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THINK ABOUT IT

People have known for thousands of years that vinegar, lemon juice, green apples, and many other foods taste sour. However, it is only recently that scientists have presented theories to explain what all these substances have in common. In what ways are these sour-tasting substances similar, and why are they defined as acids?

How are acids and bases defined?

To answer this question, you will explore

EXPLORING THE TOPIC

Chemical Makeup of Acids and Bases

Many substances dissolve in water to produce solutions that are acidic. Because the behavior of all acidic solutions is similar, you might expect some similarity in the chemical makeup of these substances.

Take a moment to look for patterns in the table of common acids.

Here are some patterns you might notice.

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You might also notice that some of the molecular formulas are written in a way that gives some information about the molecular structure. For example, acetic acid, C₂H₄O₂, is written as CH₃COOH.

Many other substances dissolve in water to produce solutions that are basic. If acidic solutions have H⁺ ions in common, what is similar in the chemical makeup of basic solutions?

Take a moment to look for patterns in the names and chemical formulas of the common bases listed in this table.

Here are some things you might notice.

Acid and Base Theories

ARRHENIUS THEORY OF ACIDS AND BASES

One of the early theories of acids and bases was first proposed in 1884 by a Swedish chemist named Svante Arrhenius. He defined an acid as a substance that adds hydrogen ions, H⁺, to an aqueous solution. He defined a base as a substance that adds hydroxide ions, OH⁻, to an aqueous solution.

BRØNSTED-LOWRY THEORY OF ACIDS AND BASES

The Arrhenius definitions of acids and bases do not explain how substances without hydroxide ions, OH⁻, in their formula, like ammonia, NH₃, can be bases. In the early 1920s, a Danish chemist, Johannes Brønsted, and an English chemist, Thomas Lowry, proposed a slightly different definition for acids and bases to account for this chemical behavior. They defined an acid as a substance that can donate a proton to another substance. They defined a base as a substance that can accept a proton from another substance. A proton in this case is actually a hydrogen ion, H⁺. Because a hydrogen atom typically has no neutrons, if you remove the electron from a hydrogen atom to form an ion, all that’s left is a proton.

When ammonia, NH₃, is added to water, it removes a hydrogen ion, H⁺, from some of the water molecules, leaving behind some hydroxide ions, OH⁻. An NH₃ molecule accepts a hydrogen ion and becomes NH₄⁺. This makes NH₃ a Brønsted-Lowry base.

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NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq)

So dissolved ammonia really consists of ammonium and hydroxide ions. For this reason, it is often referred to as ammonium hydroxide solution. Many substances that do not contain OH⁻ can act as bases.

Strong and Weak Acids and Bases

These two illustrations show particle views of a 0.010 M hydrochloric acid, HCl, solution and a 0.010 M formic acid, HCOOH, solution. The water molecules are not shown. Take a moment to examine them.

Notice that there are no molecules of HCl in the solution on the left. The HCl has dissociated completely into H⁺ and Cl⁻ ions. However, the solution on the right contains formic acid molecules. Only some of the HCOOH molecules have dissociated into ions. This means that the concentration of H⁺ ions is smaller in the formic acid solution than in the hydrochloric acid solution, even though the solutions have the same molarities.

Acids that dissociate completely in solution are called strong acids. Strong acids include hydrochloric acid, HCl, nitric acid, HNO₃, sulfuric acid, H₂SO₄, and hydrobromic acid, HBr. Strong acids are good conductors of electricity.

Acids that dissociate only partially in solution are called weak acids. These solutions are only moderate conductors of electricity. Some common weak acids are formic acid, HCOOH, acetic acid (vinegar), CH₃COOH, citric acid, C₃H₅O(COOH)₃, and phosphoric acid, H₃PO₄. Weak acids tend to be less corrosive because they do not dissociate completely into ions.

Bases can also be classified as strong or weak. A strong base dissociates completely into ions in solution and weak bases dissociate only partially. Some examples of strong bases include sodium hydroxide, NaOH, and barium hydroxide, Ba(OH)₂. Examples of weak bases include ammonia, NH₃, and aniline, C₆H₅NH₂.

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LESSON SUMMARY

How are acids and bases defined?

The Arrhenius definition of an acid is a compound that dissociates in solution to form a hydrogen ion, H⁺, and an anion. The Arrhenius definition of a base is a substance that dissociates in solution to form a hydroxide ion, OH⁻, and a cation. However, not all bases contain OH⁻. Brønsted and Lowry defined acids as proton donors and bases as proton acceptors. The word “proton” in this case refers to an H⁺ ion. A strong acid or base dissociates completely into ions in solution. A weak acid or base dissociates only partially into ions in solution.