11.4    Thermal Decomposition

Thermal decomposition of  Group II  salts

• In general, compounds with high charge density cation and large anion size tend to decompose more easily(less stable on heat) due to the greater polarisation of anion by the cation.
• Thermal decomposition of Group II salts decreases down the In other words, the thermal stability of Group II salts increases down the Group.
• This is because going down the Group, the cation size increases while the anion size remains unchanged. Therefore the charge density and polarising power of cation decreases and the anion cloud is less polarised. The compound is more stable on heating
• Group II salts are less stable compared to Group I salts due to the higher charge density of M²⁺ ion

Thermal decomposition of Group II nitrates

• All Group II nitrates decompose on heating to give the corresponding metal oxide, brown nitrogen monoxide gas and oxygen gas

2M(NO3)2(s) → 2MO(s) + 4NO2(g) + O2(g)  ; where M = A Group II  element

• Thermal stability of Group II nitrates increases down the This is because the cation size increases down the Group, this reduces the charge density and polarising power of cation. The nitrate ion is less polarised and the compound is more stable.

Thermal decomposition of Group II carbonates

• All Group II carbonates decompose on heating to give the corresponding metal oxide and carbon dioxide gas

MCO3(s) → MO(s) + CO2(g)  ; where M = A Group II element

• Thermal stability of Group II carbonates increases down the This is because the cation size increases down the Group, this reduces the charge density and polarising power of cation. The carbonate ion is less polarised and the compound is more stable.