Чамберс К., Холлидей А.К. Современная неорганическая химия, 1975

346 GROUP V I I - T H F HALOGENS

only commercially available interhalogen compound) and bromine. BrF3. These compounds, which react explosively with water, wood, rubber and other organic material—and even with concrete and asbestos—are used to fluorinate compounds, for example actinides to produce the hexafiuorides (the most important being uranium hexafluoride, UF6) and chlorinated hydrocarbons to produce chlorofluorocarbon lubricating oils. Bromine trifluoride has interesting properties as a polar solvent; it undergoes slight ionisation thus:

2BrF3 =± BrF2+

Polyhalides

The best known polyhalide is the triiodide ion, 1^, found when iodine dissolves in the aqueous solution of the iodide of a large unipositive cation (usually K+ ):

Iodine monochloride, formed when iodine reacts with the iodate(V) ion in the presence of an excess of concentrated hydrochloric acid,

IOJ 4- 2I2 + 6H+ + 5CT -» 5IC1 4- 3H2O

dissolves in the presence of excess chloride:

ici +cr ^ ici;

Other polyhalides, all singly charged, are formed from one halide ion together with other halogen or interhalogen molecules adding on, for example [ClIBr]~, [IC14]~. Many of these ions give salts with the alkali metal cations which, if the metal ion is large (for example the rubidium or caesium ion), can be crystallised from solution. The ion ICl^ is known in the solid acid, HIC14.4H2O, formed by adding iodine trichloride to hydrochloric acid. Many other polyhalide ions are less stable and tend to dissociate into the halide and interhalogen compound.

USE OF HALOGENS AND THEIR COMPOUNDS

FLUORINE

Fluorine in the free state is too reactive to be of a direct practical value, but it may be used to prepare other compounds of fluorine, which are then used as fluorinating agents, for example chlorine

GROUP VII: THE HALOGENS 347

trifluoride, C1F3, cobalt(III) fluoride, CoF3, silver difluoride, AgF2. Hydrofluoric acid is used to etch glass, to remove sand from precision castings, in the manufacture of synthetic cryolite, NaAlF6, and as a preservative for yeast and anatomical specimens. Hydrogen fluoride is a catalyst in the alkylation of butane to give higher hydrocarbons, and in the presence of a catalyst is itself used to prepare fluorocarbons. A wide variety of iluorocarbons are known and used extensively as refrigerants, lubricants and as aerosol propellants. Tetrafluoroethene (tetrafluoroethylene), C2F4, is readily polymerised to give polytetrafluoroethylene, PTFE, a plastic of high thermal stability and one not subject to chemical attack by most reagents which finds considerable use not only in the chemical industry but also in the manufacture of knon-stick' pans and oven ware. Calcium fluoride, and other fluorides, are used as fluxes in making vitreous enamels.

CHLORINE

World production of chlorine in 1965 was 14 million tons and the production has risen steadily each year since. Most of it is now used for chemical processes involving the introduction of chlorine into organic compounds, for example the ehlorination of olefins, manufacture of carbon tetrachloride, ehlorination of paraffins to make grease solvents, and the manufacture of plastics and synthetic rubber. Hydrogen chloride is the by-product of many of these processes. Much goes into use for sterilising water and sewage, and it is used directly or indirectly as a bleaching agent. The use of soluble chlorates(I) is replacing bleaching powder for such purposes as bleaching paper pulp and cotton.

Chlorine is also used in the manufacture of hydrochloric acid, the extraction of titanium, and the removing of tin from old tinplate Cde-tinning').

BROMINE

Bromine is used in the manufacture of many important organic compounds including 1,2-dibromoethane (ethylene dibromide), added to petrol to prevent lead deposition which occurs by decomposition of the "anti-knock'—lead tetraethyl; bromomethane (methyl bromide), a fumigating agent, and several compounds used to reduce flammability of polyester plastics and epoxide resins. Silver(I) bromide is used extensively in the photographic industry

348 GROUP VII: THE HALOGENS

whilst calcium and potassium bromates(V) are used in the malting industry to suppress root formation after germination of barley. Bromine is sometimes used in place of chlorine for sterilising water.

