Acids and Bases - Definitions, Properties, and Reactions

Definitions - Arrhenius

There are three different definitions for an acid. The one that is most frequently used in introductory chemistry courses is named after the Swedish chemist Svante Arrhenius. Arrhenius acids are compounds that dissociate in water and yield hydrogen ions (H+). Common compounds that fit this definition include hydrochloric acid and sulfuric acid. In fact, it is safe to say that inorganic compounds that have a formula that begins with "H" are Arrhenius acids. Conversely, bases are compounds that dissociate in water and yield hydroxide (OH-) ions. Examples of Arrhenius bases are sodium hydroxide and calcium hydroxide.

Definitions - Brønsted-Lowry

In this definition, an acid is referred to as a proton donor and a base is a proton acceptor. Unlike Arrhenius, which also defines the products as a salt and water, Brønsted-Lowry stipulates that an acid transfers a proton to base yielding a new acid/base pair. This new pair are referred to as the conjugat acid and conjugate base. This also implies that the reaction is reversible, so Brønsted-Lowry is most often discussed with regards to weak acids and bases. Since there is very little dissociation with weak acids and bases, an equilibrium is established between the Brønsted-Lowry acid/base pair and its conjugate acid/base pair. As an example, consider the reaction below:

Regarding this reaction, one may initially only recognize it as the dissociation of a weak acid in water. However, the dynamics of this dissociation are what are important. HNO2, or nitrous acid, gives up a proton (H+) to the water molecule. Since water is the proton acceptor, it is the base in this reaction. This is a clear difference from the Arrhenius definition which stipulates that bases produce hydroxide ions in solution. Also unique to the Bronsted-Lowry theory are conjugate acid/base pairs. In order to determine the conjugate acid and base, it is best to view the reaction "in reverse," which is fully plausible since the reaction is an equilibrium:

When viewed in reverse, it can be seen that the hydronium ion (H3O+) donates a proton to the nitrite ion (NO2-). In fact, there is no other option, since nitrite has no hydrogen in its formula! For this reason, hydronium is the conjugate acid and nitrite is the conjugate base.

Organic Acids

Recall that organic compounds are those that feature carbon, with emphasis on carbon-carbon and carbon-hydrogen bonds. Some common organic acids are acetic acid and citric acid. Organic acids are commonly referred to as carboxylic acids, and they feature the -COOH functional group. In many formulas, the -COOH appears at the end of the formula. The aforementioned acetic acid is often written as CH3COOH. It should be noted that although these compounds also end in -OH like bases, the -COOH group is a different functional group. If the -OH group is attached to metals and polyatomic ions, and do not contain the C-C and C-H bonds the compound would be inorganic. Therefore, compounds that contain only -OH are not organic acids.

Reactions - Neutralization

The Arrhenius definition of acids and bases provides the basis for a simple double displacement reaction. Consider the reaction shown below:

H3PO4 (aq) + 3 NaOH (aq) → 3 H2O (ℓ) + Na3PO4 (aq)

In this reaction, the Arrhenius acid is H3PO4, or phosphoric acid. The presence of H at the beginning of the formula is a clue that it is the acid. Similarly, the second compound, NaOH (sodium hydroxide), must be the base because it features a metal (Na) bonded to the hydroxide (OH-) ion. In the reaction between an Arrhenius acid and base, water will always be produced. This is because the reaction is double displacement and the arrangement of the acid's cation (always H+) with the base's anion (always OH-) yields the compound HOH, or H2O. The second product is referred to as the salt, and is made of acid's anion and the base's cation. It is often, but not always aqueous. Recall that double displacement reactions only work when one of the products are not aqueous. This is true of the product water, which is always a liquid.

Reactions - Acids and Metals

The reactions of acids with metals are single displacement reactions. There are some metals that do not react with acids, namely those located below hydrogen on the activity series. The second product in the reaction is an ionic compound that is usually aqueous. The reaction below is an example of a metal reacting with an acid.

Zn (s) + 2 HBr (aq) → H2 (g) + ZnBr2

Reactions - Acids and Carbonates

Carbonates will react with acids in a double displacement reaction that produces H2CO3, carbonic acid. This compound is very unstable at room temperature and pressure, and automatically decomposes into carbon dioxide and water. This reaction occurs naturally when acid rain erodes away minerals that contain the carbonate ion, like calcite, CaCO3. Consider the reaction below, which illustrates the reaction of sulfuric acid (which makes rain water acidic) with calcium carbonate:

H2SO4 (aq) + CaCO3 (s) → H2O (ℓ) + CO2 (g) + CaSO4 (s)

Reactions - Acids and Sulfites

Like carbonic acid, sulfurous acid, the product of reactions between sulfites and acids, is unstable. Sulfurous acid will immediately decompose into sulfur dioxide and water. Compare the similarity of the reaction between calcium carbonate and acid with the reaction between calcium sulfite and acid. When the acid attacks the sulfite, the distinct foul odor of sulfur dioxide is evident.

H2SO4 (aq) + CaSO3 (s) → H2O (ℓ) + SO2 (g) + CaSO4 (s)