The Kapustinskii equation shows that lattice enthalpies are inversely proportional to the interionic distances. Later work has shown that further simplification of the Kapustinskii equation allows lattice enthalpies to be estimated from the molecular (formula) unit volume (the unit cell volume divided by the number of formula units, Z, that it contains) or the mass density (see, for example, H.D.B. Jenkins and D. Tudela, J. Chem. Educ., 2003, 80, 1482). How would you expect the lattice enthalpy to vary as a function of 

(a) The molecular unit volume 

(b) The mass density? Given the following unit cell volumes (all in cubic angstroms, ų; 1 Å = 10⁻¹⁰ m) for the alkaline earth carbonates MCO₃ and oxides, predict the observed decomposition behaviour of the carbonates.

Transcribed Image Text:

MgCO3 47 MgO 19 CaCO3 61 CaO 28 SrCO, 64 SrO 34 BaCO, 76 BaO 42