Acids and Bases (Previous Version): _{An Introduction}

Neutralization

As you can see from the equations, acids release H⁺ into solution and bases release OH^-. If we were to mix an acid and base together, the H⁺ ion would combine with the OH^- ion to make the molecule H₂O, or plain water:

The neutralization reaction of an acid with a base will always produce water and a salt, as shown below:

Though Arrhenius helped explain the fundamentals of acid/base chemistry, unfortunately his theories have limits. For example, the Arrhenius definition does not explain why some substances, such as common baking soda (NaHCO₃), can act like a base even though they do not contain hydroxide ions.

In 1923, the Danish scientist Johannes Brønsted and the Englishman Thomas Lowry published independent yet similar papers that refined Arrhenius' theory.  In Brønsted's words, "... acids and bases are substances that are capable of splitting off or taking up hydrogen ions, respectively."  The Brønsted-Lowry definition broadened the Arrhenius concept of acids and bases.  

The Brønsted-Lowry definition of acids is very similar to the Arrhenius definition: Any substance that can donate a hydrogen ion is an acid. (Under the Brønsted definition, acids are often referred to as proton donors because an H⁺ ion, hydrogen minus its electron, is simply a proton).

The Brønsted definition of bases is, however, quite different from the Arrhenius definition.  The Brønsted base is defined as any substance that can accept a hydrogen ion.  In essence, a base is the opposite of an acid.  NaOH and KOH, as we saw above, would still be considered bases because they can accept an H⁺ from an acid to form water. However, the Brønsted-Lowry definition also explains why substances that do not contain OH^- can act like bases.  Baking soda (NaHCO₃), for example, acts like a base by accepting a hydrogen ion from an acid as illustrated below:

In this example, the carbonic acid formed (H₂CO₃) undergoes rapid decomposition to water and gaseous carbon dioxide, and so the solution bubbles as CO₂ gas is released.

pH

Under the Brønsted-Lowry definition, both acids and bases are related to the concentration of hydrogen ions present. Acids increase the concentration of hydrogen ions, while bases decrease the concentration of hydrogen ions (by accepting them). The acidity or basicity of something, therefore, can be measured by its hydrogen ion concentration.

In 1909, the Danish biochemist Sören Sörensen invented the pH scale for measuring acidity. The pH scale is described by the formula:

For example, a solution with [H⁺] = 1 x 10^-7 moles/liter has a pH equal to 7 (a simpler way to think about pH is that it equals the exponent on the H⁺ concentration, ignoring the minus sign). The pH scale ranges from 0 to 14. Substances with a pH between 0 and less than 7 are acids (pH and [H⁺] are inversely related - lower pH means higher [H⁺]). Substances with a pH greater than 7 and up to 14 are bases (higher pH means lower [H⁺]). Right in the middle, at pH = 7, are neutral substances, for example, pure water. The relationship between [H⁺] and pH is shown in the table below alongside some common examples of acids and bases in everyday life.