A general discussion of the second and third law methods, including their advantages and limitations relative to first law techniques, was presented in sections 2.9 and 2.10. Now, after a summary of that introduction, we examine some examples that apply the second law method to the thermochemical study of reactions in solution. Recall that the third law method is only practical for reactions in the gas phase. Both the second and third law methods rely on the experimental determination of equilibrium constants. As shown in section 2.9, the equilibrium constant (K) of a reaction is defined in terms of the activities (ai) of reactants and products: where νi are the stoichiometric coefficients of the reaction. In most real situations, the activity values are difficult to obtain, so they are replaced by other quantities. In the case of reactions in solution, if the ideal model is assumed, we have seen that K is identified with Km, the equilibrium constant defined in terms of the molalities (mi) of reactants and products: mo being the standard molality, equal to 1 mol kg−1. Although molalities are simple experimental quantities (recall that the molality of a solute is given by the amount of substance dissolved in 1 kg of solvent) and have the additional advantage of being temperature-independent, most second law thermochemical data reported in the literature rely on equilibrium concentrations. This often stems from the fact that many analytical methods use laws that relate the measured physical parameters with concentrations, rather than molalities, as for example the Lambert-Beer law (see following discussion). As explained in section 2.9, the equilibrium constant defined in terms of concentrations (Kc) is related to Km by equation 14.3, which assumes that the solutes are present in very small amounts, so their concentrations (ci) are proportional to their molalities: mi = ci/ρ (ρ is the density of the solution).
UV spectroscopic determination of the chlorine monoxide (ClO) ∕ chlorine peroxide (ClOOCl) thermal equilibrium constant
