Naming acids and bases Knowledge-Based Study

Study notes

1. Overview of Acid and Base Nomenclature

   In chemical nomenclature, acids and bases represent specific classes of compounds with systematic naming rules governed by their structural composition. Recognized in aqueous solutions, traditional acids release hydrogen ions, $\text{H}^+$, while Arrhenius bases release hydroxide ions, $\text{OH}^-$.

   Understanding how to name these substances requires identifying whether the compound is:

   • A binary acid: composed of hydrogen and one other nonmetal element.
   • An oxyacid (oxoacid): composed of hydrogen, oxygen, and a central nonmetal or metalloid atom.
   • An inorganic base: typically an ionic compound composed of a metal cation and hydroxide anions.

   Rules for Binary Acids

   Binary acids consist of hydrogen bonded to a single nonmetallic element in aqueous solution, represented generally as $\text{HX(aq)}$.

   To name a binary acid:

   1. Begin with the prefix hydro-.
   2. Follow with the root name of the nonmetal.
   3. Add the suffix -ic to the nonmetal root.
   4. Conclude with the word acid as a separate word.

   Examples

   • $\text{HCl(aq)}$: hydro + chlor + ic acid = hydrochloric acid
   • $\text{HF(aq)}$: hydro + fluor + ic acid = hydrofluoric acid
   • $\text{H}_2\text{S(aq)}$: hydro + sulfur + ic acid = hydrosulfuric acid
   • $\text{HBr(aq)}$: hydro + brom + ic acid = hydrobromic acid

   Note: In the gas phase without water, these compounds are named as standard covalent compounds, such as hydrogen chloride for $\text{HCl(g)}$.

   Rules for Oxyacids

   Oxyacids contain hydrogen, oxygen, and a central nonmetallic atom, essentially consisting of hydrogen ions combined with polyatomic oxoanions. Unlike binary acids, the prefix hydro- is never used for oxyacids.

   The name of an oxyacid is directly derived from the suffix of its polyatomic anion:

   • If the anion ends in -ate, change the ending to -ic acid.
   • If the anion ends in -ite, change the ending to -ous acid.
   • Any prefixes attached to the anion (such as per- or hypo-) are retained in the acid's name.

   Common Series Examples

   • $\text{HClO}_4$ contains perchlorate $\rightarrow$ perchloric acid
   • $\text{HClO}_3$ contains chlorate $\rightarrow$ chloric acid
   • $\text{HClO}_2$ contains chlorite $\rightarrow$ chlorous acid
   • $\text{HClO}$ contains hypochlorite $\rightarrow$ hypochlorous acid

   For sulfur- and phosphorus-containing oxyacids, extra syllables are restored: sulfate forms sulfuric acid ($\text{H}_2\text{SO}_4$), sulfite forms sulfurous acid ($\text{H}_2\text{SO}_3$), and phosphate forms phosphoric acid ($\text{H}_3\text{PO}_4$).

   Nomenclature of Inorganic Bases

   Most common laboratory bases are ionic compounds formed by a metal cation paired with one or more hydroxide anions ($\text{OH}^-$).

   Because these compounds follow standard ionic nomenclature, they do not require unique acid-style prefixes or suffixes:

   1. State the name of the metal cation first.
   2. If the metal forms cations of variable charge (such as transition metals), indicate its oxidation state using a Roman numeral in parentheses.
   3. Follow with the word hydroxide.

   Examples of Common Bases

   • $\text{NaOH}$: sodium hydroxide
   • $\text{Ca(OH)}_2$: calcium hydroxide
   • $\text{KOH}$: potassium hydroxide
   • $\text{Fe(OH)}_3$: iron(III) hydroxide
   • $\text{Cu(OH)}_2$: copper(II) hydroxide

   Important exception: Ammonia ($\text{NH}_3$) acts as a common molecular base in aqueous solution, producing ammonium and hydroxide ions, but retains its traditional molecular name.

   Systematic Decision Pathway

   Use this reference framework when identifying an unknown formula:

   • Starts with H and contains only one other element?
     Use hydro- + root + -ic acid (e.g., $\text{HI}$ = hydriodic acid).
   • Starts with H and contains a polyatomic ion with oxygen?
     No hydro- prefix. Identify the anion:
     • -ate becomes -ic acid (e.g., $\text{HNO}_3$ from nitrate = nitric acid).
     • -ite becomes -ous acid (e.g., $\text{HNO}_2$ from nitrite = nitrous acid).
   • Contains a metal and hydroxide (OH)?
     Name cation + hydroxide (e.g., $\text{Mg(OH)}_2$ = magnesium hydroxide).