IODINE

Iodine as such finds few uses but a solution in alcohol and water, also containing potassium iodide ('tincture of iodine5) was commonly used as an antiseptic for cuts and wounds, but had rather an irritant action. lodoform (triiodomethane), CHI3, is also an antiseptic, but newer compounds of iodine are now in use. Silver iodide, like silver bromide, is extensively used in the photographic industry.

TESTS FOR HALIDES

TESTS FOR FLUORIDE

Most fluorine-containing compounds can be reduced to the fluoride ion, F~, which can be detected by the tests given below.

1.The action of concentrated sulphuric acid liberates hydrogen fluoride, which attacks glass, forming silicon tetrafluoride; the latter is hydrolysed to "silicic acid' by water, which therefore becomes turbid.

2.Addition of calcium nitrate solution to a fluoride gives a white

precipitate of calcium fluoride, CaF2. If the latter is precipitated slowly, it can be filtered off and weighed to estimate the fluoride. Fluoride can also be determined by the addition of sodium chloride and lead nitrate which precipitate lead chlorofluoride, PbClF. This

is filtered off and weighed.

TESTS FOR CHLORIDE

Most chlorine-containing compounds can be converted to give chloride ions, for example covalent chlorides by hydrolysis,chlorates by reduction. The chloride ion is then tested for thus:

1. Addition of silver nitrate to a solution of a chloride in dilute nitric acid gives a white precipitate of silver chloride, AgCl, soluble in ammonia solution. This test may be used for gravimetric or volumetric estimation of chloride; the silver chloride can be filtered off, dried and weighed, or the chloride titrated with standard silver nitrate using potassium chromate(VI) or fluorescein as indicator.

GROUP VII: THE HALOGENS 349

2. If a chloride is heated with manganese(IV)oxide and concentrated sulphuric acid, chlorine is evolved.

3. If the chloride is heated with sodium or potassium dichromate- (VI) and concentrated sulphuric acid, a red gas, chromium(VI) dichloride dioxide, CrO2Cl2, is evolved; if this is passed into water, a yellow solution of a chromate(VI) is formed.

TESTS FOR BROMIDE

1.Addition of silver nitrate to a solution of a bromide in nitric acid produces a cream-coloured precipitate of silver bromide, soluble in ammonia (but not so readily as silver chloride). The reaction may be used quantitatively, as for a chloride.

2.Addition of concentrated sulphuric acid to a solid bromide produces hydrobromic acid, but also some bromine (brown vapour).

3.Addition of chlorine water to a bromide solution liberates bromine, winch colours the solution brown.

TESTS FOR IODIDE

1.Addition of silver nitrate to a solution of an iodide in dilute nitric acid, yields a yellow precipitate of silver iodide practically insoluble in ammonia.

2.Addition of an oxidising agent to a solution of an iodide (for example concentrated sulphuric acid, hydrogen peroxide, potassium dichromate) yields iodine; the iodine can be recognised by extracting the solution with carbon tetrachloride which gives a purple solution of iodine.

3.Addition of mercury(II) chloride solution to a solution of an iodide gives a scarlet precipitate of mercury(II) iodide, soluble in excess of iodide:

21- 4- HgCl2 -> HgI2i + 2Cr

IiKlication of the presence of a given halide ion can be obtained by the series of tests given in Table 11.4. Confirmatory tests can then be performed.

350 GROUP VII: THE HALOGENS

Table 11.4

PRELIMINARY TESTS FOR HALIDE IONS

QUESTIONS

1. Give a comparative account of the oxo-acids of the halogens from the viewpoint of:

2. lodic acid may be made by oxidising iodine with excess fuming nitric acid according to the equation

I2 + 10HNO3 -> 2HIO3 + 10NO2 + 4H2O

The iodic acid may then be dehydrated by heat, giving iodine pentoxide

2HIO3 -> I2O5 + H2O

are added. The mixture is then heated on a water bath at 385-390 K in a fume cupboard until the reaction seems to be complete. This takes about an hour. The solution is then transferred to an evaporating basin and evaporated to dryness on a steam bath. The iodic acid

GROUP VII:THE HALOGENS 351

is then recrystallised from 50% nitric acid. The iodic acid is then heated at a temperature maintained between 500 K and 550 K in order to dehydrate it to iodine pentoxide.

(a)Indicate which elements change in oxidation number during this set of reactions, and the changes involved.

(b)Why is it necessary to perform the oxidation of iodine in a fume cupboard?

