In Group 2 carbonates, what is the trend in thermal stability as you move down the group?

In Group 2 carbonates, what is the trend in thermal stability as you move down the group?

• A. It increases
• B. It decreases
• C. It remains constant
• D. It varies unpredictably

Answer: (A)

The idea being tested is how readily a carbonate releases CO2 when heated, which hinges on how strongly the cation polarizes the carbonate ion. In Group 2 carbonates, small, highly charged cations such as Be2+ and Mg2+ distort the carbonate ion more, weakening the C–O bonds and making decomposition to MO and CO2 easier. As you move down the group, the cations get larger and their polarizing power decreases, so the carbonate ion is less distorted and more stable to heat. Therefore, the energy (temperature) required to decompose the carbonate increases down the group. BeCO3 and MgCO3 decompose at relatively lower temperatures, while CaCO3, SrCO3, and BaCO3 require higher temperatures to decompose, reflecting greater thermal stability.

The idea being tested is how readily a carbonate releases CO2 when heated, which hinges on how strongly the cation polarizes the carbonate ion. In Group 2 carbonates, small, highly charged cations such as Be2+ and Mg2+ distort the carbonate ion more, weakening the C–O bonds and making decomposition to MO and CO2 easier. As you move down the group, the cations get larger and their polarizing power decreases, so the carbonate ion is less distorted and more stable to heat. Therefore, the energy (temperature) required to decompose the carbonate increases down the group. BeCO3 and MgCO3 decompose at relatively lower temperatures, while CaCO3, SrCO3, and BaCO3 require higher temperatures to decompose, reflecting greater thermal stability.