L. HELLEMANS AND C. JONCKHEERE
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Kinetics of the Chlorination of Mercuric Chloride in Acetone at a Solid Surface
by L. Hellemans and C. Jonckheere Departamendo de Qutmica, Universidad del Valle, Cali, Colombia, South America
(Received November 88,1967)
Although potassium chloride is insoluble in acetone, it will dissolve in a mercury(I1) chloride solution through formation of charged complexes. The rate of this heterogeneous reaction has been followed conductometrically at 25”. An attempt was made to calculate the area for sifted fractions of the potassium salt. It has been shown that in the concentration range up to 2 X M HgC12, the only product is potassium trichloromercurate, formed by means of an adsorption process. The adsorption constant K is (2.1 & 0.4) x 102 1. mol-’, and the product with the rate constant of complexation kzK is (6.19 A 0.30) X 10-2 min-1 cm-2.
Introduction The velocity of the reactions between charged species in water is very high and requires special measuring techniques. I n less polar solvents, such as acetone, inorganic substances are often incompletely dissociated and undergo slower reactions of the type moleculemolecule or ion-molecule. It was found that KC1 did not precipitate quantitatively when HgC12 was added to a KI-acetone solution. However, KC1 is almost insoluble in this solvent: the solubility is 0.92 X M at 1 8 O . l On the other hand, solutions up to 58.5% in weight of HgClz in acetone can be obtained a t the same temperature.l This substance practically does not dissociate because of its covalent character. The conductivity of the solution is consequently very low. When solid KCI is now mixed with an HgC12 solution, it will be noted that the conductivity of the liquid increases considerably and that KC1 dissolves. These facts can be understood if the formation of charged complexes is considered. It has been proven by several methods that chloromercurate complexes arise in the system HgC12-IXClHzOa4 and their stability constants have been determined.6-7 Strocchi found that the solubility of mercury(I1) halides in water is markedly increased upon addition of the corresponding potassium halide, while the conductivity of the alkali halide solution drops when the corresponding mercury(I1) halide is added.8 I n acetone solution the reverse takes place. This paper will attempt to give a kinetic description of the dissolution of KCI in an HgClz-acetone mixture at 25’.
Experimental Section Reagent grade acetone from Matheson was used without further purification* and I t s measured conductivity was about 2 X ohm-’ The Journal of Phvsical Chemistry
cm‘l a t 25’. This is to be compared with the lowest of the International Critical Tables values9 of 6 X lo-* ohm-’ cm-’. The density of all acetone solutions was taken to be 0.7855 g cm4. Some runs were duplicated, where water was added purposely to the acetone up to 0.5% in volume, but no significant differences were noted. For HgC12,Carlo Erba reagent grade material of 99.9% purity was used without purification. It was dried for 4-5 hr at 45” until weight constancy was obtained. KC1 was also Carlo Erba reagent grade with maximum amounts of impurities: Br, 0.01; I, 0.002; and Na, 0.02%. It had been recrystallized in water twice and dried a t 110” until the weight was constant. The density was taken as 1.98 g ~ m - ~ Fractions . of powder with particle size within a known range were obtained by passing the pulverized KC1 through Tyler sieves. The weight distribution over the different fractions was fairly constant in distinct grinding operations. The samples were dried again after sifting. Some of their characteristics can be found in Table I. Also, mixtures with grain sizes in the range of 0-105 p were prepared. Apparatus. A Pyrex conductivity cell with platinumblack-coated electrodes was used. The cell constant was found to be 1.373 & 0,001 cm-’. A screw-shaped glass bar could be introduced as a stirrer with a variable (1) H. Stephen and T. Stephen, Ed., “Solubilities of Inorganic and Organic Compounds,” Vol. E, The Macmillan Go., Inc., New Work, N. Y., 1963. (2) T. Moeller, “Inorganic Chemistry,” John Wiley and Sons, Inc., New York, N. Y., 1952, p 865. (3) A. K. Dey, Proc. Natt. Acad. Sci., India, A16, 61 (1947); Chem.
