Thermodynamic Analysis of Ionic Compounds: synthetic ~pplications Claude H. Yoder Franklin and Marshall College. Lancaster, PA 17604 One of the most serious problems to confront the instructor, not to mention the student, of inorganic chemistry is the great diversity of facts associated with the more than 100 elements. Hidden beneath these facts are generalizations that pro\.idr wherency and sirnplitj. the studv of rhernistry. The uncovering and applicntion of these aenrralizarions is the raison d'etie of both the researcher and teacher. A particularly useful set of generalizations is based upon the application of thermodynamic cycles. These can he used to rationalize variations in stability, solubility, electrode potentials, Lowry-Bronsted and Lewis acidity, etc. (1,2).I t is the purpose of this article to show how thermodynamic cycles can he used to understand trends in heats of formation and aqueous solubilities and, most importantly, how they may he used to choose synthetic routes to new ionic compounds. Analysts of Heats of Formatlon First, consider the thermodynamic steps involved in the formation of an alkali halide, MX, from the elements in their standard states:
Although the ionization energy (step 21, electron affinity (step 41, and the lattice energy (step 5) are usually tabulated as energy changes rather than enthalpy changes, this difference is small and AH0f is obtained by summation of steps 1-5. If the compound is more complex, e.g., an alkali sulfate, then the cycle is best given as
+ 20,(g) + 2e2M+(g)+ SO?-(g) S(s)
-
.
S0,2-(g) M,SO,(s)
AH", (anion(g))
-LE
where AHof(anion(g)) can he determined if AHof(cation(g)), and the lattice energy for a t least one compound are known (2,3). The lattice energy can he ohtained experimentally from AHof,or calculated with some accuracy with an expression such as the Born equation (2), or, if the structure of the solid is unknown, recourse to a more approximate equation, such as the Kapustinskii equation (4)
is necessary. Here u is the numher of moles of ions per mole of compound, the Q's are the integral charges on the ions 232
Journal of Chemical Education
(Q(SOa2-) = 2, Q(K+) = I), and r is the sum of the sixcoordinate ionic radii in Anestr~ms.The resultine lattice energy is in kcallmole. The analysis of the AHo
