Joan Mason
Open Jn \ e r r * ) M~ltonKeynes, MK7 6AA, Great Britain
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The Polyhalide Experiment and the Open University Summer school
The polyhalide experiment is one of those devised for the second-level summer school in chemistry of the Open University in Great Britain. Open University students study on their own a t home after their working day, and their brief attendances at summer school may be their only experience of university residence, tuition, and laboratory work. They may be science teachers, skilled technicians, or new to chemistry, so the experiments designed to provide a grounding in chemical laboratory practice must he onen-ended to canture the interest of students to whom this'is familiar. ~ l possible i connections are made with major principles in the course; and important aims of the summer school are to develop the higher level skills that are needed for the design and interpretation of experiments. The experiment is described here as refined by the students, technicians, and tutors. The indented questions are for discussion in the laboratory and in the tutorial sessions; comments on topics not discussed in the text are appended. Preparatory Experiments
As preliminary to the polyhalide problem the students can run through standard methods by which the halogens, alkali metal halides and halates are prepared and recognized: test-tube reactions, uv-visible spectra of the halogens in CCl*, etc. (In general, halogen excludes fluorine.) (1) Why does the visible absorption band move to longer wavelengths down the group of the halogens?
Then follow 'which turns out what' exoeriments: the aqueous halides in turn are shaken with an aqueous halogen and then with CC14 to extract the halogen. (2) Why does chlorine plus bromide give chloride pius bromine, and so on? (This question can be discussed with related questions raised by the polyhalide experiment below.)
The student is then asked how to distinguish between aqueous chloride and iodate; and to predict and investigate the reaction of iodine with aqueous alkali. The standard tests do not necessarily show which halides or halates are present in mixtures. For these the stu-
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dent uses paper chromatography, and begins by measuring Ri values for solutions of the halides and halates, singly and mixed together. Vogel (I) gives methods for the halides. For the halates we use acetone containing 30 vol % of water to elute (30 min), and as locating agent, a solution of methyl red in 1 M HzSOl (aq), the paper being dried briefly at 70°C. R, values are 0.80 for chlorate, 0.62 for bromate, and 0.21 for iodate, i.e., the order is inverted compared with that for the halides. C1-, I-, and lo3- are well separated. The Polyhalide Experiment
The problem is to make a polyhalide and identify it by simple quantitative tests. All three halogens are present in the preparation and some is washed away in the purification of the product. When the student heats the polyhalide it decomposes to leave a halide which he identifies, but the nature of the halogen driven off is less obvious (not to say misleading). Finally he must explain the reactions and deduce the structure of the product from principles studied in the course. This involves thermodynamic cycles, the Gihhs-Helmholtz equation, donor-acceptor (charge transfer) complexes, ionic equilibria and Le Chatelier's principle, arguments as to oxidation states, valence shell repulsion theory, Fajans' rules, theory of the bonding of heavier elements ( d orbitals versus p-electron delocalization), and the bases of electronegativity arguments. Since a polyhalide behaves as interhalogen or halogen plus halide when it decomposes or reacts, the initial examination of IC1 helps the student in the solution of the problem. Chlorine is passed into a trap containing powdered iodine. with shakinz. ... until the increase in weieht correspends to equimolar amounts. (All apparatus and chemicals must be scrupulously dry.) Reaction is exothermic 1 usually ~ and some yellow crystals of 1 ~ are The student measures the uv-visible spectrum of the product i"CC14.
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(3) Why does iodine react with chlorine? (4) Which halogen spectrum does the ICI spectrum resemble and why?
(5) How can the composition of a sample of IC1 be demonstrated? Here the tutor checks that the student expects an interhaloeen to be hvdrolvzed . . bv alkali to form halide olus halate, the latter to contain the larger halogen. The conditions of the nolvhalide oreoaration should be specified as they may deteimine the nature of the polyhalide. KBr (0.13 mole per mole of iodine taken) is well mixed with the ICI; then chloroform is added to precipitate the polyhalide from ICI and prevent the formation of polyhalates. The yellow polyhalide is filtered at the pump and washed with chloroform. It is stable indefinitely if kept dry. When the polyhalide is heated a brown vapor that looks like bromine is given off, and the residue is easily identified as KC1. By various routes the students discover that alkaline hydrolysis of the polyhalide gives I-, 108-, and C1- onlv. If the brown vaoor is nassed into CCL for snectroscopic identification, mixtures may be obtained hecause of decomposition of the interhalogen. Some students obtained an ICI spectrum from the polyhalide in spectroscooic chloroform. perhaps because this contains 2% ethan o h h i c h s o l v a