HEATSOF FORMATION OF EUROPIUM(II) CHLORIDE A N D EUROPIUM(II) ION
991
The Heats of Formation of Anhydrous Europium(I1) Chloride and of the Aqueous Europium(I1) Ion''
by C. T. Stubblefield, J. L. Rutledge, and R. Phillipdb Department of Chemistry, Prairie View Agricultural and Mechanical College, Prairie View, Tezas (Received October 30, 196'4)
The heats of reaction of metallic Eu and of EuClz(s) in hydrogen-saturated 6.00 A4 HCl were found to be - 141.0 and -21.7 kcal./mole, respectively, by adiabatic calorimetry. The reactions followed virtually oxygen-free paths and were catalyzed by platinum black. Calculations for the heats of formation of EuClz(s) and Eu(II)(aq) yielded the values of - 195.8 and - 120.9 kcal./mole, respectively.
All 14 of the lanthanon nietals can exist in the trivalent state in solution, but only three of them, namely, Sm, Eu, and Yb, have been definitely proved to be capable of possessing the divalent state in solution. Sm(II), Eu(II), and Yb(I1) are easily oxidized to the trivalent state in acid solution according to path A Ln+2
+ H+
=
Ln+3
+ I/zHz
(A)
A second path is also available for the oxidation of EU
(11)2 , 3
Eu+'
+ H+ +
'/40z
=
+
E u + ~ '/zHzO
(B)
Previous attempts to measure the heats of solution and oxidation of EuClz in which the reaction proceeded by only one path, A or B, gave inconsistent result^.^^^ When emphasis was placed on path A, the EuClz was oxidized in hydrogen-saturated 6.00 M HC1 yielding -27 to -36 k(~al./mole. Similarly, the range of results for which the reaction proceeded predominantly by path B in oxygen-saturated 0.015 M HC1 was -51 to -60 kcal.,/niole. Since the oxidation of Eu(I1) by path B liberates iiiore heat and proceeds more rapidly than by path A, the very low hydrogen ion concentratioii in the latter case would favor path B. Likewise, it is to be expected that the oxidation of metallic europium in acid solution is dependent on the availability of oxygen. If oxygen is absent, the oxidation of the metal follows pat'h C Eu
+ 3H+ = E u + +~ 3/~Hz
(C)
Burnet t and Cunninghani4 reported the heat of re-
action of Eu metal in oxygen-saturated 0.1 11 HC1 to be -164.57 kcal./mole a t 298.2'. The reaction followed path D
EU
+ 3H+ +
+
E u + ~ '/zHzO
+ Hz
(D) In this paper the techniques which have been devised to measure the heats of solution of EuClz and Eu metal in 6.00 M HC1 are presented, and the results are tabulated. The calorimetric data thus obtained are combined with other published data to obtain the standard heats of formation of solid EuClz and of the aqueous Eu(I1) ion. '/40z
=
The Adiabatic Calorimeter This 200-nil. solution calorinieter was designed for these experiiiients and was operated near room temperature and at atmospheric pressure. The assembly had many of the features which were described previously. The major differences are given below. Tantaluin Calorimeter: (1) spherical bottom ; (2) four tubes-two to accommodate the thennocouple junctions at the calorimeter end of the main therinel, and the other two to contain the two calibration heaters; (3) a platinized platinum stirring propeller (the (1) (a) This research was supported by a Robert A. Welch Foundation grant; (b) National Science Foundation Graduate Research Participant, 1904. (2) G. R . Machlan, C. T. Stuhblefield, and L. Ej.ring, J . A m . Chem. SOC.,7 7 , 2975 (1955). (3) C . T. Stubblefield and L. Eyring, ibid.,77, 3004 (1955). (4) J. L. Burnett and B. B. Cunninghtim, paper presented at the Fourth Rare Earth Research Conference. Phoenix. Arin.. April 22, 1904.
