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ELMERJ. HUBER,JR., AND CHARLES E. HOLLEY, JR.
VOl. 60
THE HEAT OF COMBUSTION OF CALCIUM1s2 BY ELMER J. HUBER,JR., BND CEKARLES E. HOLLEY, JR. Contributionfrom the University of California,Los Alamos ScientiJic Laboratory, Los Alamos, New Mexico RecsiOed November 18, 1966
Calorimetric combustions of calcium metal were conducted at an initial temperature of 25' under an oxygen pressure of 50 atm. The energy of combustion under these conditions was found to be 15,806 joules/g. The corresponding standard heat of formation of the oxide, CaO, from the elements is calculated to be -635.09 =k 0.89 kjodes/mole. This value agrees with the presently accepted value.
Introduction.-The data in the literature on the heat of combustion of calcium are very limited. Muthman, Weiss and Metzger' found a value of - 162.64 kcal./mole for the heat of formation of CaO based on combustion measurements. This differs significantly from the value of -151.9 kcal./mole selected by the National Bureau of Standards4 from results of solution calorimetry. Guntz and Benoits were unable t o get consistent results for the heat of combustion of calcium. Since calcium metal of high purity has been prepared in connection with the research program of this Laboratory, it was thought desirable to attempt an accurate measurement of its heat of combustion. The heat of formation of CaO derived from this value serves as an independent check of the value obtained from solution calorimetry. Three independent series of combustion measurements on calcium metal have been carried out at this Laboratory. The first series, the results of which have been reportedj2 used calcium metal which had not been analyzed for oxygen or hydrogen. The resulting value for the heat of formation of CaO, -1150.67 i 0.21 kcal./mole, is believed to be too low on the basis of the present work. I n the second series an attempt was made to analyze the calcium for oxygen using a method based on dissolution of the metal in mercury to effect a separation from an insoluble residue. The residue was analyzed for calcium and, after correction for CaC2 and Ca3N0,the oxygen was determined by difference. This method could probably be developed to give reliable results as no fundamental difficulties appear to be involved. However, after the work had been completed an unexplained trend was discovered in the data6and it was decided to repeat the work. The repetition was desirable also because the calcium used in the second series contained several tenths of a per cent of oxygen. In the third series, calcium of very high purity was available and a recently reported method was used for analyzing for oxygen which is believed to be reliable within the limit of its sensitivity. The results of the third series are given in this paper. (1) This work was performed under the auspices of the A.E.C. (2) Presented in part before the Section on Phyaical and Inorganic Chemistry, 123rd Meeting A.C.S., LOBAngeles. Calif., March 16,
1953. (3) W. Muthman, L. Weins and J. Metxger, Ann.. 866, 137 (1907). (4) Selected Values of Chemical Thermodynamic Properties. N.B.P. Circular 500. 1952, p. 386. (5) A. Guntz and F. Benoit, Ann. Chim.. 10, 5 (1023). (6) We are indebted to Dr. Guy Waddington, Bureau of Mines, Bartlesville, Oklahoma, for pointing out this trend.
Method.-The method involved the determination of the heat evolved from the burning of a weighed sample of the metal in a bomb calorimeter at a known initial pressure of oxygen. The energy equivalent of the calorimeter was determined from the heat of combustion of benzoic acid (NBS Sam le 39g). The completeness of combustion was dehr"fby treating the combustion products with 8 N hydrochloric acid and measuring the amount of hydrogen evolved from any unburned metal present. The uncertainties given are twice the corresponding standard deviations. The units of energy used are the absolute joule and the defined calorie: 1 defined calorie = 4.1840 absolute joules. The unit of mass is the gram mass zn vc". The calcium metal was weighed to the nearest O.OOO1 g. The water for the calorimeter was weighed to the nearest 0.01 g. Apparatus.-The details of the construction and calibration of the calorimeter have been described.' The bomb used in this work is the same as there described. Ita volume is approximately 360 ml. The calorimeter can is new so that a redetermination of the energy equivalent was necessary. The energy equivalent of the calorimeter with oxygen at 25 atm. pressure was 10,006.0 f 3.8 joules/degee, based on 22 runs. When calcium was burned at 25 atm. pressure the amount of unburned metal was quite large (u to 7%) so runs were made at 50 atm. oxygen pressure. Tge energy equivalent was redetermined at this pressure and was found to be 10,009.8 f 3.8 joules/degree for 20 runs. The calculated value from the energy equivalent at 25 atm. and the specific heat of the extra oxygen at 50 atm. is 10,013.9 f 3.8 joules/degree (V,. = 355 ml.). For this work the average value of 10,011.8 f 2.7 joules/degree was used. Calcium Metal.-The calcium metal was m the form of chunks. It was analyzed at this laboratory with the following results: Mg, 0.01%; C, 0.011%; .H! 0.025%; 0, 0.02%; N,
