Nomenclature of Ionic Compounds and Acids

Chapter 6: Nomenclature of Ionic Compounds

Acids

Acids are chemical compounds that dissociate (ionize) in water to produce a hydrogen cation (H+) and an anion. The general dissociation of an acid can be represented as:

• General equation:

$HA \rightarrow H^+_{(aq)} + A^-_{(aq)}$

• Composition: Acids are composed of hydrogen (written first in the formula) and one or more nonmetals (written second).

• Classification: Acids are typically divided into two groups:

  1. Binary acids: Contain only hydrogen and a nonmetal.

  2. Oxyacids: Contain hydrogen, a nonmetal, and oxygen.

Binary Acids

Binary acids consist of hydrogen and a single nonmetal element. Their names follow a specific pattern:

• Examples: HCl (hydrochloric acid), HBr (hydrobromic acid)

• Naming rule: Add the prefix "hydro-" before the root of the nonmetal's name, add the suffix "-ic," and end with "acid."

Formula: hydro + base name of nonmetal + -ic + acid

Oxyacids

Oxyacids contain hydrogen, a nonmetal, and oxygen. Their general formula is:

$H_x(\text{2nd element})O_y$

• x = number of hydrogens (usually 1-3)

• 2nd element: N, S, P, C, Cl, Br, Se, Cr, As

• y = number of oxygens (usually 1-4)

Common oxyacids (ending in "-ic") and their properties:

• Nitric Acid ($HNO_3$): 1 hydrogen, monoprotic

• Chloric Acid ($HClO_3$): 1 hydrogen, monoprotic

• Sulfuric Acid ($H_2SO_4$): 2 hydrogens, diprotic

• Carbonic Acid ($H_2CO_3$): 2 hydrogens, diprotic

• Phosphoric Acid ($H_3PO_4$): 3 hydrogens, triprotic

Note: The charge of the corresponding anion is equal to the number of hydrogens in the acid.

Other Oxyacids and Naming Variations

Oxyacids often have related acids with different numbers of oxygen atoms. The naming depends on the number of oxygens:

• More oxygen than common oxyacid: Add "per-" before the root and use "-ic" suffix. Example: $HClO_4$ (perchloric acid)

• Less oxygen than common oxyacid: Change "-ic" to "-ous." Example: $HClO_2$ (chlorous acid)

• Even less oxygen: Add "hypo-" before the root and use "-ous." Example: $HClO$ (hypochlorous acid)

Examples of related oxyacids:

• $HNO_3$ (nitric acid) → $HNO_2$ (nitrous acid)

• $H_2SO_4$ (sulfuric acid) → $H_2SO_3$ (sulfurous acid)

• $H_3PO_4$ (phosphoric acid) → $H_3PO_3$ (phosphorous acid) → $H_3PO_2$ (hypophosphorous acid)

Oxyacid Families

Oxyacids often have related acids within the same group of the periodic table. For example:

Oxyacid	Related Oxyacid
HClO3 (Chloric Acid)	HBrO3 (Bromic Acid)
H3PO4 (Phosphoric Acid)	H3AsO4 (Arsenic Acid)
H2SO4 (Sulfuric Acid)	H2SeO4 (Selenic Acid)

Polyatomic Anions

Oxyacids react with active metals to form ionic compounds (salts) and hydrogen gas. The general reaction is:

$H_2SO_4 + Cu_{(m)} \rightarrow CuSO_4 + H_2_{(g)}$

• Salt: Composed of a metal cation and a polyatomic anion derived from the acid.

Naming Oxyanions

The name of the oxyanion is based on the oxyacid from which it is derived:

• Acids ending in "-ic": Oxyanion ends in "-ate" Example: $SO_4^{2-}$ (sulfate) from sulfuric acid; $ClO_4^-$ (perchlorate) from perchloric acid

• Acids ending in "-ous": Oxyanion ends in "-ite" Example: $NO_2^-$ (nitrite) from nitrous acid; $PO_2^{3-}$ (hypophosphite) from hypophosphorous acid

• Anion charge: Equal to the number of hydrogens in the acid

Compounds with Oxyanions

Ionic compounds containing oxyanions are named like binary ionic compounds:

• Naming rule: Name the cation first, then the anion.

• Examples:

  • $Na_2SO_4$ — Sodium sulfate

  • $Cu(NO_3)_2$ — Copper(II) nitrate

  • $NH_4ClO_2$ — Ammonium chlorite

  • $FePO_2$ — Iron(III) hypophosphite

Acid Salts

Some salts retain one or more hydrogens from the parent oxyacid. These are called acid salts.

• Examples:

  • $HCO_3^-$ — Hydrogen carbonate (bicarbonate)

  • $HSO_4^-$ — Hydrogen sulfate

  • $H_2PO_4^-$ — Dihydrogen phosphate

  • $HPO_4^{2-}$ — Hydrogen phosphate

• Naming rule: Name the cation, then the acid anion (e.g., sodium hydrogen carbonate)

Common Polyatomic Ions

Below is a table of common polyatomic ions, their formulas, and names:

Formula	Name	Formula	Name
ClO-	Hypochlorite	ClO2-	Chlorite
ClO3-	Chlorate	ClO4-	Perchlorate
CO32-	Carbonate	HCO3-	Hydrogen carbonate (bicarbonate)
SO42-	Sulfate	SO32-	Sulfite
HSO4-	Hydrogen sulfate (bisulfate)	HSO3-	Hydrogen sulfite (bisulfite)
NO3-	Nitrate	NO2-	Nitrite
PO43-	Phosphate	HPO42-	Hydrogen phosphate
H2PO4-	Dihydrogen phosphate	NH4+	Ammonium
CN-	Cyanide	MnO4-	Permanganate
CrO42-	Chromate	Cr2O72-	Dichromate
O22-	Peroxide

Binary Molecular (Covalent) Compounds

Binary molecular compounds are composed of two nonmetal elements and are named using prefixes to indicate the number of each atom:

• Naming rule: Name the first element, then the second element ending in "-ide." Use prefixes (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-) to indicate the number of atoms.

• Examples:

  • $PBr_3$ — Phosphorus tribromide

  • $P_2O_5$ — Phosphorus pentoxide

  • $N_2O_4$ — Dinitrogen tetroxide

  • $SF_6$ — Sulfur hexafluoride

  • $CCl_4$ — Carbon tetrachloride

Note: Prefixes are rarely used for ionic compounds, except for acid salts. Organic hydrocarbons use different naming conventions.

Summary of Naming Practices

• Binary Ionic Compounds: Name cation, then anion (no prefixes).

• Type II Cations: Name cation, indicate charge in Roman numerals, then anion.

• Oxyacids: Name cation, then anion (no prefixes except for acid salts).

• Binary Molecular Compounds: Use number prefixes for both elements, ending second element with "-ide."