Trends in Chemical Properties of Group 17 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Group 17 Elements and Their Compounds

5 lessons

Lesson 5 of 5

Oxoacids and Oxo-salts of the Halogens, and Their Uses and Hazards

25 min

From bleach to rocket fuel

The bottle of bleach under the sink, the iodised salt on the table, the head of a match and the fuel that lifted space shuttles off the ground all contain a halogen combined with oxygen. In these oxoanions the halogen has a positive oxidation state, from +1 to +7, which is why so many of them are oxidising agents. This lesson surveys the chlorates, iodates, perchlorates and periodates, then sums up the uses and hazards of the halogens and their compounds.

Definition

Oxoacid: an acid containing oxygen, with the acidic hydrogen bonded to oxygen. Example: chloric(I) acid (hypochlorous acid), HOCl.

Systematic names: the oxidation state of the halogen is shown in Roman numerals. Chlorate(I) = $\mathrm{ClO^-}$ (hypochlorite); chlorate(V) = $\mathrm{ClO_3^-}$ ; chlorate(VII) = $\mathrm{ClO_4^-}$ (perchlorate); iodate(V) = $\mathrm{IO_3^-}$ ; iodate(VII) = $\mathrm{IO_4^-}$ (periodate).

The oxoacids of chlorine

Figure 12.7: Oxidation states of chlorine, with examples and uses.

Acid strength rises sharply as oxygen atoms are added, from the weak acid HOCl ( $\mathrm{p}K_a = 7.53$ ) to $\mathrm{HClO_4}$ , one of the strongest acids known. Each extra oxygen pulls electron density away from the O–H bond, making the proton easier to lose, and spreads the negative charge of the anion over more oxygen atoms, making it more stable.

Chlorate(I): bleach

Sodium chlorate(I), NaClO, is made by passing chlorine into cold, dilute sodium hydroxide (Lesson 3): $\mathrm{Cl_2 + 2NaOH \rightarrow NaCl + NaClO + H_2O}$ . It is a strong oxidising agent that kills microbes and destroys coloured compounds. It slowly decomposes, faster when warm or in light, by disproportionation: $\mathrm{3ClO^- \rightarrow 2Cl^- + ClO_3^-}$ . With acids it releases chlorine, $\mathrm{ClO^- + Cl^- + 2H^+ \rightarrow Cl_2 + H_2O}$ , which is why bleach must never be mixed with acidic cleaners.

Chlorate(V)

Potassium chlorate(V), $\mathrm{KClO_3}$ , is made from chlorine and hot potassium hydroxide, or by electrolysing hot brine. When heated with a manganese(IV) oxide catalyst it gives oxygen; heated alone, it first disproportionates to chlorate(VII):

\mathrm{2KClO_3(s) \xrightarrow{\text{MnO}_2,\ \text{heat}} 2KCl(s) + 3O_2(g)} \qquad \mathrm{4KClO_3 \xrightarrow{\text{heat}} 3KClO_4 + KCl}

Mixed with sugar, sulphur or other fuels, chlorates burn violently; they are used in matches and fireworks and are dangerous to store carelessly.

Chlorate(VII): perchlorates

In $\mathrm{ClO_4^-}$ chlorine has its highest oxidation state, +7. Perchlorates are surprisingly unreactive at room temperature, but when heated with a fuel they release their oxygen very fast: ammonium perchlorate mixed with aluminium powder was the solid rocket propellant of the space shuttle boosters.

Iodates and periodates

Potassium iodate(V), $\mathrm{KIO_3}$ , is the iodine compound most commonly added to table salt, because it is more stable than iodide. In acid it oxidises iodide to iodine, $\mathrm{IO_3^- + 5I^- + 6H^+ \rightarrow 3I_2 + 3H_2O}$ , which is used to test iodised salt and to standardise thiosulphate solutions.
Periodates (iodate(VII), such as $\mathrm{NaIO_4}$ ) are powerful oxidising agents: periodate oxidises manganese(II) to purple manganate(VII), a reaction used to measure small amounts of manganese.

Worked example 1: how strong is a bleach?

10.0 cm³ of bleach is diluted to 250 cm³. A 25.0 cm³ portion is added to excess potassium iodide and acid, $\mathrm{ClO^- + 2I^- + 2H^+ \rightarrow Cl^- + I_2 + H_2O}$ , and the iodine formed needs 21.0 cm³ of 0.100 mol dm⁻³ sodium thiosulphate, $\mathrm{2S_2O_3^{2-} + I_2 \rightarrow S_4O_6^{2-} + 2I^-}$ . Calculate the concentration of $\mathrm{ClO^-}$ in the original bleach.

