Why does CdCO3 have a significantly lower decomposition temperature than CaCO3, despite the similarity in the ionic radii of Ca2+ and Cd2+?
I think they both decompose to give the metal oxide and carbon dioxide. Could the polarising ability of Ca compared with Cd have something to do with the difference in temperature? I really don't know where to start with this problem.
To understand why CdCO3 has a lower decomposition temperature than CaCO3, we need to consider the key factors that influence the stability of carbonate compounds.
Firstly, let's address the similarities in the ionic radii of Ca2+ and Cd2+. Both calcium ion (Ca2+) and cadmium ion (Cd2+) have similar sizes because they belong to the same group in the periodic table. Therefore, the difference in their ionic radii is not the primary reason for the difference in decomposition temperature.
The decomposition of carbonate compounds occurs when the metal carbonate is heated, leading to the formation of the metal oxide and carbon dioxide gas. The stability of the carbonate compound depends on the strength of the metal-carbonate bond, which is influenced by the polarizing ability of the metal cation.
The polarizing ability of a cation is determined by its charge and size. A metal cation with a higher charge or a smaller size has a stronger polarizing ability. In the case of Ca2+ and Cd2+, both ions have a +2 charge, so the difference in their polarizing ability can be attributed to their respective ionic sizes.
The primary reason for CdCO3 having a lower decomposition temperature compared to CaCO3 is the higher polarizing ability of the Ca2+ ion compared to the Cd2+ ion. The smaller size of the Ca2+ ion leads to a stronger polarization of the carbonate (CO3 2-) ions, resulting in stronger metal-carbonate bonds in CaCO3. Therefore, CaCO3 is more stable and requires a higher temperature to decompose.
On the other hand, the larger Cd2+ ion has a weaker polarizing ability, resulting in weaker metal-carbonate bonds in CdCO3. This lower bond strength makes CdCO3 less stable, allowing it to decompose at a lower temperature than CaCO3.
In summary, the difference in decomposition temperature between CdCO3 and CaCO3 can be attributed to the polarizing ability of the respective metal cations. The higher polarizing ability of Ca2+ leads to stronger metal-carbonate bonds in CaCO3, making it more stable and requiring a higher temperature for decomposition. In contrast, the weaker polarizing ability of Cd2+ results in weaker metal-carbonate bonds in CdCO3, making it less stable and decomposing at a lower temperature.