Chemistry · Ch 7 — Elements of Groups 16, 17 and 18

Oxoacids of halogens

7.8.2

Oxoacids of halogens

Note

The book prints this heading with a stray double colon — "7.8.2 Oxoacids of halogens : :".

Halogens form several oxoacids (see Table 7.8). Only four oxoacids have been isolated in pure form: hypofluorous acid (HOF), perchloric acid ( $\mathrm{HClO_4}$ ), iodic acid ( $\mathrm{HIO_3}$ ) and metaperiodic acid — printed in the book with the formula " $\mathrm{H_2IO_6}$ ". The others are stable only in aqueous solutions or in the form of their salts.

Note

The printed formula " $\mathrm{H_2IO_6}$ " for metaperiodic acid is the book's own misprint: metaperiodic acid is $\mathrm{HIO_4}$ (the related orthoperiodic acid is $\mathrm{H_5IO_6}$ ). Neither corresponds to $\mathrm{H_2IO_6}$ .

The acid strength of the halogen oxoacids increases with the increasing oxidation state of the halogen. For example, acid strength increases from HClO, a weak acid ( $K_a = 3.5 \times 10^{-8}$ ), to $\mathrm{HClO_4}$ , a very strong acid ( $K_a \gg 1$ ).

Table 7.8 Oxoacids of halogens

oxidation state of X	Generic name	Oxoacids of fluorine	Oxoacids of chlorine	Oxoacids of bromine	Oxoacids of iodine
+1	Hypohalous acid (HXO)	HOF	HOCl	HOBr	HOI
+3	Halous acid ( $\mathrm{HXO_2}$ )	−	HOClO	−	−
+5	Halic acid ( $\mathrm{HXO_3}$ )	−	$\mathrm{HOClO_2}$	$\mathrm{HOBrO_2}$	$\mathrm{HOIO_2}$
+7	Perhalic acid ( $\mathrm{HXO_4}$ )	−	$\mathrm{HOClO_3}$	$\mathrm{HOBrO_3}$	$\mathrm{HOIO_3}$

Figure 7.8.2aStructural formula of hypochlorous acid HOCl drawn as a bent H-O-Cl chain.

Fig. 7.8.2a — Structural formula of hypochlorous acid HOCl drawn as a bent H-O-Cl chain.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Hypochlorous acid, HOCl - just H-O-Cl, drawn angular at the oxygen. Chlorine's oxidation state here is +1, the lowest of the four oxoacids. HOCl has never been isolated pure - it exists only in aqueous solution - and of the series it is the weakest acid, consistent with the acid-strength tren …

Figure 7.8.2bStructural formula of chlorous acid HClO2: an H-O-Cl chain with one chlorine-oxygen double bond.

Fig. 7.8.2b — Structural formula of chlorous acid HClO2: an H-O-Cl chain with one chlorine-oxygen double bond.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Chlorous acid, HOClO or $\mathrm{HClO_2}$ - the H-O-Cl chain now carries one Cl=O; chlorine is at +3. Like HOCl it is known only in solution (and as its salts, the chlorites), and the added terminal oxygen already makes it a disti …

Figure 7.8.2cStructural formula of chloric acid HClO3: an H-O-Cl chain with two chlorine-oxygen double bonds.

Fig. 7.8.2c — Structural formula of chloric acid HClO3: an H-O-Cl chain with two chlorine-oxygen double bonds.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Chloric acid, $\mathrm{HClO_3}$ - two Cl=O bonds beside the -OH; chlorine is at +5. Chloric acid too is handled only in aqueous solution; its salts, the chlorates (like the $\mathrm{KClO_3}$ used to prepare oxygen in section 7.9.1) …

Figure 7.8.2dStructural formula of perchloric acid HClO4: an H-O-Cl chain with three chlorine-oxygen double bonds.

Fig. 7.8.2d — Structural formula of perchloric acid HClO4: an H-O-Cl chain with three chlorine-oxygen double bonds.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Perchloric acid, $\mathrm{HClO_4}$ - three Cl=O bonds; chlorine reaches +7. Notice the ladder across the four drawings: each step up in oxidation state adds one more Cl=O, and with it the acid strength climbs from wea …

Start your 14-day free trial to unlock the full solution →

← Previous · 7.8.1Oxoacids of sulfur Next · 7.9 →Oxygen and Compounds of oxygen