Solution. $n(\mathrm{S_2O_3^{2-}}) = 0.0210 \times 0.100 = 2.10 \times 10^{-3}\ \mathrm{mol} \qquad n(\mathrm{I_2}) = 1.05 \times 10^{-3}\ \mathrm{mol} = n(\mathrm{ClO^-})\text{ in }25.0\ \mathrm{cm^3}$ In 250 cm³: $1.05 \times 10^{-2}$ mol, all from the 10.0 cm³ of bleach: $c = \frac{1.05 \times 10^{-2}}{0.0100} = 1.05\ \mathrm{mol\,dm^{-3}}$

Worked example 2: oxygen from potassium chlorate(V)

2.45 g of potassium chlorate(V) is heated with a little manganese(IV) oxide until no more gas is given off. Calculate the volume of oxygen at room temperature and pressure (24.0 dm³ mol⁻¹), and give the role of the $\mathrm{MnO_2}$ . $M(\mathrm{KClO_3}) = 122.55$ .

Solution. $\mathrm{2KClO_3 \rightarrow 2KCl + 3O_2}$ . $n(\mathrm{KClO_3}) = \frac{2.45}{122.55} = 0.0200\ \mathrm{mol} \qquad n(\mathrm{O_2}) = 0.0200 \times \frac{3}{2} = 0.0300\ \mathrm{mol} \qquad V = 0.0300 \times 24.0 = 0.720\ \mathrm{dm^3}$ $\mathrm{MnO_2}$ is a catalyst: it lowers the decomposition temperature and is unchanged at the end. Chlorine goes from +5 to −1 and oxygen from −2 to 0: an internal redox reaction.

Uses of the halogens and their compounds

Table 12.4: Uses and the properties behind them

Element or compound	Use	Property
Chlorine	disinfecting drinking water and swimming pools; making PVC, solvents and hydrochloric acid	strong oxidising agent that kills microbes; reacts readily with organic compounds
Sodium chlorate(I)	household bleach and disinfectant	oxidises dyes and microbes
Hydrochloric acid	cleaning (pickling) steel; making chemicals	strong acid that dissolves metal oxides
Fluorides (NaF, $\mathrm{SnF_2}$ )	toothpaste; fluoridated water in some countries	fluoride strengthens tooth enamel against acid
Fluorine compounds	PTFE (non-stick coatings); refrigerants	very strong, unreactive C–F bonds
Bromine compounds	flame retardants; silver bromide in photographic film	interfere with burning; light-sensitive silver salt
Iodine and iodine solutions	antiseptic for wounds	oxidising agent that kills microbes
Potassium iodate	iodised salt	stable source of iodine for the thyroid gland
Potassium chlorate and ammonium perchlorate	matches, fireworks, rocket propellant	release oxygen rapidly when heated with a fuel

Hazards

Toxicity. Chlorine is so poisonous that it was used as a weapon in the First World War. All the free halogens attack the eyes and lungs.
Bleach and acid. Mixing bleach with acidic toilet cleaners releases chlorine gas in a small room.
The ozone layer. Chlorofluorocarbons (CFCs), once used in refrigerators and aerosol sprays, release chlorine atoms in the upper atmosphere that destroy ozone. The Montreal Protocol of 1987 phased them out, and the ozone layer is slowly recovering.
Too little or too much. Iodine deficiency causes goitre and is the leading preventable cause of intellectual and developmental disability in the world, which is why salt is iodised. Too much fluoride, on the other hand, can mottle teeth (dental fluorosis).

Warning

Common mistakes. (1) Giving chlorine a negative oxidation state in $\mathrm{ClO_3^-}$ : it is +5. (2) Saying the acid with more oxygen atoms is weaker: $\mathrm{HClO_4}$ is the strongest. (3) Writing $\mathrm{2KClO_3 \rightarrow 2KCl + 3O_2}$ without noting the $\mathrm{MnO_2}$ catalyst (without it, heating first gives $\mathrm{KClO_4}$ ). (4) Confusing iodate ( $\mathrm{IO_3^-}$ ) with iodide ( $\mathrm{I^-}$ ).

Safety. Never mix bleach with acids or ammonia-based cleaners. Chlorates and perchlorates can explode when mixed with fuels such as sugar, sulphur or powdered metals, or when ground; only small amounts are heated, by a teacher, behind a screen. If chlorine is breathed in, move the person to fresh air and get medical help.

Tip

More oxygen, higher oxidation state, stronger acid: HOCl (+1) < $\mathrm{HClO_2}$ (+3) < $\mathrm{HClO_3}$ (+5) < $\mathrm{HClO_4}$ (+7). For hazards remember toxic gases, bleach + acid, CFCs and ozone, iodine deficiency.

