Acids and Bases

Lesson Objectives Define acids and bases in terms of the ions that are produced when each type of compound is dissolved in water. Be able to name acids when given their chemical formulas. Write the correct formula for an acid when given its name. Name and write formulas for bases.

Acids and bases are two very important classes of chemicals that you probably use every day. Many foods and beverages contain acids, such as citrus fruits and juices, soda, and vinegar. Bases are very prevalent in cleaners such as ammonia, and most drain cleaners use a strong base as their active ingredient. Acid-base chemistry will be covered in great detail in a later chapter. In this lesson, we will focus on the nomenclature used for these two types of compounds.

Acids An acid can be defined in several ways. For the purposes of this cLASS, the most straightforward description is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H+) when dissolved in water. This is a different type of compound than the others we have seen. Acids are molecular, which means that in their pure state, they exist as individual molecules and do not adopt the extended three-dimensional structures exhibited by ionic compounds like NaCl.

However, when these molecules are dissolved in water, the chemical bond between the hydrogen atom and the rest of the molecule breaks, leaving a positively-charged hydrogen ion and an anion. For example, the following chemical equation describes what happens when the acid HCl is dissolved in water: Since acids produce H+ cations upon dissolving in water, the H of an acid is written first in the formula of an inorganic acid.

The remainder of the acid (everything except the H) constitutes the anion that is formed after the acid dissolves. Organic acids are also an important class of compounds, but the rules for naming them are different, and they will primarily be discussed in a later chapter. A binary acid is an acid that consists of hydrogen and one other element. The most common binary acids contain a halogen. An oxoacid is an acid that consists of hydrogen, oxygen, and a third element. The third element is usually a nonmetal.

Naming Acids Since all acids contain hydrogen, the name of an acid is based on the anion that goes with it. Recall from earlier in the chapter that anions can either be monatomic or polyatomic. The names of all monatomic ions end in –ide. The majority of polyatomic ions end in either –ate or –ite, though there are a few exceptions, such as the cyanide ion (CN−). It is the suffix of the anion that determines how the acid is named, as described in the rules: 1. When the anion ends in –ide, the acid name begins with the prefix hydro-. The root of the anion name goes in the blank (e.g., chlor- for chloride), followed by the suffix –ic acid. HCl is named hydrochloric acid, because Cl− is a chloride ion. HCN is hydrocyanic acid because CN− is a cyanide ion. 2. When the anion ends in –ate, the name of the acid is the root of the anion followed by the suffix –ic acid. There is no prefix. H2SO4 is sulfuric acid (not sulfic) because SO is the sulfate ion. 3. When the anion ends in –ite, the name of the acid is the root of the anion followed by the suffix –ous acid. Again, there is no prefix. HNO2 is nitrous acid because NO2− is the nitrite ion.

Note that the root for a sulfur-containing oxoacid is sulfur- instead of just sulf-. The same is true for a phosphorus containing oxoacid, which uses phosphor- as its root instead of simply phosph-. Anion Suffix Example Name of acid -ide chloride (Cl−) hydro_____ic acid hydrochloric acid (HCl) -ate sulfate (SO42−) _____ic acid sulfuric acid H2SO4) -ite nitrite (NO2−) _____ous acid nitrous acid (HNO2)

Writing Formulas for Acids Like other compounds that we have studied, acids are electrically neutral. Therefore, the charge of the anion part of the formula must be exactly balanced out by the H+ ions. Since H+ ions carry a single positive charge, the number of H+ ions in the formula is equal to the magnitude of the negative charge on the anion. The chloride ion carries a 1− charge, so only one H is needed in the formula of the acid (HCl). The sulfate ion carries a 2− charge, so two hydrogen atoms are needed in the formula of the acid (H2SO4). Another way to think about writing the correct formula is to utilize the crisscross method, as shown below for sulfuric acid.

Bases The simplest way to define a base is an ionic compound that produces hydroxide ions when dissolved in water. One of the most commonly used bases is sodium hydroxide.

Inorganic bases are named in the same way as other ionic compounds Inorganic bases are named in the same way as other ionic compounds. Since they all contain the OH− anion, the names of these bases end in hydroxide. Some examples of names and formulas for bases are shown here:

Lesson Summary Acids are molecular compounds that dissolve in water to produce hydrogen ions and an anion. Bases are ionic compounds consisting of hydroxide ions and a cation. The naming rules for acids are based on the suffix of the anion. Formulas for acids are written by balancing out the charge of the anion with the appropriate number of hydrogen ions. Naming and formula writing for bases follows the same guidelines as for other ionic compounds.