t e s ~ ~ -l . They concluded that the polyhalide is KICIz, IBr being washed away. Some students confirmed this conclusion by making the same product (mp. 195"C, sealed tuhe) fmm ICI and KCI. With chlorine this gives the orange-red chloriodate KIC4, as does the combination of KC1 and ICk. If, however, KC1 is dissolved in ICI and left to crystallize out below 45"C, dark red or black needles of the unsymmetrical compound KIzCIs are obtained. This decomposes to give KIClz unless kept in a limited volume (2). (6) Why is KIClz formed under our conditions and not a hromide? (I) Why does thermal decomposition of the polyhalide give KC1 and no KI? (8) Why do we use potassium rather than some other cation for the bromide in this experiment? Suggest good cations for making stahle polyhalides. (9) How would you stabilize your polyhalide in aqueous solution? (10) What are the expected shapes of I C L and ICln-? In what sense are these ions Lewis aeid-base adducts? How can the bonding in these ions he described? Some students were provoked by finding that the polyhalide contains iodine and chlorine hut not bromine, and tried to make one containing bromine by treating KBr with a smaller proportion of ICI, or by making IBr and adding it to KBr or CsBr. KIBrz and KIBrCl (which tends to disproportionate to KIClz and KIBrz) are not very stable, hut CsIBrCl was obtained as a stable bright yellow solid, giving CsCl,Iz, and Brz on heating. To investigate the importance of cation size, stable polyhalides and polyhalates were made using NEtrBr. Attemots to make oolvhalides from NaBr failed. A staff ~ ~ tutor remarked that some students 'were getting a whiff of real research.' This imnression was heiehtened bv the unhelpfulness of some o< the standard literature. he Rubber Handbook (3) describes KICh as colorless, mp 60% whereas it is bright yellow and melts at 195°C (sealed tube); the hydrate is orange (mp 43"C, sealed tuhe) (2). Wells' sueeestion in "Structural Inorganic Chemistrv" " 14) , that KIcY;~~ unstable at room temperature is not true. A general account of nolvhalides and oolvhalates is lacking. -. for the recent review Hrtic~es(5) c o k e ;mainly the recent literature, while the groundwork was laid 25-50 years ago (2, 6). The uv-visible absorption of IClz- (aq) is to be 243 a t 343 found in a paper on I & - : IClz- (aq) has ,,c, nm, and 47,000 at 224 nm (7). The uv-visible absorption of solutions in acetonitrile and ethylene dichloride have been reported (8). Polyhalogen acids, made from a halogen or interhalogen plus the aqueous halogen acid, have been known for some time (6) but not much is known ~~
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about them. CsIClz and CsICla (aq) can be made by chlorination of a heated suspension of Iz in CsCl (aq) (9). Other extensions of the experiment included the crystallization of ICI by use of a freezing mixture (the stable a form melts a t 27°C and boils at 97.4"C); and examination of the yellow IC18 or rather IzCls crystals, which give the absorption hands of IC1 and Clz in solution. A natural extension is to include fluorides; Vogel gives information on the paper chromatography ( I ) . Principles Involved in the Polyhalide Experiment; Comments on ln-Text Questions
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1) The halogen visible absorption is due to an n a* transition which decreases in energy as the halogen increases in size, i.e. as the lone pair electrons are held more loosely and the a a* splitting decreases. 2) We can see how it is that the smaller halogen turns out the larger (as aqueous halide ion) by considering the thermodynamic cycle, for which values of the quantities involved are given in the table:
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The overall entropy change is of course small. The electron affinity and bond energy terms almost exactly halance, and we can say that the smaller halogen turns out the larger (as halide ion) because of the larger (more negative) enthalpy of hydration of the smaller ion compared with the larger. 3) The table shows that iodine and chlorine gases combine exothermically to form ICI gas; note the connection with Pauling's electrongativity scale. Reaction of solid iodine with gaseous chlorine to form liquid IC1 involves enthalpies of vaporization which tend to cancel, as do the entropy changes. 4) The Brz and IC1 visible absorption bands are rather similar in energy since the IC1 and Brz molecular orbitals are intermediate between those of iodine and chlorine. 6 ) We can imagine the formation of KIClz in our experiment to involve these steps: (a) KBr(s) + ICI(1) = KCl(s) + IBr(1) and (b) KCl(s) + ICI(1) = KIClz(s). These processes can be broken down as in the cycles below, the data for which are in the table. ~
Step fa)
As before, the electron affinity and bond energy terms roughly balance, as do the enthalpies of evaporation AHz*. Thus the higher lattice energy of the salt containing the smaller halide ensures that C1 in ICI turns out Brfrom KBr(s). Volume 52, Number 4 . April 1975
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Thermodvnamic Data on the Halogens and Halides
Step fbJ
D(X2 or XY)
243
193
151
208
(estimated) 178 (estimated)