,Vol~~m 69, e Number 3
Maich 1965
992
deposited platinum catalyzes the oxidation of E u metal or Eu(I1) by hydrogen ions). Copper Adiabatic Shield: (1) spherical bottom; (2) a new type thermocouple support niade of nickelplated copper extending three-eighths of the distance around the circumference of the adiabatic shield, permanently attached to the top, but removable from the niain part of the shield, and having 164 holes (No. 61 drill) to acconiniodate all the junctions of those ends of the two B and S . 36 chroniel-aluniel therinels which serve the shitdd ; (3) a platinum resistance thermometer noninductively wound with high grade, hand-drawn B. and S. 36 wire on a thin strip of mica, adjusted for 25.5 olinis at O', and connected to a Leeds and n'orthrup ;\lode1 G1 JIueller bridge; (4) an automatically controlled heater wound on the outside of the adiabatic shield to supplement the other manually controlled heater; ( 5 ) it similar such arrangement operated independently on the top. Brass Isotherirzal Shield: (1) spherical bottom; (2) a 0.64-cni. 0.d. copper tube welded into the housing of the stirrer niechanisin for attachment of a gas line to control the atniosphere above the contents of the calorimeter. Energy-Input Circuzt: (1) the Leeds and Korthrup Type IC3 potentiometer used to monitor the current through the two calibration heaters, as well as to measure the potential across them; (2) an American Time Products, Inc., Type 2001-2L frequency standard (60 C.P.S. + 1 p.p.in.) to activate the relays in the push-button system so that accurate intervals of 0.2 niin. of calibration heat to the calorimeter could be supplied; (3') a Kepco Model CIC 40-0.811 d.c. power supply to furnish the current to the heaters. Characteristics: (1) 76.53 cal./niin. of heat supplied by the energy-input circuit ; (2) a heat capacity of 48.6 cal./deg.; (3) thermometer sensitivity of loe4 deg.; inin.-'. (4)a thermal leakage modulus of
Preparation and Analysis of Samples Anhydrous EuC12. Eu203, obtained from Research Chemicals arid labeled 99.9% pure, was first ignited in a platinum crucible at 900' for 1 hr. in an electric furnace to rid it of any oxalate which niight have been present. After cooling, the oxide was transferred to a Pyrex crucible and suspended in a chlorinating apparatus which was similar to, but larger than, the one described previously. The oxide was completely chlorinated to anhydrous EuC13 within 24 to 48 hr. by cycling (XI, vapor over it a t 450'. After this period of time, the electric power to the furnace tube was removed, and the cycling of CC1, was continued until the temperature of the sample was low enough to The Journal of Physical Chemistry
C. T. STUBBLEFIELD, J. L. RUTLEDGE, AND R. PHILLIPS
allow some condensation of liquid over the sample. This liquid, together with the dense vapor of CC14, served to protect the bright yellow EuC13 from exposure to the atmosphere during its transfer to a simple hydrogen reduction apparatus which had been filled previously with dry nitrogen gas. The reduction apparatus was inmediately attached to a vacuuni line, and the CCIJ was pumped into a trap which was cooled by liquid nitrogen, care being taken to prevent the saniple from being pulled out of the crucible during the process. In order to eliminate the possibility of carbon formation during the reduction of EuC13, the sample was then heated for about 30 min. at 250' in vacuo to sublime any hexachloroethane which occurred upon pyrolysis of CCL. Next, it was allowed to cool to room temperature, and atmospheric pressure was restored with dry nitrogen. Pure hydrogen was then passed over the EuC13, and the temperature was slowly increased to 450' where reduction to EuC12 was conipleted within 6 to 8 hr. The pure white EuCl2 was then cooled to rooin temperature; the stopcocks were closed, and the reduction apparatus was iminediately taken to the drybox. In an argon atmosphere, the EuC12 was removed from the reduction apparatus, ground to a fine powder in an agate mortar, and stored in a glass-stoppered bottle until ready for use. Qualitatively, the anhydrous EuCh was shown to be free of oxide or oxychloride when a test portion produced a clear yellow solution in water. Subsequently, the solution became turbid white owing to oxidation and hydrolysis. Addition of HCI until acidic resulted in a water-clear solution. Powder X-ray patterns showed the same d-spacings reported in the ASTJI data file. Gravimetric quantitative determinations were made on each preparation for C1 and Eu. A weighed portion of the sample was dissolved in 0.1 M HSO3 and precipitated as AgC1. After all excess Ag was removed from the filtrate with 6.00 II4 HC1, Eu was precipitated with either oxalic or tricarballylic acid,5and the residue was ignited to Eu203 in a platinum crucible. The analyses for three different preparations are given in Table I. The theoretical yields are 68.19% Eu and 31.81% C1. Eu Metal. The metal obtained from Research Cheiiiicals was shipped under oil and labeled 99.9% pure. A thick piece was taken from the stock bottle in the argon atmosphere of the drybox and washed with anhydrous benzene. The yellowish coating on the metal was removed with an abrasive wheel, leaving the characteristic metallic appearance. The ( 5 ) A. K. Gupta and J. E. Powell, paper presented a t the Fourth Rare Earth Research Conference, Phoenix, 4-iz., April 1964.
