The Thermochemistry of Inorganic Solids IV. Enthalpies of Formation of Compounds of the Formula MXaYb

Abstract

It is found that the standard enthalpies of formation ΔfH○298 of double salts of the type MXaYb are related by a simple additivity relation to ΔfH○298 of their binary salts MXc and MYd. For divalent metals M this relation takes the form, ΔfH○298(MXY) = 1/2 ΔfH○298(MX2)+ 1/2 ΔfH○298(MY2)+C, with C=−13.4 or −17.6 kJ/mol giving equally good fits to the data. From a lesser number of data for trivalent and tetravalent metals M, one finds again a simple additivity relation of the form ΔfH○298(MXaYb) =(ax/z)ΔfH○298(MXz/x) +(by/z)ΔfH○298(MYz/y)+C, where x, y, and z are the formal valences of X, Y, and M, respectively, so that z=ax+by, and C=0. For 16 divalent metal compounds average deviations are 5.5 kJ/mol with a maximum deviation of 10.7 kJ/mol. For eight trivalent metal compounds the average deviations are 13.9 kJ/mol with a maximum of 50.6 kJ/mol. For five tetravalent compounds, the average deviations are 3.5 kJ/mol with a maximum of 6.3 kJ/mol.