Determination of Bond Dissociation Energies by Thermal Equilibrium Methods

by knowing the relationship between the equilibrium constant and the temperature we can obtain A//°. the change in heat content at that temperature. If the temperature is sufficiently low, this may not differ much from D, as discussed in Section 2.1. If the heat capacity data are known, A//° may be converted to AHq. This introduction of the experimental data by equation 3.1.5 depends on the second law of thermodynamics. 

from a knowledge of the equilibrium constant at a single temperature, and the first member of the right-hand side of equation 3.1.6, we can obtain or D. The first member of the right-hand side is the function (C known to American authors 

This function may be calculated from molecular data by tlie methods of quantum statistical mechanics. It may also be obtained from experimentally determined heat capacity data, from which the required entropies are deduced using the third law of thermodynamics. The calculation may be put into several equivalent forms, and is discussed in the text-books. An account of the theory is given by Fowler and Guggenheim 7 its application by 

The measurement of the variation of dissociation pressure with temperature is less accurate than that of the quantity itself, and if suitable subsidiary data are available, third law procedures give more accurate results. The most satisfactory situation is, of course, when third and second law treatments give the same answer. Where they differ, third law values are to be preferred. 

CIlearly the equilibrium constant for a dissociation reaction can be obtained directly from the pressure of a given mass of gas, provided the effect of gas imperfection is known, and provided it is certain that the molecule concerned and its primary dissociation products are the only species present. The first condition is fulfilled in almost all work at pressures near to or less than atmospheric, and only for work of the highest accuracy is it necessary to take gas imperfections into account. The second can be checked by the internal consistency of the results, if the equilibrium constant is determined over a range of temperature. 

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DETERMINATION OF BOND DISSOCIATION ENERGIES BY THERMAL EQUILIBRIUM METHODS... 

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