Standard states for bomb calorimetry

TLDR

Abstract

An examination of the thermodynamics of the conditions existing in bomb calorimetry shows that the heat of combustion per unit mass of substance burned is a function of the mass of sample used, of the initial oxygen pressure, of the amount of water placed in the bomb, and of the volume of the bomb.In order to eliminate the effects of these at present unstandardized variables and to obtain a more generally useful thermal quantity which characterizes the pure chemical reaction for stated conditions, it is suggested that every bomb-calorimetric determination be first corrected (where such correction is significant) so as to give the value of A£/r, the change of ''intrinsic" energy for the pure isother- mal reaction under the pressure condition of 1 normal atmosphere for both re- actants and products.From this value the more generally useful quantity, Q v , the heat of the pure reaction at a constant pressure of 1 atmosphere is readily calculable.An equation for calculating the correction is given and illustrated by examples.The magnitude of the correction varies from a few hundredths of 1 per cent up to several tenths of 1 per cent according to the nature of the substance burned and the conditions prevailing in the bomb during the combustion.It is further recommended that, in approving, for the purpose of standardizing a calorimeter, a particular value for.the heat of combustion (in the bomb) of a standardizing substance, such as benzoic acid, the value approved be accompanied by specification of the oxygen concentration and of the ratios to the bomb volume of (1) the mass of the sample and (2) the mass of water, together with appropriate tolerances.An equation is given for correcting to any desired standard temperature the heat measured in the bomb calorimeter.CONTENTS Page I. Nomenclature 526 1 1 .Introduction 527 III.Calorimetry and the first law of thermodynamics 528 IV.The nature of the bomb process 529 V. Proposed standard states for constant-volume combustion reactions 530 VI.Comparison of the actual bomb process with that defined by the proposed standard states 531 VII.The total energy of combustion defined by the proposed standard states 531 VIII.Definitions of some auxiliary quantities 534 1.The initial system 534 2. The final system 534 IX.Correction for dissolved carbon dioxide 535 X.The energy content of the gases as a function of the pressure 536 XI.Correction for the change in energy content of the gases 537 XII.Calculation of the change in pressure resulting from the combustion 538 XIII.The negligible energy quantities 540 1 .The energy content of the water 540 (a) The change in the energy content of the water vapor 540 (b) The change in the energy content of the liquid water 540 2. Combined energy corrections for water vapor and for dis- solved carbon dioxide 541 525 526 Bureau of Standards Journal of Research [Voi.ioXIII.The negligible energy quantities-Continued. page 3. The energy content of the dissolved oxygen 4. The energy content of the substance (a) The energy of compression of the substance (6) The energy of vaporization of the substance XIV.The total correction for reduction to the standard states 1.The general correction equation 2.An approximate correction equation XV.The magnitude of the correction in relation to the type of sub- stance burned XVI.Computation of the correction 1.General remarks 2. Computation for benzoic acid 3. Computation for a mixture XVII.Corrections for iron wire and for nitrogen XVIII.Reduction of bomb calorimetric data to a common temperature-.XIX.The temperature coefficient of the heat of combustion XX.Standardizing substances XXI.Standard conditions for calorimetric standardizations XXII.The heat of combustion of standard benzoic acid Appendix I. Concentration of saturated water vapor in gases at various pressures Appendix II.Empirical formula of a mixture R Gas constant.S Solubility.s Heat capacity, (s) Solid state.T Absolute centigrade temperature.t Centigrade temperature.t H Standard temperature °C.U Total or intrinsic energy content.washbum} Standard States jor Bomb Calorimetry 527 -AU B Heat evolved during the combustion, per g.f. w. of substance burned.-AUr Decrease in intrinsic energy for the pure chemical reaction under standard conditions.