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JOHN
P. HUNTAND ROBERTA. PLANE
inole-', respectively, a t 298.16'K. These heats of formations lead to AN&3.1614 = -669 + 45 cal. The close agreement between the calorimetric heat and that calculated from b In Kl/b 1 / T substantiates our conclusion that the free energy difference between UH.3 and UH2D does not appreciably depend on the relative amounts of CY and p forms of UH3. Acknowledgrnent.-II-e thank LIr. A. P. Irsa of this Laboratory for carrying out the mass
[CONTRIBUlION FROM
BAKERLABORATOR\'
Vol. 'TO:
spectrometer analyses. We also wish to thank Dr. B. SI. dbraham and Mr. H. Flotow of the Argonne National Laboratory for sending us the results of their calorimetric measurements prior to publication. In addition it is a pleasure to acknowledge a stimulating conversation with Dr. Abraham on the heats of formation of the isotopic uranium hydrides. UPTON,
L. 1 , s Y
O F CHEMISTRY, C O R K E L L U A I V E R S I T Y ]
The Kinetics of the Exchange of Water between Cr(H,0),+3and Solvent B Y JOHN
P. HUNTAND ROBERT.I.PLANE RECEIVED JULY I , 1954
~ l e a s u r c i ~ i c noft sthe exchange of water between solvent and C I - ( H ~ O )have ~ + ~ been extended to dilute solution. The rate i. best expressed as rate = kl(Cr(Hz0)6+3)over a wide range of ionic strength. kl increases slightly with ionic strength and hiis an average value a t infinite dilution of 2.OT X inin.-' a t 27" in both nitrate and perchlorate solutions. An activation energy of 2T.6 kcal./mole is found a t 0.7 molar ionic strength.
+
The ignition of C r P 0 4 6 H z 0 to CrP04 6H20 was Investigation of the rate of substitution of water for water in aquo ions is of fundamental importance carried out as follows: 0.25 g. in a small Pt boat was placed a horizontal Pt crucible. The latter was inserted in a in understanding substitution rezctions in complex in horizontal Vycor tube which was then attached t o a vacuum ions. ; i start has been made by studying the line. The Vycor tube could be heated by means of a tube mater exchange reaction of Cr(H20)6+3.1,2These furnace. Next, the solid was outgassed for 20 min. by mm. Dry, oxygen-free, Nz was then studies, limited to concentrated solutions, suggested pumping a t admitted t o 1 atm. pressure (this was necessary to prevent the rate law: rate = k2(Cr(H20)6+")(anion). loss of sample from the boat during the initial heating). \ ' a l ~ c s of k? were surprisingly independent of The temperature of the sample was raised t o 160" in 15 ionic environment. The present authors have min. following a time schedule, then held constant a t 160" extended the measurements to more dilute solu- for 20 min. Ca. 4HzO/Cr had been evolved a t this stage collected in a cool portion of the Vycor tube. The NZ tions enabling tnore rigorous testing of rate laws. and and HzO were pumped off through a - 180" trap, collecting Ahhas been pointed out,' comparisons of rates of the HzO. Ignition was continued a t 10-5 mm., raising the water exchange with rates of complex ion forma- temperature to 750' in 30 min. according t o a fixed time tion reactions can give information regarding schedule. This temperature was maintained for 10 min. The final 2HzO/Cr were trapped out continuously during mechanisms and the present data should be more this 40-min. period. Total HzO was transferred t o a sample useful in this connection. tube and equilibrated with COz for mass spectrometer
Experimental Thc general procedure and methods have been described previously.2 H?018again \?-asused as the tracer. The significant innovation in experimental procedure involved sampling by precipitation and ignition of CrPO4. BH2O. The basic process used was as follows. T o 10 ml. of solution a t room temperature and 0.1 M in Cr+3, 0.1 M in H - was added solid ?;nHzPOa,H20 in 1070 excess with vigorous stirring. After solution occurred (ca. 1 min.) solid SaC2HsO: rras added until the PH was 5. The reiulting slurry was stirred 5 min., the lavender solid filtered on it Hirsch funnel, washed 5 times with ordinary distilled ij-ater, and dried 20 min. by sucking air through the filter. The poivdcr was further dried over Cas04 in a vacuum desiccator for 40 min. hlodiiication of the precipitation was necessary when Cr-3 WLS < 0.1 21 or H L > 0.1 JI or total anion concentratioii > 1.0 -11. For sonic samples dilution with ordinary tiistilled water w-as sufficient to obtain the basic conditions. If Cr-8 wis < 0.1 AI, NaJ-IP04 replaced NaH?POd,H20. \\-ith H - > 0.1 d1 or anion concentration > 1.0 M after dilution t o 0.1 AI Cr+3, Sa.HPOI again replaced hTaH2P04. H,O. Chemical analyses of the dry solids showed that they were indeed C r P 0 4 6 H 2 0 . Tracer experiments showed that ca. 17~"induced exchange" occurred during precipitation, the amount of exchange being reproducible with constant conditions of precipitation, varying slightly from run t o run as these conditions changccl. I ) j. 1 2 ) I
