Estimating thermochemical properties by analogy or educated guessing

4 Estimating thermochemical properties by analogy or educated guessing 

When thermochemical data for a substance or group of substances are not available in the literature, group additivity is not applicable, and quantum chemistry calculations are not feasible, then one can start by simply making some educated guesses. Sometimes, this can be done using analogies to similar systems, often moving up the periodic table to smaller, better studied elements. In other cases, one can interpolate  

Suppose we knew heats of formation for the gaseous aluminum atom and chlorine atom and for AICI3, but had no information at all about AlCl or AlCb- Then we might simply guess that the bond strengths for all three Al-Cl bonds were equal. We could write the hypothetical reaction 

Since this is the total enthalpy change for breaking all three Al-Cl bonds, the average enthalpy change for breaking an Al l bond is 1278.19/3 = 426.06 kJ/mol. If we assume that all three bonds are equivalent, then the reactions 

While these are obviously very coarse estimates, they are not ridiculously far off. In a case where one had no idea, a priori, whether such species might be important, something like this might be appropriate for generating a first 

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