The Trend in Stabilities of Metal Carbonates and their Thermal Decomposition

All metal carbonates are compounds that contain a metal, carbon and oxygen atoms, possessing the general formula MCO₃, where M represents a metal.
Metal carbonates tend to decompose (break down) when heated, producing metal oxides and carbon dioxide. The general equation for this reaction can be written as: MCO₃(s) → MO(s) + CO₂(g), where MO represents a metal oxide.
Thermal decomposition of metal carbonates is a key concept in the Limestone topic, as limestone is primarily calcium carbonate (CaCO₃). When heated, it decomposes to produce calcium oxide (lime) and carbon dioxide: CaCO₃(s) → CaO(s) + CO₂(g). This reaction is significant to the manufacture of cement and lime for mortars.
The thermal stability of a metal carbonate increases as you move down a group in the Periodic Table. For instance, Group 2 metal carbonates such as calcium carbonate are more stable than those of Group 1 like sodium carbonate.
The trend in stability can also be linked to the size and charge of the cations (positive ions) found in the different metal carbonates. As you move down a group in the Periodic Table, the metal ions get larger but maintain the same charge, and thus their ability to ‘polarise’ the negatively charged carbonate ion and initiate decomposition decreases.
High temperatures are needed to decompose metal carbonates, hence they are classified as thermally stable. The decomposition of metal carbonates and resulting chemical changes contribute to environmental issues like global warming due to the release of CO₂.
Experiments can be carried out to study the effect of heat on metal carbonates using techniques like Limewater test - the production of CO₂ can be confirmed as it forms a milky-white precipitate (calcium carbonate) when bubbled through Limewater.
Remember that exception to these trends exist. For instance, unlike rest of the Group 2 carbonate, Magnesium carbonate decomposes at normal atmospheric pressure.
Understanding the factors affecting the thermal decomposition of metal carbonates is vital, not just for understanding limestone, but for wider aspects of chemistry including industrial processes and environmental science.
Don’t forget to keep practising questions on this subject to reinforce your understanding of the topic and to become familiar with how these concepts can be applied. Practice is the key to efficient learning.