Acta Chim. Slov. 1998, 45(2), pp

Transcription

1 Acta Chm. Slov. 1998, 45(2), pp (Receved ) EXCESS MOLAR VOLUMES OF BINARY LIQUID MIXTURES OF CYCLOHEXANE - CARBON TETRACHLORIDE AND TOLUENE - BENZENE AT VARIOUS TEMPERATURES Faculty of Chemstry and Chemcal Engneerng, Unversty of Marbor, Smetanova 17, 2000 Marbor, Slovena Abstract: Excess molar volumes of bnary mxtures of cyclohexane - carbon tetrachlorde at , , K and of toluene - benzene at , , K have been determned usng a vbratng tube densmeter. Flory s theory of lqud mxture has been appled to calculate the excess enthalpy of cyclohexane - carbon tetrachlorde at 298,15 K; the calculated value for the equmolar mxture s n farly good agreement wth expermental results. INTRODUCTION Excess molar volumes of bnary mxtures of cyclohexane - carbon tetrachlorde and toluene - benzene are presented n ths paper. The toluene - benzene system whch forms nearly deal solutons has been used as reference system. The am of our work was to calculate the excess enthalpy from measured excess volumes, usng the statstcal Flory s theory of lqud mxtures [1]. Flory has developed an approach whch relates the excess propertes of a mxture to measurable macroscopc propertes of pure lqud components by a partton functon. Ths theory has been very useful n predctng the thermodynamc propertes of bnary mxtures of nonpolar

2 molecules such as hydrocarbons (normal, branched, cyclc and aromatc) and halocarbons [2,3]. In ths work we use ths approach to the mxture of cyclohexane - carbon tetrachlorde wth a moderate polarty. EXPERIMENTAL Cyclohexane (Kemka, Zagreb), carbon tetrachlorde (Carlo Erba), toluene (Kemka) and benzene (Redel - de Haen), wth p.a. stated purty, were used wthout further purfcaton. The nvestgaton of sources of errors n V E by Lepory et al. [4] showed namely, that purty of substances was not a crucal factor n V E measurements. Denstes were measured, usng a vbratng tube densmeter A.Paar DMA 60/602, at , , and K. Temperature control of the cell was ± C. Before each seres of measurements, the nstrument was calbrated wth doubly dstlled water and dry ar at atmospherc pressure. The densty d, of any lqud relatve to the densty of pure water d w, s gven by d = d w + k ( T 2 - T 2 w ) (1) where k s the characterstc of a partcular oscllator. T 2 and T 2 w are vbraton perods of the tube, flled wth lqud and wth water, respectvely. The determned denstes [5,6] are accurate to at least ± 10-5 g cm -3. For pure cyclohexane, benzene, toluene and carbon tetrachlorde they agreed well wth lterature values [7-10,12-14]. The mxtures were prepared by weght. The values of V E determned from densty measurements are accurate wthn ± cm 3 mol -1. RESULTS AND DISCUSION Excess molar volumes V E of bnary mxtures of cyclohexane (1)- carbon tetrachlorde (2) and toluene (1) - benzene (2) were calculated from the correspondng densty measurements usng the equaton V E (cm 3 mol -1 ) = x 1 M 1 (d -1 m - d -1 1 ) + x 1 M 1 (d -1 m - d -1 2 ) (2) where x s the molar fracton of component, and d m and d are the denstes of the mxture and pure component, respectvely. M are the molar masses of the pure components. The obtaned results are lsted n Table 1 and Table 2.

3 Table 1: Denstes and excess molar volumes V E of bnary mxtures of toluene(1) - benzene(2) at , and K K K K (g cm -3 ) (cm 3 mol -1 ) (g cm -3 ) (cm 3 mol -1 ) (g cm -3 ) ( cm 3 mol -1 ) Table 2: Denstes and excess molar volumes V E of bnary mxtures of cyclohexane(1) - carbon tetrachlorde(2) at , and K K K K (g cm -3 ) (cm 3 mol -1 ) (g cm -3 ) (cm 3 mol -1 ) (g cm -3 ) ( cm 3 mol -1 )

