An acid’s name comes from its anion
As lesson 1.8.4 showed, an acid releases H⁺ in water. Think of an acid as H⁺ plus an anion, and its English name follows directly from the anion’s name: how the anion’s name ends decides how the acid is named. So know the anion names from lessons 2.5.1 and 2.5.2 well before you start.
Three rules
| Anion ending | Acid name | Example | Chinese |
|---|---|---|---|
| -ide | hydro-…-ic acid | Cl⁻ chloride → HCl hydrochloric acid | 氢某酸 (HCl is 盐酸) |
| -ate | …-ic acid | SO₄²⁻ sulfate → H₂SO₄ sulfuric acid | 某酸 |
| -ite | …-ous acid | SO₃²⁻ sulfite → H₂SO₃ sulfurous acid | 亚某酸 |
- Memory aid: -ate goes with -ic (“I ate something icky”), and -ite goes with -ous (“a bite of something delicious”).
- Prefixes carry over: per- and hypo- stay in the acid’s name: ClO₄⁻ perchlorate → HClO₄ perchloric acid; ClO⁻ hypochlorite → HClO hypochlorous acid.
- Root changes: sulfur acids use the root sulfur- (sulfuric, sulfurous), and phosphorus acids use phosphor- (phosphoric).
- Formulas: an anion with a charge of n− pairs with n H⁺ ions: HNO₃, H₂SO₄, H₃PO₄.
- Acid names are for solutions: pure HCl is a gas called hydrogen chloride; only dissolved in water is it hydrochloric acid. Likewise H₂S gas is hydrogen sulfide, and its solution is hydrosulfuric acid.
Chinese acid names match the English patterns almost one for one: -ide acids are 氢某酸, -ate acids 某酸, -ite acids 亚某酸, hypo-…-ous acids 次某酸, and per-…-ic acids 高某酸.
Choose an anion to see which rule it follows; the practice below goes from formula to name or from name to formula:
Choose an anion:
Chinese: 氢…酸
Chinese: …酸
Chinese: 亚…酸
Practice:
Example 1: From formulas to names
Name H₂SO₃ and HClO₄.
- Find the anion first. Remove all the H⁺ from an oxyacid to get its anion: H₂SO₃ minus 2 H⁺ leaves SO₃²⁻, sulfite; HClO₄ minus 1 H⁺ leaves ClO₄⁻, perchlorate.
- Change the ending. The rule: -ate → -ic acid, -ite → -ous acid, and prefixes like per- and hypo- stay.
- H₂SO₃. Sulfite ends in -ite, which becomes -ous, with the root sulfur-: sulfurous acid (亚硫酸).
- HClO₄. Perchlorate ends in -ate, which becomes -ic, and per- stays: perchloric acid (高氯酸).
- A common mistake. Adding hydro- to an oxyacid (“hydrosulfurous acid”). Hydro- is only for acids without oxygen, from -ide anions; HCl is hydrochloric acid.
Example 2: From names to formulas
Write formulas for hydrobromic acid, chlorous acid, and phosphoric acid.
-
Hydrobromic acid. The hydro- means an oxygen-free -ide acid, so the anion is bromide, Br⁻. The number of H⁺ equals the anion’s charge, here 1: HBr.
-
Chlorous acid. The -ous comes from -ite, so the anion is chlorite, ClO₂⁻, with 1 H⁺: HClO₂.
-
Phosphoric acid. The -ic comes from -ate, so the anion is phosphate, PO₄³⁻; its 3− charge needs 3 H⁺:
- chloride
-ide → hydro-…-ic: hydrochloric acid, HCl.
- nitrate
-ate → -ic: nitric acid, HNO₃.
- sulfite
-ite → -ous: sulfurous acid, H₂SO₃.
- sulfide
-ide → hydro-…-ic: hydrosulfuric acid, H₂S in water.
- carbonate
-ate → -ic: carbonic acid, H₂CO₃.
- hypochlorite
-ite → -ous, keeping hypo-: hypochlorous acid, HClO.
- perchlorate
-ate → -ic, keeping per-: perchloric acid, HClO₄.
- nitrite
-ite → -ous: nitrous acid, HNO₂.
- cyanide
Cyanide is polyatomic but ends in -ide: hydrocyanic acid, HCN.
Common mistakes
Mistake 1: “Acids contain hydrogen, so every acid name starts with hydro-.” Only acids from -ide anions take hydro-. H₂SO₄ is sulfuric acid; hydrosulfuric acid is H₂S dissolved in water.
Mistake 2: “-ic and -ous are interchangeable.” They come from different anions: sulfuric acid (H₂SO₄) comes from sulfate and sulfurous acid (H₂SO₃) from sulfite. They are two different acids.
Mistake 3: “HCl gas is hydrochloric acid.” Acid names apply only to solutions. The gas is hydrogen chloride, HCl(g); its solution, HCl(aq), is hydrochloric acid.
Quick check
What is the name of HNO₂?
The anion in HNO₂ is NO₂⁻, nitrite, and -ite becomes -ous: nitrous acid. Nitric acid is HNO₃.
What is the formula of hypochlorous acid?
The -ous comes from -ite, and hypo- carries over, so the anion is hypochlorite, ClO⁻, with 1 H⁺: HClO.
Summary
- -ide anion → hydro-…-ic acid; -ate → -ic acid; -ite → -ous acid. Per- and hypo- carry over to the acid’s name.
- An anion with an n− charge gives an acid with n H atoms: HNO₃, H₂SO₄, H₃PO₄.
- Acid names apply only to solutions: HCl(g) is hydrogen chloride, and HCl(aq) is hydrochloric acid.
On to AP (for later)
In Unit 8, AP expects you to know the common strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄. When a question gives only names, you need to write at once that hydrobromic acid is HBr and perchloric acid is HClO₄. These rules save you from memorizing every acid name separately.