Das, I.

Dubey, B. L.

Nita

English

NOPR

Indian Journal of ChemistryVol. 21A, February 1982, pp. 118-120--_ .._-----Solid State Reaction between Mercury(I) Dicarboxy lates & Halogens:Part I-Reaction between Mercury(I) Oxalate & IodineB. L. DUBEY*, I. DAS & NITADepartment of Chemistry, Gorakhpur University, GorakhpurReceived 28 March 1981; revised and accepted 14 September 1981Reaction of iodine with solid mercury(I) oxalate has been studied. An yellow coloured intennediate is firstformed which immediately changes into a red coloured final product. The red product has been identified as Hglz.The kinetics of the bulk diffusion of iodine into solid mercury(1) oxalate has been studied, and the mechanism of pro-pagation of the reaction has been established.•THE essential features of solid state reactionsare : the destruction of the crystal lattices ofthe reactants, the breaking and redistributionof chemical bonds, the formation of the' crystallattices of the reaction products, formation of productlayer between the reactants and lastly the diffusionof one component or the other through the productlayer for further propagation of the reaction. Inprevious investigations=! from our laboratoryseveral reactions of the type A(s) + B(s) ---7- C(s)have been studied. The present work relates to anexchange reaction of the type A(s) + B(g) --+C(s) + D(g) involving mercury (I) oxalate (solid)and iodine (vapour). The interesting feature of thisreaction is that iodine causes evolution of carbondioxide from the oxalate at a temperature muchlower than the decomposition temperature (140°C)of mercury(I) oxalate.Materials and MethodsMercury(I) nitrate (S. Merck, GR), oxalic acid(E. Merck, AR), nitric acid (AR), iodine (S. Merck,GR) were used without further purification. Mercury(I) oxalate was prepared by adding oxalic acid solu-tion to mercury (I) nitrate solution.The stoichiometry of the reaction was establishedby a method reported earlier". The mercury(I)oxalate and iodine of particle size> 140 mesh wereused and the experiment was carried out at tempe-rature 48 ± 1°C [mass gain : obs. 84.4 ± 0.6; calc.for HgI2 : 85.7%].Chemical analysis of reaction products : Gaseousproducts - Mercury(I) oxalate was intimately mixedwith iodine in 1:2 molar ratio and the mixture wastaken in a pyrex tube which was connected withanother tube by a standard joint. The system waskept at 60 ± 1°C for 24 hr and the gas collected inempty tube was tested with lime water and foundto be carbon dioxide.Solid products - The qualitative chemical analysisof the solid reaction product showed presence ofHg(II) and complete absence of Hg(I). Mercury wasestimated as HgS3 [Found : Hg, 43.2 ± 0.5, calc.for HgI2 44.1 %].Powder X-ray diffraction pattern of the crystalline118//reaction product was taken with an Xvray diffracto-meter using Cu K~ radiation (wavelength J .5418 A).Kinetic studies - Kinetic studies were performedusing a set-up identical to that used for stoichiometricstudies. Increase in the mass of the tube contain-ing mercury(I) oxalate was noted at different timeintervals. The experiments were performed at 33°,42°,48° and 59° ± 1°C using tubes of equal diameter(0.7 em), Results are shown in Fig. 1.Study of mode of diffusion of iodine - The experi-ment for stoichiometric studies was repeated at tem-perature 31 ± 1"e. The distance between the iodinesurface and mercury(I) oxalate was known before-hand. The mass of iodine diffused in mercury(I)oxalate was determined by measuring the increasein mass of mercury(I) oxalate at different time inter-vals. The experiment was repeated with tubes ofdifferent diameters to investigate the dependence ofthe diffusion coefficient of iodine on the diameterof the tube. Results are plotted in Fig. 2.Results and DiscussionThe results of the stoichiometric studies show thatmercury(I) oxalate and iodine react in 1:2 molarratio.The stoichiometric data and qualitative and quanti-tative analyses of the reaction product indicate thatthe solid reaction product is mercury(II) iodide.This was corroborated by the powder X-ray diffrac-tion pattern of the reaction product also. The dhklvalues of the reaction product were in good agree-ment with the dhk! values for mercury(II) iodide asreported in ASTM file.On the basis of stoichiometric data and charac-terization of the reaction product, reaction may bepostulated to proceed through any of the followingtwo chemical pathways (A and B) :Hg2C204 + 12 ----+ Hg2I2 + 2C02Hg212 + 12 ---7- 2HgI~(A)---7- Hg2C204.I2---7- Hg2C204.212---7- 2HgI2 + 2C02(B)(i)(ii)(i)(ii)(iii)DUBEY et al. : SOLID STATE REACfION BETWEEN Hg(l) SALTS & HALOGENS0.2300t (hr)Fig. 1 - Kinetic data for the reaction between Hg2C20 4 andI. [1, 59°C; 2, 48°C; 3, 4rC; 4, 33°C].•t (~,)Fig. 2 - Kinetic data for the study of diffusion of iodine[1, r = 1.2 cm; 2, r = 0.9 cm; 3, r = 0.7 em; 4. r = 0.45].