4 Each set of expermental results as ftted to the Redlch - Kster equaton [11] V E (cm 3 mol -1 ) = x 1 x 2 n = 0 A (1-2 x 2 ) (3) Values of coeffcents A are lsted n Table 3, together wth the standard devaton of the ft, σv E, defned as σv E E = ( Vexp V E calc ) /( N n ) 2 1/ 2, (4) where N s the number of data ponts and n s the number of coeffcents. Table 3: Coeffcents A and standard devaton σv E of equaton (3) cyclohexane-carbon tetrachlorde toluene-benzene T (K) A 0 A 1 A 2 A 3 σv E (cm 3 mol -1 ) The V E of both mxtures are postve throughout. However, they are sgnfcantly smaller for toluene-benzene mxture, whch s nearly deal soluton, than for the cyclohexanecarbon tetrachlorde system. Postve values can be explaned by the predomnance of expanson n volume, caused by the loss of dpolar assocaton and dfference n sze and shape of component molecules, over contracton n volumes, due to the dpole-dpole and dpole-nduced dpole nteractons. Ocon, Tojo, Espada [12] have determned V E of cyclohexane-carbon tetrachlorde at K. Our results for ths mxture at K are compared wth lterature ones n Fg.1; t can be seen, that our values are hgher n a part of the curve (X 2 = ). The reason may be n addtonal purfcaton of chemcals [12].

5 Fg.1: Expermental V E for cyclohexane-carbon tetrachlorde at K ( seres 1); Ocon, Tojo, Espada (seres 3); calculated values based upon equ. 3 (seres 2) Analyss n terms of the Flory theory The nteracton parameter χ 12 n Flory s theory can be calculated from expermental excess volume data and should be used further to obtan H E. Detals were gven by Flory (1965) and Abe, Flory (1965). Symbols n the followng equatons have ther usual meanng and are explaned at the end of the text: V E = ( v ~ ) 0 7/ 3 ( 4/ 3) ( v ~ ) 0 1/ 3 ( T ~ T ~ 0 * * )( X V + X V ), (5) H E * * * * = X P V ( 1/ v ~ 1/ v ~ ) + X P V ( 1/ v ~ 1/ v ~ ) + X V Θ χ / v ~, (6) where v ~ 0 = Φ v ~ + Φ v ~, (7) ~ ~ Φ P T + Φ P T T = Φ Φ Φ Θ v~ 13 / * * * * 1 P1 + 2 P2 1 2 χ12 α T 1= 31 ( + α T ), (8), (9) Φ = 1 Φ = 1 2 * XV 1 1 * * (10) X V + X V The key quantty of the theory, the nterchange energy parameter χ 12, was ftted to equmolar expermental values of V E for cyclohexane-carbon tetrachlorde at K. The resultng χ 12 was used to calculate H E, along wth the pure component parameters

6 (α from Wood, Gray [14] and γ from Holder, Whalley [15] ) as gven n Table 4. Comparson between calculated and expermental (Adcock and McGlashan [16] ) excess enthalpy, and TS E s gven n Table 5. Table 4: The parameters of pure lqud component used n Flory s theory component c-c 6H 12 CCl 4 3 V ( cm / mol) ( K ) α γ ( J/ cm 3 K) ~v ~ T P * ( J / 3 cm ) V * 3 ( cm / mol) T ( K) Φ Θ Table 5: Calculated (Flory s theory) and expermental H E (X=0.5) and TS E (X=0.5) values for the cyclohexane-carbon tetrachlorde system, at K χ 12 H E (J/mol) T S E (J/mol) calc. exp. calc. exp From Table 5 t can be seen, that the theory predcts H E reasonably well, even though the calculated value exceeds the expermental by 12%. The agreement n excess entropy s worse; but t s well known, that Flory s theory can not provde a better conformty for a moderate polarty of molecules.

7 LIST OF SYMBOLS H E P * T * ~ T V E V V * excess molar enthalpy characterstc pressure of pure component characterstc temperature of pure component reduced temperature of component I excess molar volume molar volume of pure component characterstc volume of pure component ~v reduced volume of pure component X 2 mole fracton of component 2 Θ 2 ste fracton of component 2 Φ 2 segment fracton of component 2 γ thermal pressure coeffcent α thermal volume coeffcent χ 12 nteracton parameter!" # $$%! &&"'(! &!) #*## # +,"-../ &! 0 4.)../ &! # & 11 2.!3 ( 4#*! $$%! &! #4!" (! 0* * 5!22!) 56"' "#$% %& (" 5! 7 82!)(.!9 '% %$ ( #

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