NAMING INORGANIC ACIDS

6.4. NAMING INORGANIC ACIDS

The anions described in the preceding sections may be formed by reaction of the corresponding acids with hydroxides:

HCl + NaOH −→ NaCl + H 2 O

H 2 SO 4 + 2 NaOH −→ Na 2 SO 4 + 2H 2 O

H 3 PO 4 + 3 NaOH −→ Na 3 PO 4 + 3H 2 O

The salts formed by these reactions consist of cations and anions. The cation in each of these cases is Na + , and the anions are Cl − , SO 2− , and PO 3− 4 4 , respectively. In these examples, the chloride ion, sulfate ion, and phosphate ion are formed from their parent acids. Thus the acids and anions are related, and so are their names.

When they are pure, acids are not ionic. When we put them into water solution, seven become fully ionized: HCl, HClO 3 , HClO 4 , HBr, HI, HNO 3 , and H 2 SO 4 . They are called strong acids. All other acids ionize at least to some extent and are called weak acids. Strong acids react completely with water to form ions, and weak acids react to some extent to form ions; but both types react completely with hydroxides to form ions. Formulas for acids conventionally are written with the hydrogen atoms which can ionize placed first. Different names for some acids are given when the compound is pure and when it is dissolved in water. For example, HCl is called hydrogen chloride when it is in the gas phase, but in water it ionizes to give hydrogen ions and chloride ions and is called hydrochloric acid. The names for all the acids corresponding to the anions in Table 6-5 can be deduced by the following simple rule. Note that the number of hydrogen atoms in the acid is the same as the number of negative charges on the anion.

Replace the -ate ending of an anion with -ic acid or replace the -ite ending with -ous acid. This rule does not change if the anion has a prefix per- or hypo-; if the anion has such a prefix, so does the

acid. If not, the acid does not either.

If the anion ends in -ide, add the prefix hydro- and change the ending to -ic acid.

Ion Ending

Acid Name Components

-ate

-ic acid

-ite

-ous acid

-ide

hydro—ic acid

EXAMPLE 6.15. Name the following as acids: (a) HBr, (b) HNO 2 , (c) H 3 PO 4 , (d) HClO, and (e) HClO 4 . Ans.

(a) HBr is related to Br − , the bromide ion. The -ide ending is changed to -ic acid and the prefix hydro- is added. The name is hydrobromic acid.

(b) HNO 2 − is related to NO 2 , the nitrite ion. The -ite ending is changed to -ous acid. The name is nitrous acid. (c) H 3 PO 4 is related to PO 4 3− , the phosphate ion. The -ate ending is changed to -ic acid, so the name is phosphoric

acid.

94 INORGANIC NOMENCLATURE

[ CHAP. 6

(d) HClO is related to ClO − , the hypochlorite ion. The prefix hypo- is not changed, but the -ite ending is changed to -ous acid. The name is hypochlorous acid.

(e) HClO 4 − is related to ClO 4 , the perchlorate ion. The prefix per- is not changed, but the ending is changed to -ic acid. The name is perchloric acid.

EXAMPLE 6.16. Write formulas for the following acids: (a) nitric acid, (b) chloric acid, (c) hypophosphorous acid, and (d) perbromic acid.

Ans. (a) Nitric acid is related to the nitrate ion, NO − 3 . The acid has one hydrogen ion, corresponding to the one negative

charge on the nitrate ion. The formula is HNO 3 . (b) The acid is related to the chlorate ion, ClO − 3 . The acid has one hydrogen atom, because the ion has one

negative charge. The formula is HClO 3 .

(c) The acid is related to hypophosphite, PO 2 3− . The acid has three hydrogen atoms, corresponding to the three

negative charges on the ion. Its formula is H 3 PO 2 .

(d) HBrO 4 .