(c)State one observation which would tell you the oxidation of iodine is complete.

(d)Iodine vapourises readily. Explain how loss of iodine from the reaction mixture is prevented in this experiment.

(e)Describe briefly how you would recrystallise iodic acid from 50% nitric acid.

(f)How would you heat iodic acid in such a way as to maintain its temperature between 500 K and 550 K?

(N,A)

3. By considering the trends in the vertical groups of the Periodic Table, deduce possible answers to the following questions concerning the element astatine (At), atomic number 85.

(a)State, giving an equation, how astatine could be prepared from an aqueous solution of potassium astatide K+ At~~.

(b)State what you expect to observe when concentrated sulphuric acid is added to solid potassium astatide.

(c)Name an insoluble astatide, and write its formula.

(d)State, giving a reason, whether ethyl astatide would be more or less reactive than ethyl chloride, when heated with a nucleophilic reagent.

(e)The isotope is0At is formed by the emission of one jSparticle from an unstable nucleus. Give the mass number and the number of neutrons in this parent element.

(f)State two reasons why you are unlikely to perform (or see performed) experiments involving astatine.

(JMB, A)

4. The following table shows the atomic numbers of the elements in Group VII of the Period Table and the melting points of their hydrides.

352GROUP VII: THE HALOGENS

(a)(i) What is the general chemical formula of the hydrides?

(ii)What is the type of chemical bonding encountered in the pure hydrides?

(b)Refer to the data in the above table and explain briefly

(i)the increase in melting point of the hydrides along the series chlorine, bromine and iodine,

(ii)the relatively high melting point of the hydride of fluorine.

(c)Give balanced ionic equations describing the reaction(s) between concentrated sulphuric acid and

(i)solid sodium chloride,

(ii)solid sodium iodide.

(L,A)

5. Comment on the following:

(a)The electron affinities of fluorine and chlorine are —333 and

—364kJmol"1 respectively; but their standard electrode

potentials are H-2.87 and -f 1.36V respectively.

(b)Iodine forms some electropositive compounds.

(c)In dilute aqueous solution hydrogen fluoride is a weak acid but the acid strength increases with the concentration of hydrogen fluoride.

(d)Elements exhibit their highest oxidation state whencombined with fluorine.

(e)NaF is slightly alkaline in aqueous solution.

354THE NOBLE GASES

2.The increase in melting point and boiling point, and the very narrow liquid range.

3.The large ionisation energies, as expected for atoms with com

plete quantum levels.

4. The small negative electron affinities of helium and neon.

The increases in melting point and boiling point arise because of increased attraction between the/ree atoms; these forces ofattraction are van der Waal's forces (p. 47) and they increase with increase of size. These forces are at their weakest between helium atoms, and helium approaches most closely to the 'ideal gas'; liquid helium has some notable characteristics, for example it expands on cooling and has very high thermal conductivity.

CHEMICAL PROPERTIES (1)

The simple fact that the noble gases exist as separate atoms—a uniqiie property at ordinary temperatures—is sufficient indication of their chemical inactivity. Calculations of the heats of formation of hypothetical noble gas ionic compounds have been made, using methods similar to those described in Chapter 3 for kNaC!3' or %MgCl'; they indicated that, if the noble gases are to form cations X+ , then the anion must have a large electron affinity to "compensate" for the large ionisation energy of X (Table 12.1). The discovery by Bartlett that the compound platinum(VI) fluoride, PtF6, had a sufficiently large electron affinity to unite directly with molecular oxygen O2 (first ionisation energy 1176 kJ mol ) to form the essentially ionic compound O2PtF6 (i.e. O2[PtF6]~), suggested that xenon (1st ionisation energy 1169 kJ moP1) might form a similar compound XePtF6, and this compound was made by direct reaction of xenon with platinum(VI)fluoride.The further chemistry of the noble gases is described later.

OCCURRENCE AND ISOLATION

The most important source of helium is the natural gas from certain petroleum wells in the United States and Canada. This gas may contain as much as 8 % of helium. Because helium has a lower boiling point (Table 12.1)than any other gas, it is readily obtained by cooling natural gas to a temperature at which all the other gases are liquid (77 K); almost pure helium can then be pumped off. The yearly production in this way may be many millions of m3 of gas, but something like 1011 m3 per year is still wasted.