HEATSOF FORMATION OF EUROPIUM(II) CHLORIDE ASD EmoPrmi(I1) ION
Table I : Analyses of EuC12 Wt.,
g.
44 5 8
5 0
o/c Eu
c/o CI
% tot81
67 82 68 17 68 18
31 33 31 71 31 74
99 15 99 88 99 92
metal was then cut into pieces of suitable size for the calorinieter.
Experimental Methods The procedure for assembling the calorimeter preparatory to carrying out a nieasurenient on EuC12 is presented in detail because of the extreme importance of the eliniination of oxygen froni the system. When it was deeined necessary, the platinum black deposit on the stirring propeller was dissolved in aqua regia, and a fresh coating was restored by electrolysis. It was difficult to determine when best to renew the deposit because soinetinies the older job served to catalyze better the oxidation of Euo or E u + ~by hydrogen ions. The conditions under which the poisoning of the catalyst occurred were never clearly understood; however, the usual precautions concerning deposition and protection of platinum black were observed. Fragile calorimeter bulbs similar in shape to those described by Westruni and Eyring6 were blown from flint glass, and a glass bead was fitted to each tapered neck. The bulbs were cleaned with dichromate solution and thoroughly rinsed with water. Their approximate volunies were measured by buret for later weight corrections. The bulbs were then dried in an oven a t 110' and cooled in a desiccator containing Drierite. Bulbs having volunies of 1-3 nil. were selected for the EuCl2 runs. Each was weighed together with its glass bead and a globule of Apiezon W wax on the Jlettler B6 semimicro balance (0.03 nig./div.). They were taken into the drybox, filled with EuC12, sealed with a hot-wire heater, and finally returned to the same balance and reweighed. The cleaned Eu nietal samples were weighed directly on the Cahn gram electrobalance (precision: 0.05 mg. on 500-nig. range), which was set up in the drybox, and these were then sealed into bulbs having volunies 0.3 to 0.4 nil. A bulb containing the sample to be run was firmly sealed below the platinized stirrer with additional Apiezon W wax. The calorimeter was almost filled with 215 g. of 6.00 M HCI. To increase the solubility of hydrogen, the acid was cooled to 15' in an ice bath
993
while being saturated with hydrogen by nieans of a fritted glass dispersion tube inside a fume hood. This hydrogen had previously been passed through a Deoxo to remove oxygen and also through a gas-washing bottle having a fritted glass disk, containing 6.00 M HCI to saturate it. After 5 niin. the dispersion tube was renioved from the calorimeter, and the calorinieter was loosely bolted to its top, leaving a sniall gap. Dry hydrogen was passed through the gas inlet of the stirrer housing, and ilpiezon Q putty was packed over the opening a t the top of the housing to force the hydrogen streani downward into the space above the acid and out through the gap. At the end of 10 niin. it was assumed that ail the air above the acid had been flushed out. The top was then tightened against the O-ring gasket of the calorinieter, and the putty was removed froni the stirrer housing. The hydrogen then flowed upward through the stirrer niechanisni and prevented the entrance of air to the calorimeter. This flow was not interrupted for the duration of the run. The cooled calorinieter was wiped