Abstr., 45, 9414e (1951). (4) R. C . Aggarwal, 2. Anorg. Allgem. Chem., 290, 352 (1957). (6) B. Lindgren, A. Johnson, and L. G. S i l k , Acta Chem. Scand., 1, 479 (1947). (6) K. Damm and A. Weiss, 2.Naturforsch., B10, 534 (1955). (7) Y . Marcus, Acta Chem. Scand., 11, 699 (1957). (8) P. M. Strocchi, Gazz. Chim. Ital., 79, 41 (1949). Book (9) e t ~ ~ t ~ Critical ~ ~ Tables,” ~ t i Val, ~ ~VI,~ McGraw-Hill l Co., Inc., New YoYk, N. Y . , 1929, p 143.
CHLORINATION OF MERCURIC CHLORIDE IN ACETONEAT -~
~~
~~~
Table I : Dimensions of Sifted Fractions of KCl A. Diameter Variation (Ad), approximately 50 p Powder no.
1 2 3 4 5
Grain size, P
0-62 62-105 105-149 149-210 210-250
P
P
P -1
Surface to weight ratio, om2 mg -1
62 43 44 61 40
31.0 83.5 127 179 230
32.3 12.0 7.87 5.57 4.35
0.976 0.362 0.238 0.168 0.132
Av size,
Ad,
Reciprocal of av, 10-3 X
B. Diameter Variation (Ad), 105 p Powder no.
6 7
-Composition, No. 1
66.7 41.1
No. 2
Reciprocal of ay, 10-8 X p - 1
Surface to weight ratio, cmzrng-1
33.3 58.9
25.5 20.3
0.771 0.615
%--
velocity. It closed almost completely the narrow neck of' the cell, so that evaporation of the acetone could be neglected. The conductivity was measured continuously by a 60-cycle Leeds and Northrup conductivity monitor nlodel No. 4958 in the range of 1/R values of 0-100 pohm-l. The signal (maximum, 10 mV) was attenuated by a potentiometer and transferred to a graphical recorder (maximum, 2 mV). The chart paper had variable rates: 2.54, 5.07, or 10.13 cm min-'. The response of the whole apparatus was linear to within 1% and was much faster than any variation produced by the reaction. The readings of final conductivities and calibration measurements were made on an Industrial Instruments Inc. RC-18 conductivity bridge operating at 1 kc. A precision of 0.5 ,&ohm-' could be obtained. The measuring leads were blinded cables. Potassium Content. The quantitative analysis of potassium was made on a Process and Instruments Corp. flame spectrophotometer with direct reading for samples prepared as follows: 0.1 ml of an aqueous solution in the range of 0-10-2 M K + and 0.5 ml of a 0.6 M Li+ solution, further diluted with distilled water to a total volume of 10.0 ml. Calibration solutions were prepared by dissolving equimolar amounts of HgC12 and KC1 in water. Procedure. ]Before each run the conductivity cell was carefully cleaned and dried. Then the HgC12-acetone solution was introduced to half-height. Both cell and solution were kept at 25.0 f 0.1" in a water bath. The KCl powder was weighed and transferred into the partially filled cell in order to obtain a good dispersion of the solid. The cell was immediately filled with the solution to standard height; the total volume was 41.0 f. 0.5 ml. The reaction proceeds slowly without stirring. The conductivity of the heterogeneous mixture has been taken equal to zero a t zero
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reaction time, neglecting the very poor conductivity of the HgCla-acetone solution (at the most 2 X 10-6 ohm-I cm-l) and any contribution from dissolved KCl. If necessary, the curve was extrapolated back until it crossed the zero conductivity line. Then the stirrer was set on. The conductivity changes were recorded, and once the reaction was completed, the precise final 1/R value was taken. The presence of the stirrer lowered this value by about 201,. Finally, the ordinate on the chart paper could be given numerical 1/R values using these results.
