical formula. Some non-hydroxide bases may not even be considered in this category, such as CH3NH2, which is methylamine.

What Happens When Arrhenius Acids and Bases are Combined?

When an Arrhenius acid reacts with an Arrhenius base, there are two items that are typically produced. The first is water and the second is a salt. This reaction is often called a “neutralization reaction.”

As the water and salt form, the nature of the acid is reduced. It is no longer as caustic as it is in its pure state. The nature of the base is also reduced, which creates substances that are essentially neutral from a pH standpoint.

A complete neutralization can occur when the pH of the acid is equalized to the pH of the base. If a neutral pH is 7, then an example of this would be an acid with a pH of 4 being combined with a base that has a pH of 10.

Items that are bases are sometimes referred to as being alkaline.

Many items that we use every day fit into the categories of acids or bases. Many grains, seafood items, and some cooked vegetables are slightly acidic. Many fruits, some vegetables, and wild rice are slightly alkaline. One interesting note here is that distilled water is considered an acid, but spring water or sea water is considered either neutral or alkaline.

Limitations of the Arrhenius Acid Base Theory

The primary limitation of this particular theory on acids and bases is that it can only describe the chemistry within an aqueous solution. Similar reactions between an acid and a base can occur in certain solvents and with molecules when in a “vapor,” or gaseous state.

Because of these limitations, modernized theories about acids and bases are typically used as they allow for additional chemical reactions within a broader range.

To summarize the Arrhenius acid base theory:

  • Acids are any compound that will increase their concentration of hydrogen ions within water.
  • Bases are any compound that will increase their concentration of hydroxide ions within water.
  • When placed in water, hydrogen ions will immediately react with the molecules of the water and this forms hydronium ions, which are classified as being H3O.
  • In a neutralization reaction between acids and bases, most Arrhenius acid and base combinations will usually react in a way that forms a salt with the water.

Although there is a cutoff point between weak and strong acids, but there are always exceptions to the rule as well. By understanding how each compound or substance will react, it allows us to know what to expect from a chemistry standpoint.