fairly free of dew; the adiabatic shield was attached and cooled to about 23.5'. After niouriting the isothernial shield, the assembly was taken froni the hood to the isothernial bath where the rest of the connections were niade. A portable exhaust hood was inounted just above the stirrer housing to reniove hydrogen froin the rootii. It required about 1 hr. to evacuate the space between the calorinieter and the isothernial shield owing to the dew accumulated on the cold calorinieter. This time was utilized to warni the adiabatic shield to the desired starting temperature of 24.2 or 24.3'. The rise in temperature of the c*alorinieter, together with the stirring of the acid, caused a further slight flushing of the acid as the solubility of the hydrogen decreased. Throughout the run, the teniperature of the adiabatic shield was kept as near as possible to that of the calorimeter, using both the nianual and the autoniatic heater controls. While observing the natural drift in teniperature of the calorinieter, the precision of control was about but during the two 2-niin. period when c,alibration heat was supplied or during the initial few niinutes of the satiiple reaction, the precision of control was sonietiiiies not better than 0.1". A run consisted of (1) plotting a 20- to 40min. teniperature drift at the initial teniperature, ( 2 ) supplying exactly 2 niin. of calibration heat to the calorinieter by nieans of the heat-input circuit, whkh raised the temperature to about 25', (3) plotting a serond temperature drift for 40 to 100 inin., (4) initiating the ( 6 ) E. F. Westrurn. Jr., and L. Eyring, J . Am. Chem. Soc.. 74, 2045 ( 1952).
Volume 69. Siimher 3
.Marrh 1.986
C. T. STUBBLEFIELD, J. L. RUTLEDGE, A N D R. PHILLIPS
994
reaction by depressing the stirring shaft to break the bulb (the time required for completion of the reaction depended upon the nature and weight of the sample, and also upon the condition of the platinum catalyst on the propeller), (5) plotting a third temperature drift of the calorimeter for about 1 hr., (6) supplying another calibration heat for exactly 2 min. to the calorimeter, and ( 7 ) plotting a final temperature drift. Steps 6 and 7 were optional.
Results The results of five runs to determine the heat of reaction of Xu metal with 6.00 M HCI are shown in Table 11, and those for 11 runs on EuClz in 6.00 M HCl are shown in Table 111. These data were combined with other published thermodynamic quantities to obtain thcb standard heats of formation of EuCh(s) and of aqueous Eu(I1). The proper combinations of the following therrnocheniical equations yielded the desired values
Eu(s)
+ zHC1.yHzO
=
EuCI~.(Z- 3)HCl.yHzO
+
3/2H2(g) (1)
Table 11: Heat of Reaction of Eu Metal in 6.00 M HCI Wt., g.
Time, min.’
0.09620 0.2777 0.2314 0.2523 0,1993 Av.
A H , kcal./moleb
180 -141.1 180 -140.7 30 -141.9 100 -139.9 110 -141.5 -141 . O f 0 . 8 ‘ kcal./mole
Estimated time for completion of reaction. Includes -0.45 kcal./mole for evaporation of water by the liberated hydrogen. < Standard deviation.
Table I11 : Heat of Reaction of EuC12 in 6.00 M HCI Wt., g.
Time, m h a
0 09805 0 79189 0 55963
75 40 30 45 45 20 20 20 15 10 15
2 98691 1 07343 0 63925 0 73342 0 64129 1 48597 0 43040 3 93607 Av.
A H , kcal./moleh
-22 -21 -22 -21 -21 -22 -22 -21 -20 -22 -21
7 4 8 3 0 2
2 4
8 2
2
-- 2 1 . 7 f 0.7