//It is observed that when iodine comes in contactwith the solid mercury(l) oxalate, an yellow inter-mediate is formed which subsequently changes tored Hg12, the final reaction product. This yellowintermediate may be Hg212 [mechanism A(i)] or anaddition complex [mechanism B]. The Hg-Hgbond in mercury(l) oxalate is weaker than Hg-Obonds. Therefore, when iodine approaches mer-cury(l) oxalate, Hg-Hg bond is likely to break pre-ferentially. Thus, the probability of formation ofHg212 is ruled out and the reaction is likely to takeplace through mechanism B. Since the intermediatecomplex [mechanism B(i) or B(ii)] is very unstableand is immediately converted to the final products,the isolation and identification of the intermediateis not possible. The formation of the addition productcan easily be understood if we take into considerationthe structure of mercury(l) oxalate (I),Hg-Hg/ "-o 0"- /C--CII IIo 0(I)It is evident from structure (I) that the linearity ofO-Hg-Hg bonds (due to the inherent sp hybridisationof mercury atoms) is disturbed considerably. Onaccount of this, the molecule would be highly strainedand thus possess extreme chemical reactivity. Thechemical reactivity is increased due to high polari-sing power of mercury atoms in the molecule. Whenhighly polarisable iodine atoms attack mercury (I)oxalate molecule the mercury atoms extend theircoordination number from two to three and finallyto four giving the unstable addition intermediatewith mercurytl) oxalate and iodine in 1:2 molarratio. The formation of new Hg-I bonds weakensHg-Hg, Hg-O and I-I bonds to such an extent thatthe intermediate complex breaks to give the finalreaction products.To gain an insight into the mode of the propagationof reaction in solid state, kinetic studies were under-taken. Kinetic data at all temperatures (Fig.l) obeyan equation of the type (1),6 w = kt .. (1)where 6 w is the increase in mass of mercury(l)oxalate at time t and k is the rate constant. Thevalue of k was found to increase with temperaturein accordance with the Arrhenius equation,k = A e-EIRT •• (2)From the Arrhenius plot energy of activation fordiffusion was calculated to be 15.2± 0.6 kcal Imol.Since the reaction is diffusion-controlled, it ispertinent to comment on the mode of the diffusionof iodine into the bulk of solid mercury(I) oxalate.The diffusion may be controlled either by surfacemigration or by vapour-phase migration. Since theenergy of activation is very close to the heat of1190.60.5'fu 0.4'"~E....~ 0.3~INDIAN J. CHEM., VOL. 41A, FEBRUARY 19820.20.1~3XOL-__L-__~ __~ __~ __~ __~ __-L__~0.0 0.2 0.4 0.6 0.9 1.0 1.2 1.4 I.--[ (eM)Fig. 3 - Plot of k,fr against r.where r is the radius of the tube. (L and () are givenby the relations: (l = 7TCe Dv/l, and ()= 2 reCeDe.Here I is the distance between iodine surface and themercury(I) oxalate layer, Ce is the equilibrium con-centration of iodine just above the surface, D; isvapour-phase diffusion coefficient and D, is thediffusion coefficient for surface migration. Eq. (4)has been tested by plotting kl/r vs r in Fig 3. Ce,the concentration of iodine just above the iodinesurface was calculated from the vapour pressure ofiodine by using the equation",Blog.; P = A - C' + l'where t' is temperature in °C, P is vapour pressureof iodine in mm, A,B and C' are constants equal to7.26304, 1697.87, 204.00 respectively. The vapourpressure of iodine at the experimental temperaturewas calculated to be 1.091 mm. Values of D; andD, have been evaluated from Fig. 3 and are foundto be 9.76 X 10-4 and 1.12 x 10-5 respectively.These results indicate that although the diffusionof iodine in the solid mercury (l) oxalate is controlledby both vapour phase and surface migration, vapourphase migration plays a predominant role... (4)AcknowledgmentThe authors thank Prof. R. P. Rastogi, Head,Chemistry Department for providing necessaryfacilities and encouragement. Authors thank Dr V.Amirthalingham, BARC, Bombay for facilities torecord powder X-ray diffraction patterns. Thanks arealso due to UGC, New Delhi for financial support.References1. RASTOGI,R. P., J. scient. indo Res.,29 (1970), 177.2. RASTOGI,R. P. & DUBEY, B. L., J. Am. chem. Soc., 89(1967), 200.3. VOGEL,A. I., A text book of quantitative inorganic analysis(ELBS and Longmans Green, London), 1961, 423.4. BAILAR, J. C., Comprehensive inorganic chemistry, Vol. II(Pergamon Press, London), 1973, 1174.5. JOST, W., Diffusion in solids, liquids and gases (AcademicPress Inc., New York), 1960, 8-9.6. LANGE, N. A., Handbook of chemistry, (Handbook Publi-shers Inc., N. Y.), 1956, 1424 & 1431,sublimation of iodine (14.48 kcal/mol)", the modeof diffusion appears to be predominantly controlledby vapour phase; however, diffusion by surfacemigration cannot be ruled out. The relative contri-butions of these can be known from the diffusionexperiment.The data of the diffusion experiment obey Eq. 3,S = kl t .. (3)where S is proportional to the amount of iodinediffused in solid mercury (I) oxalate and is equal tothe increase in mass of mercuryfl) oxalate, t is time.and ki is the rate constant. The value of ki dependson the diameter of the tube according to the following.equation",120/

Solid State Reaction between Mercury(I) Dicarboxylates & Halogens: Part I-Reaction between Mercury(I) Oxalate & Iodine

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Solid State Reaction between Mercury(I) Dicarboxylates & Halogens: Part I-Reaction between Mercury(I) Oxalate & Iodine

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