Thermal stability of carbonates and nitrates of group-II elements.

WHY DOWN THE GROUP THE THERMAL STABILITY OF CARBONATES AND NITRATES OF GROUP A-II ELEMENTS INCREASE.

The carbonate and nitrates of group A-II elements decomposes on heating. Carbonates decomposes to form corresponding oxides and carbon oxide while nitrates decomposes to form corresponding oxides, nitrogen dioxides and oxygen

                    XCO_3(s)→XO_(s)+CO_2(g)

Decomposition of carbonates where “X” is an group (ii) element

                           2X(NO₃)_2(s)→2XO_(s)+4NO_2(g)+O_2(g)

Decomposition of nitrates where “X” is an group (ii) element

Their thermal stability of carbonates and nitrates of group elements increase down the group. This means that lower member carbonates of group A-II elements are more stable and require more energy to be decompose

REASON

The group A-II of the periodic table have the following elements and their size increases down the group.

After losing the two valance electrons they form M⁺²  ions. The M⁺² ions of these elements have high charge density, a small 2+ ion has a very large charge in a very small volume. Down the group the size of M⁺² ions also increases and charge density decreases. M⁺² ion with high Charge density have high polarizing power than M⁺² ions with low charge density.Cation with high polarizing power distorts the anion easily. In the group A-II of periodic table as we go down the group the polarizing power of group (II) elements decreases and so their distorting effect on the large carbonate ions and nitrates ions get reduced and stability of carbonate and nitrates increases. In short more polarization requires less heat.  So they become thermally more stable. This means that MgCO₃ is thermally less stable than CaCO₃

ENERGETICS OF THE PROCESS

The data in the above table shows that the above reaction becomes more endothermic down the group and thermal stability increases down the group. To explain the cause of increasing endothermic changes down the group we will focus on the enthalpy cycle involving the lattice enthalpies of the metal carbonates and the metal oxides. The amount of heat required to convert one mole of crystal into gaseous component ions is called lattice enthalpy. In BaCO₃ the distance between the center of negative carbonate ion and positive Ba⁺² is greater than the distance in MgCO₃ case. So greater energy is required in case of in case of MgCO₃ to separate into ions having lager lattice energy. The same rule is followed for nitrates.