AHbyao ( X 3 -25 -4 6 +21 The lattice energy Lo is smaller for KIClz (because of the Lo (KX) -708 -679 -641 larger anion) than for KCI, and this must he more than All quantities refer to a m o l e of the a p e i e s as written, and are measured compensated for by the exothermic combination of ICI i n kJmole-'at29S"1K. and C1- (there is some loss of entropy in the overall proBr- (aq) > CI- (aq) > HzO(l), which are given also by cess). Lo is smaller still for KIBrC1, or KIBrz (and in electronegativity arguments. An experimental determinathese cases the enthalpy of formation of the polyhalide is. tion (12) of the order of acid strength, from formation less favorable than for ICL-). constants of the interhalogen complexes with pyridine and 7) The thermal decomposition of polyhalides to give the methylpyridines, gives the order: IC1 > IBr > BrC1. halide containing the smallest halogen can he understood The central position of the larger (less electronegative) from the two alternative cycles for the decomposition of KIClz
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Entropy changes are similar for corresponding processes in the two cycles. The values of the table give the decomposition to KC1 IC1 as the more exothermic by 85 kJ mole-', the lattice energy terms heing decisive. 8) Polyhalides which are stable as solids at room temperature have the cations K + , Rh+, Cs+, NH4+ or alkylammonium; some lattice energy relationships have been given. Note the connection with Fajan's rules; cations are unsuitable if small and/or highly charged, or if the nuclear shielding is reduced'heca&esome of the electrons are in d orbitals. Although the thermal stability increases with the size of the cation, we use the potassium salt because it is stable enough and cheap; rubidium and cesium are very expensive. The tetraalkylammonium (Me4N+, Et4N+) polyhalides are relatively very stable and not unduly expensive. 9) Polyhalide ions are stabilized in aqueous solution by the addition of the appropriate halide ions, as explained by the ionic equilibria and Le Chatelier's principle. 10) The observed shapes of I C l z (linear, cf. XeFd and IC14 (square planar, cf. XeF4) are as predicted by valence shell repulsion theory. I2CI6 is planar with chlorine hridgine. dissociatine to ICl and Clz in fluid phases; IC13 has notieen observed. The larger (or largest) halogen is always central in the polyhalides as in the interhalogen compounds. The thermodynamics of this have been examined (10) for the aqueous reactions
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Interpolation and extrapolation of the known values of the Gibbs function give values for the hypothetical ions such as I B r C l in which the central atom is not the largest. The isomerization to the ion that is ohserved, e.g., BrICI-, is then seen to involve a large decrease in AG. In these reactions the simple halide ion acts as Lewis base, and the halogen or interhalogen as Lewis acid (11). The A G values give the orders (10) of: acid strength; IC1 >> BrCl > IBr >> Iz > Br2 >> Clz, base strength; I (aq) >> 246
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Journal of Chemical Education
halogen is often rationalized in terms of electronegativities, or of the higher oxidation states heing relatively more stable for the larger elements. These arguments gain suhstance if the nature of the bonding is examined in more detail. The physical evidence on the interhalogens and polyhalides and related compounds (such as the halides of tellurium and xenon) is against earlier suggestions that d orbitals of the central atom are used to a considerable extent, as is the magnitude of the promotion energies that are estimated. The bonding can he well described (13-15) by delocalization of p electrons in multicenter orbitals, leaving a partial positive charge on the central atom if this differs from the ligands. The larger, more polarizable halogen is more suited to this position. The relatively long and weak interhalogen bonds that are observed (5) support this description. Recently MO calculations on this basis have successfully reproduced the mu* o, electronic transitionsof1~-,IBrz,I C l z , Bra-, andBrC12- (15). Sharpe's review (16) contains useful material for discussion of the principles involved in the polyhalide experiment.
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Literature Cited (11 Voeel. A . I . . "A Textbook of Macm and Semimicro Qualitative lnoresnie A n d y & " a t h Ed.. Longmans, Landon, 1961, p 362-3.626. (2) Cornog.J.. and Rauer, E.E..J.Amer. Chrm Soc , 61,2620l19421. (8) "Handbook of Chenlirtry and Physics." 54th Ed.. Chemical Rubber Co.. Ohio.
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,C,.tA . , p" . ~,9" ~
111 Wells. A . F.. "Structural lnoreanie Chemi~trv."3rd Ed.. Oxford University Press, Oxford. 1162.p. 325. I61 Popov. A . I., "Halopen Chemistry,' Vol. 1. (Editor Gutmann. V I , Academic PIPII. New York, 1967, p. 225: "MTP International Review of Science." V o l 3 1Edilor:Gutmann. V.).Butterworths. London, 1972p.53. 16) Chattaway, F. D.. and Hoyie. G . . J Chem Soc., 123. 654 119231; Cmmer. H . W.. andDuncan. D. H . J . Chem Sac.. 1357 (19311: 181 119351. (7) Csnon.D.L..andNeumann. H. M , J Arner. Cham S o r . 83.1822I19611. 181 Popov. A.1.. and Swenaen. R. F..J.Amrr. Chrm S o r . 77.3724 l19551. (91 Popov, A. I.. and Buekler. R. E.. Inore. Sunth., i.167 11957). This contain3 a useful survey of preparative methods for polyhaiides. 1101 S c o t f , R . L . . J Amer C'hem..Soe.. 75.155011363). ( I l l Mulliken.R.S..J.Arnrr. Chpm Sor.12.6WI19501. 1121 Sur1er.T.. ~. . andPomv.. A.I..lnor@. Chem.. 8.2049 11969). 113) Pimentel. G . C . . J . Chrm. Phvr.. 19.446119511 ~
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Prom New York. 1967, p I