Results and Discussion 1. Elimination of Diffusion Effects. The diffusion of reagents toward the solid surface and of products toward the bulk of the solution is the velocity-controlling step, when the mixture is not stirred. Therefore, the reaction velocity was followed as a function of the transformer voltage, which supplied energy to the stirrer. Half-life times (based on 1/R readings) were compared with the half-lives obtained a t 70 V for runs at well-determined experimental conditions. Throughout this work the stirrer's energy input was kept at 65 V. Figure 1 shows that experimentation a t this voltage and temperature will be free from limiting diffusion effects. 2. Calibration Curve of Conductivity vs. Product Concentration. To find a relation between product concentration and conductivity, the final 1/R value (with stirrer present) for a number of reactions was measured and plotted against the initial concentration of HgClzin Figure 2. KC1 was always present in excess. Some intermediate values were obtained by diluting the 0.939 X M HgClz mixture after completion of the reaction. I n order to decide on the nature of the products, several liquid samples of roughly 20 ml were collected from distinct reaction mixtures, at the end as well as during the course of the reaction, The acetone of the solutions was slowly vaporized at 54" until dry, white, needle-like crystals formed. Then weighed amounts of distilled water were added in order to bring the concentration of the dissolved salt into
I
I
50
I
60 Transformer voltage, V.
'
I
70
Figure 1. Relative half-lives v8. transformer voltage for the reaction of 33.0 mg of KCI powder no. 6 with 0.939 X 10-8 M HgC12-acetone solution at 25'. Volume '79, Number 6 June 1968
L. HELLEMANS AND C. JONCKHEEE
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1
A
t
I
I
I
lEO
140 3.
a c
8 6
100
\ 3
60
20
0.2
0.6 1.0 108 Ci(HgCls), M .
1.4
1.8 2
Figure 2. Conductivity of products (with stirrer present) us. initial concentration of HgClz: 0, final 1/R values; 0, obtained by dilution of the 0.939 X 10-8 M reaction mixture. The cell constant is 1.373 cm-1.
the measuring range of the flame spectrophotometer; these samples were analyzed for potassium. I n Table I1 each K+ concentration thus obtained is compared in the second column with the HgClz concentration read on the curve in Figure 2, according to the conductivity of the collected sample, and in the third column with the apparent HgClz content of the same sample, the solvent now being water instead of acetone.
Table 11: Comparison of K + Content and HgClz Converted According to the Conductivity Calibration Curve
stirrer present, #ohm -1
Converted HgClz concn (from Figure 2 ) , 10-8 M
Apparent HgCla conon (aqueous), 10-8 M
K+ ooncn (from flame speotrum), 10-8 M
7% of con-
25.7 42.0 55.0 61.3 79.8 102 0 110.3 140.0 190.0
0.217 0.364 0.485 0.545 0.725 0.939 1.025 1.322 1.853
0.87 1.39 1.90 2.09 2.55 3.67 4.08 5.19 7.52
0.72 1.11 1.52 2.76 2.54 3.57 3.90 5.15 7.63
25 40 100 60 80 100 100 100 100
Conductivity
U/R)
I
Approx version, %
It can be seen with the aid of Figure 3 that the K + concentration and the concentration of converted HgClz (both determined in aqueous solution) are substantially equal, suggesting that during the course of
4
6
108C(HgClz), M .
Figure 3. Comparison of HgCls converted and K + formed at different percentages of conversion (see Table 11).
the reaction all HgClz is progressively replaced by tl electrolyte KHgCb. The approximate degree of co version at the instant of taking the sample was ca culated from the 1/R value of this sample and tl expected final 1/R value. On the basis of these finc ings, values for the product concentration can be a signed for the conductivity vs. time data. 3. The E$ect of the Area on the Initial Rate of Rea tion. The effect of the area on the rate of reactic was investigated for the different KC1 samples reactir with HgClz-acetone solution of 0.939 X 10-3 M . I
