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7.3: Acids and Bases

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Lesson Objectives

  • Define an acid and a base in terms of ions that are present in each compound.
  • Use the three rules to name acids given the formula.
  • Write correct formulas for acids.
  • Name and write formulas for bases.

Lesson Vocabulary

  • acid
  • base
  • binary acid
  • oxoacid

Check Your Understanding

Recalling Prior Knowledge

  • What are cations and anions and how are they formed?
  • How do ionic compounds and molecular compounds compare structurally?
  • How do you make sure that formulas are electrically neutral?

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

Acids

An acid can be defined in several ways. For the purposes of this chapter, the most straightforward is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H+) when dissolved in water.

(A) Vinegar comes in a variety of types, but all contain acetic acid. (B) Citrus fruits like grapefruit contain citric and ascorbic acids.

This is a different type of compound than the others we have seen in this chapter. Acids are molecular, which means that in their pure state they are individual molecules and do not adopt the extended three-dimensional structures of 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. This can be symbolized in a chemical equation:

\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-

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 (other than the H) is the anion after the acid dissolves. Organic acids are also an important class of compounds, but 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 name of all monatomic ions ends 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 this suffix of the anion that determines how the acid is named as displayed in Table below and the rules below.

Naming System for Acids
Anion Suffix Example Name of acid Example
-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)

The three different suffixes that are possible for the anions lead to the three rules below.

  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 (chlor- for chloride), followed by the suffix –ic. HCl is hydrochloric acid because Cl- is the chloride ion. HCN is hydrocyanic acid because CN- is the 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. There is no prefix. H2SO4 is sulfuric acid (not sulfic) because SO42- 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. Again, there is no prefix. HNO2 is nitrous acid because NO2- is the nitrite ion.

Note how the root for a sulfur-containing oxoacid is sulfur- instead of just sulf-. The same is true for a phosphorus-containing oxoacid. The root is phosphor- instead of simply phosph-.

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 negative charge, the number of H+ ions in the formula is equal to the quantity of the negative charge on the anion. Two examples from Table above illustrate this point. 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 H’s are needed in the formula of the acid (H2SO4). Another way to think about writing the correct formula is to utilize the crisscross method, 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 (Figure below).

(A) Sodium hydroxide, a base, is a solid that is typically produced as small white pellets. (B) The structure of sodium hydroxide is an extended three-dimensional network. The purple spheres are the sodium ions (Na+). The red and white spheres are oxygen and hydrogen atoms respectively, which are bonded together to form hydroxide ions (OH-).

Names and Formulas of Bases

There is no special system for naming bases. Since they all contain the OH- anion, names of bases end in hydroxide. The cation is simply named first. Some examples of names and formulas for bases are shown in Figure below.

Examples of Bases
Formula Name
NaOH sodium hydroxide
Ca(OH)2 calcium hydroxide
NH4OH ammonium hydroxide

Notice that because bases are ionic compounds, the number of hydroxides in the formula does not affect the name. The compound must be neutral, so the charges of the ions are balanced just as for other ionic compounds. Sodium ion (Na+) requires one OH- ion to balance the charge, so the formula is NaOH. Calcium ion (Ca2+) requires two OH- ions to balance the charge, so the formula is Ca(OH)2. Hydroxide ion is a polyatomic ion and must be put in parentheses when there are more than on in a formula.

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 combined with a cation.
  • The three 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.

Lesson Review Questions

Reviewing Concepts

  1. What ion must be produced when an acid dissolves in water?
  2. What ion must be present for a compound to be considered a base?
  3. Identify each of the following compounds as either an acid, a base, or neither.
    1. RbOH
    2. HBr
    3. MgO
    4. CH4
    5. H3PO3
    6. Sr(OH)2

Problems

  1. Name the following acids.
    1. HF
    2. HClO2
    3. H2CrO4
    4. H2SO3
    5. H3PO4
    6. HClO4
    7. H2S
    8. HNO3
  2. Write correct formulas for the following acids.
    1. carbonic acid
    2. hydroiodic acid
    3. chloric acid
    4. phosphorous acid
    5. oxalic acid
    6. hypochlorous acid
    7. hydrobromic acid
    8. permanganic acid
  3. Write names or formulas for these compounds.
    1. LiOH
    2. Mg(OH)2
    3. Fe(OH)3
    4. nickel(II) hydroxide
    5. aluminum hydroxide
    6. silver hydroxide
  4. Identify each compound below as either a (1) ionic compound, (2) molecular compound, (3) acid, or (4) base. Then, name it properly.
    1. HCN
    2. KNO3
    3. N2O4
    4. Sr(OH)2
    5. HBrO3
    6. SO3
    7. HCH3COO
    8. CsOH
    9. Pb3(PO4)2
    10. AsH3

Further Reading / Supplemental Links

Points to Consider

The extended three-dimensional structure of ionic compounds is commonly called a crystal lattice. The interactions of the ions within a crystal lattice lead directly to many important physical and chemical properties of ionic compounds.

  • How is a crystal lattice formed?
  • Why is a crystal lattice the most stable structural form for an ionic compound?
  • What physical properties do ionic compounds have in common as a result of their structure?

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