2.4 Naming Acids
PRE-HEALTH PREP
Naming Binary Acids and Oxyacids
General Chemistry 2.4
Acid nomenclature in general chemistry follows two distinct rule sets. Binary acids—hydrogen plus one other element—use the hydro-[stem]-ic acid pattern (e.g., HCl becomes hydrochloric acid). Oxyacids derive their names from polyatomic oxyanions through the -ate → -ic and -ite → -ous suffix conversions. The hydro- prefix is reserved exclusively for binary acids and must never appear in oxyacid names, a frequently tested distinction on exams. Chlorine, bromine, and iodine each possess 4 oxyanion forms, requiring the additional per- and hypo- prefixes to name compounds such as perchloric acid and hypochlorous acid.
How to Name Binary Acids
Naming acids completes the final piece of the chemical nomenclature puzzle. After learning to name ionic compounds and molecular compounds, you need 2 more rule sets for acid nomenclature: one for binary acids and one for oxyacids (also called oxo acids). That brings the total to four naming systems, and the key challenge is always identifying which type of compound you have so you can apply the correct rules.
The word "binary" means two, just as it did with binary molecular compounds. A binary acid consists of hydrogen plus one other element. Hydrogen is always one of the two elements, though the formula may contain more than one hydrogen atom. While rare exceptions exist—HCN is considered a binary acid despite having three elements—the overwhelming majority follow the two-element pattern.
Example: The Most Famous Binary Acid
HCl is the most well-known acid in chemistry. You will find it in swimming pools, in your stomach, and in virtually every chemistry lab. Its name is hydrochloric acid—and it perfectly illustrates the binary acid naming pattern.
The 3-Part Binary Acid Naming Pattern: hydro- + stem + -ic acid
Every binary acid name follows the same template: start with the prefix hydro-, insert the root of the second element's name, and end with -ic acid. This 3-part pattern is the foundation of binary acid nomenclature—the middle portion simply identifies which element is paired with hydrogen.
Example: Naming Binary Acids Using the 3-Part Pattern
- HCl → hydrochloric acid
- H₂S → hydrosulfuric acid
- H₂Se → hydroselenic acid
In each case, set up "hydro-" and "-ic acid," then fill in the stem for the second element.
Tip: If you can remember HCl as hydrochloric acid, you can always use it as an anchor to reconstruct the binary acid naming pattern for any other compound.
Additional Common Binary Acid: Hydrogen iodide dissolved in water gives hydroiodic acid:
HI → hydroiodic acid
Acid Names vs. Molecular Names
All three examples above are also molecular compounds—they consist entirely of nonmetals. So when do you use the acid name versus the molecular compound name? The distinction comes down to state.
When the compound is dissolved in water (indicated by the "aq" label), you use the acid name. In any other state, you name it as a molecular compound. For instance, HCl(g) is simply hydrogen chloride, while HCl(aq) is hydrochloric acid.
Note: Some instructors emphasize this gas-versus-aqueous distinction heavily, while others treat it casually. Take your cue from your particular professor or textbook, but understand that the acid name is technically reserved for the aqueous form.
How to Name Oxyacids: 2 Essential Suffix Rules
Oxyacids (also called oxo acids) are a bit more involved than binary acids, but oxyacid naming becomes manageable if you already know the names of the polyatomic ions. An oxyacid is simply the acid form of an oxyanion—a polyatomic ion that contains oxygen. If you memorized those ions back in the ionic compounds lesson, you are well positioned for this section.
Example: From Oxyanion to Oxyacid
The sulfite ion is SO₃²⁻ and the sulfate ion is SO₄²⁻. Both carry a 2− charge, which is why two hydrogen ions (each +1) are needed to balance the charge, giving H₂SO₃ and H₂SO₄. For ions with a 1− charge, you need one hydrogen; for a 3− charge, three hydrogens, and so on.
Oxyacid Naming Rule 1: -ate → -ic
When a polyatomic ion ends in -ate, the corresponding oxyacid ends in -ic acid. Starting with the sulfate ion (SO₄²⁻), replace the "-ate" with "-ic" to get sulfuric acid (H₂SO₄). This is the first of the 2 core oxyacid naming rules.
Mnemonic: "I ate something disgusting and said ick." Associate -ate with -ic.
This rule applies universally across acid nomenclature. Whichever polyatomic ion carries the "-ate" ending, its oxyacid will always carry the "-ic" ending instead.
Common Mistake: Do not add "hydro-" to oxyacid names. The "hydro-" prefix is reserved exclusively for binary acids. Notice the difference: hydrosulfuric acid (H₂S, a binary acid with no oxygen) versus sulfuric acid (H₂SO₄, an oxyacid containing oxygen). Exam writers love this distinction as a wrong-answer trap.
