About MgCO3 and CaCO3

The stability of a compound is determined by the enthalpy of formation, not the lattice enthalpy.

It is wrong that you say the more exothermic the lattice enthalpy, the more stable the ionic compound.

The more exothermic lattice enthalpy only means the stronger bonds in the ionic compound.
Although ΔHlattice [MgCO3] is more exothermic than ΔHlattice [CaCO3], the MgCO3 may not be stable than CaCO3.

Actually, CaCO3 are energetically more stable than MgCO3 because
ΔHfo [CaCO3] is more exothermic than ΔHfo [MgCO3]
PS: ΔHfo [CaCO3] = -1207.60 kJ/mol
ΔHfo [MgCO3] = -1111.69 kJ/mol

With a view to the energy level diagram,

圖片參考：http://imgcld.yimg.com/8/n/HA00928059/o/700911290155613873382190.jpg


Since CaCCO3 are more stable than MgCO3, CaCO3 is more difficult to decompose than MgCO3 upon heating. The thermal stability of CaCO3 is higher than that of MgCO3.



2009-12-03 01:39:48 補充：
Apart from energetic explanation,
the reason why the CaCO3 is more stable than MgCO3 is that the electron cloud of carbonate anion is distorted more strongly by Mg2+ ion than by Ca2+ ion, making MgCO3 less stable.

thermal stability 係相對既
係relative to咩先?
要breakdown MgCO3個reaction pathway係咪一定要將MgCO3
breakdown to Mg2+同CO32- ?
如果唔係就冇得比