Periodic Videos: Chlorine (9 min). Watch the demonstrations of how reactive and dangerous chlorine gas is, and the account of its use as a weapon in the First World War.

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Practice

Give the oxidation state of the halogen in: $\mathrm{ClO^-}$ , $\mathrm{ClO_3^-}$ , $\mathrm{ClO_4^-}$ , $\mathrm{IO_3^-}$ , $\mathrm{IO_4^-}$ .
Arrange HOCl, $\mathrm{HClO_3}$ and $\mathrm{HClO_4}$ in order of increasing acid strength and explain the order.
Write equations for: (a) making sodium chlorate(I); (b) the decomposition of chlorate(I) on warming; (c) bleach reacting with hydrochloric acid.
Write the equation for heating potassium chlorate(V) with manganese(IV) oxide, and calculate the mass of $\mathrm{KClO_3}$ needed to make 1.20 dm³ of oxygen at room temperature and pressure.
Write the ionic equation for iodate reacting with iodide in acid. How many moles of iodine form from 1 mol of iodate?
Iodised salt should contain about 30 mg of iodine per kilogram. What mass of potassium iodate, $\mathrm{KIO_3}$ (M = 214.0), must be added per kilogram of salt? $A_r(\mathrm{I}) = 126.9$ .
Give one use of each: chlorine, a fluoride, silver bromide, ammonium perchlorate. State the property behind each use.
Explain how CFCs damaged the ozone layer and what was done about it.

Show answersHide answers

+1; +5; +7; +5; +7.
HOCl < $\mathrm{HClO_3}$ < $\mathrm{HClO_4}$ : each extra O withdraws electron density from the O–H bond and spreads the anion's charge over more O atoms, so the proton is lost more easily.
(a) $\mathrm{Cl_2 + 2NaOH \rightarrow NaCl + NaClO + H_2O}$ (cold, dilute); (b) $\mathrm{3ClO^- \rightarrow 2Cl^- + ClO_3^-}$ ; (c) $\mathrm{NaClO + 2HCl \rightarrow NaCl + Cl_2 + H_2O}$ .
$\mathrm{2KClO_3 \rightarrow 2KCl + 3O_2}$ . $n(\mathrm{O_2}) = \frac{1.20}{24.0} = 0.0500$ mol; $n(\mathrm{KClO_3}) = 0.0333$ mol; mass $= 0.0333 \times 122.55 = 4.09\ \mathrm{g}$ .
$\mathrm{IO_3^- + 5I^- + 6H^+ \rightarrow 3I_2 + 3H_2O}$ ; 3 mol of $\mathrm{I_2}$ .
Fraction of I in $\mathrm{KIO_3} = \frac{126.9}{214.0} = 0.593$ ; mass of $\mathrm{KIO_3} = \frac{30}{0.593} = 51\ \mathrm{mg}$ per kg.
Chlorine: water treatment (oxidising agent kills microbes). Fluoride: toothpaste (strengthens enamel). Silver bromide: photographic film (light-sensitive). Ammonium perchlorate: rocket propellant (releases oxygen rapidly when heated with fuel).
In the upper atmosphere UV light splits CFCs, releasing chlorine atoms that catalyse the breakdown of ozone; one chlorine atom can destroy many ozone molecules. The Montreal Protocol (1987) phased out CFCs, and the ozone layer is slowly recovering.

Quick check

Which is the strongest acid?

Key points

In oxoanions the halogen has a positive oxidation state: $\mathrm{ClO^-}$ +1, $\mathrm{ClO_2^-}$ +3, $\mathrm{ClO_3^-}$ +5, $\mathrm{ClO_4^-}$ +7; $\mathrm{IO_3^-}$ +5, $\mathrm{IO_4^-}$ +7. Acid strength rises with oxygen count (HOCl weak, $\mathrm{HClO_4}$ very strong).
NaClO (bleach) from $\mathrm{Cl_2}$ + cold NaOH; it disproportionates on warming and releases $\mathrm{Cl_2}$ with acid. $\mathrm{KClO_3}$ gives $\mathrm{O_2}$ with $\mathrm{MnO_2}$ ; perchlorates are rocket oxidisers; $\mathrm{KIO_3}$ iodises salt.
Uses: water treatment, PVC, bleach, fluoride toothpaste, PTFE, AgBr film, iodine antiseptic, iodised salt, matches and propellants.
Hazards: toxic halogens, bleach + acid, CFCs and the ozone layer (Montreal Protocol, 1987), iodine deficiency and excess fluoride.

1From bleach to rocket fuel

2The oxoacids of chlorine

3Chlorate(I): bleach

4Chlorate(V)

5Chlorate(VII): perchlorates

6Iodates and periodates

7Uses of the halogens and their compounds

8Hazards