Oxyacid Naming Rule 2: -ite → -ous
When a polyatomic ion ends in -ite, the corresponding oxyacid ends in -ous acid. The sulfite ion (SO₃²⁻) becomes sulfurous acid (H₂SO₃). This is the second of the 2 essential oxyacid naming rules.
Mnemonic: "I ate something disgusting and said ick" gives you -ate → -ic. For the other pair: "I got a parking cite-ation, which was ous-trageous"—or any silly association that locks -ite to -ous in your memory.
Nitrogen Oxyacids: A Worked Example
The nitrate ion (NO₃⁻) gives nitric acid (HNO₃), while the nitrite ion (NO₂⁻) gives nitrous acid (HNO₂). Both follow the same 2 suffix rules used in all oxyacid naming.
Common Mistake: Do not try to memorize the number of oxygen atoms and assume "four oxygens always means -ate." With sulfur, the -ate form has four oxygens, but with nitrogen, the -ate form has only three. The oxygen count varies by element—you must know the polyatomic ion names themselves.
| Feature | Binary Acids | Oxyacids |
|---|---|---|
| Composition | H + one other element (no oxygen) | H + oxygen + another element |
| Prefix | hydro- | None (unless per- or hypo-) |
| Suffix | -ic acid | -ic acid (from -ate) or -ous acid (from -ite) |
| Key Example | HCl → hydrochloric acid | H₂SO₄ → sulfuric acid |
The 4 Oxyacids of Chlorine, Bromine, and Iodine
Most elements have at most 2 oxyanions (-ite and -ate), but chlorine, bromine, and iodine each have 4. Naming all 4 oxyacids for these elements requires 2 additional prefixes beyond the basic -ite/-ate system.
The per- and hypo- Prefixes for 4-Oxyacid Elements
For chlorine, the 4 oxyanions are ClO⁻, ClO₂⁻, ClO₃⁻, and ClO₄⁻. The two middle ions use the standard endings: ClO₂⁻ is the chlorite ion and ClO₃⁻ is the chlorate ion.
For the ion with more oxygen than chlorate, think of a sugar-fueled child who is hyperactive. The prefix per- (short for "hyper") indicates one step above -ate. Thus ClO₄⁻ is the perchlorate ion—note it still ends in "-ate."
For the ion with fewer oxygens than chlorite, think of hypoglycemia—"hypo" means low or below. The prefix hypo- indicates one step below -ite. Thus ClO⁻ is the hypochlorite ion (sometimes written as OCl⁻—both notations are equivalent).
Naming the 4 Chlorine Oxyacids Step by Step
The same -ate → -ic and -ite → -ous conversions apply when naming oxyacids from these ions. Any prefixes (per- or hypo-) carry over unchanged—only the suffix changes.
| Oxyanion | Ion Name | Oxyacid Formula | Oxyacid Name |
|---|---|---|---|
| ClO⁻ | Hypochlorite | HClO | Hypochlorous acid |
| ClO₂⁻ | Chlorite | HClO₂ | Chlorous acid |
| ClO₃⁻ | Chlorate | HClO₃ | Chloric acid |
| ClO₄⁻ | Perchlorate | HClO₄ | Perchloric acid |
Pattern Summary: The prefixes (hypo- and per-) stay the same when converting from ion to acid name. Only the suffix changes: -ate → -ic and -ite → -ous.
Bromine and Iodine Oxyacids
Bromine and iodine follow the exact same 4-tier naming pattern as chlorine. Apply the same per- and hypo- prefixes along with the -ate/-ite suffix rules:
- Bromine: hypobromous acid, bromous acid, bromic acid, perbromic acid
- Iodine: hypoiodous acid, iodous acid, iodic acid, periodic acid
Chemistry Trivia: If you write out "periodic acid," it is spelled exactly the same as the word periodic—as in the periodic table. Context determines the meaning!
Key Takeaways
- Binary acids contain hydrogen plus one other element and follow the 3-part pattern hydro- + [stem] + -ic acid (e.g., HCl is hydrochloric acid).
- Acid names apply only when the compound is dissolved in water (aq). In the gas phase, use the molecular compound name (e.g., hydrogen chloride).
- Oxyacids are the acid forms of oxyanions. Use the 2 suffix rules: change -ate to -ic and -ite to -ous, then add "acid."
- The hydro- prefix is reserved exclusively for naming binary acids—never use it for oxyacids.
- Chlorine, bromine, and iodine each have 4 oxyacids, using the additional prefixes per- (more oxygen than -ate) and hypo- (fewer oxygen than -ite).
- Prefixes (per- and hypo-) carry over unchanged from ion to acid name; only the suffix converts (-ate → -ic, -ite → -ous).
- Knowing your polyatomic ion names is essential—oxyacid naming depends directly on them.