Nomenclature – Inorganic Acids
Acids show up everywhere in chemistry — from the hydrochloric acid in your stomach to the sulfuric acid in car batteries. To avoid confusion, chemists follow a set of naming rules recommended by IUPAC, the same organisation responsible for making chemical names unambiguous and consistent across the world (Talbot, p.172). This lesson focuses on how those rules apply specifically to inorganic acids.
Two main families of acids
Inorganic acids split neatly into two groups, and the naming pattern depends on which group they belong to.
1. Binary acids (no oxygen) These contain hydrogen plus one other non-metal element (often a halogen). The naming pattern is:
hydro- + (stem of element) + -ic acid
- HCl → hydrochloric acid
- HBr → hydrobromic acid
- H₂S → hydrosulfuric acid
Notice the "hydro-" prefix signals there's no oxygen present.
2. Oxoacids (contain oxygen) These are acids built from a non-metal, oxygen, and hydrogen (e.g. H₂SO₄, HNO₃, H₃PO₄). Here, there's no "hydro-" prefix, and the ending depends on the oxidation state of the central non-metal:
- Higher oxidation state → ends in -ic acid
- Lower oxidation state → ends in -ous acid
If a non-metal forms more than two oxoacids, extra prefixes (per- and hypo-) are added:
hypo- ___ -ous < ___ -ous < ___ -ic < per- ___ -ic
(lowest oxidation state) (highest oxidation state)
Worked Examples
Example 1: Name H₂SO₄ and H₂SO₃.
- Sulfur has oxidation state +6 in H₂SO₄ → sulfuric acid
- Sulfur has oxidation state +4 in H₂SO₃ → sulfurous acid
Example 2: Name HNO₃ and HNO₂.
- Nitrogen +5 in HNO₃ → nitric acid
- Nitrogen +3 in HNO₂ → nitrous acid
Example 3: Name HClO, HClO₂, HClO₃, HClO₄.
- HClO (Cl = +1) → hypochlorous acid
- HClO₂ (Cl = +3) → chlorous acid
- HClO₃ (Cl = +5) → chloric acid
- HClO₄ (Cl = +7) → perchloric acid
Linking acids to their anions
Once you know the acid name, you can predict the name of its conjugate base (the anion left after losing H⁺):
- -ic acid → -ate ion (e.g. sulfuric acid → sulfate, SO₄²⁻)
- -ous acid → -ite ion (e.g. sulfurous acid → sulfite, SO₃²⁻)
- hydro...ic acid → -ide ion (e.g. hydrochloric acid → chloride, Cl⁻)
This connects directly to the salts you name elsewhere in this unit — a nitrate salt, for instance, always traces back to nitric acid, and a nitrite salt back to nitrous acid.
Why bother with systematic rules?
Just like with organic compounds, where IUPAC names encode the structure of a molecule (Talbot, p.289), these rules let any chemist, anywhere, instantly know the composition and relative oxidation state of an acid just from its name — no guessing required (Talbot, p.172).
Key Takeaways
- Binary acids (no oxygen) use the pattern hydro- + stem + -ic acid (e.g. HCl → hydrochloric acid).
- Oxoacids drop "hydro-"; the ending (-ic or -ous) reflects the oxidation state of the central atom, with per- and hypo- used for extra extremes.
- Higher oxidation state = -ic; lower oxidation state = -ous.
- The matching anions follow -ate (from -ic acids), -ite (from -ous acids), and -ide (from hydro-acids).
- Systematic naming (IUPAC) ensures every chemist worldwide names and interprets acids the same way.