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A-Level Trends in the elements across a period and down a group: worked solution
3 marks. Full working, one step per line.
Question
Group 2 oxalates, MC2O4, follow a thermal-stability trend similar to that of Group 2 carbonates. Which of MgC2O4 or CaC2O4 decomposes thermally at the lower temperature? Explain your answer. [3]
Worked answer
Thermal stability of Group 2 salts of oxoanions increases DOWN the group. The cation polarises the anion; a smaller, more polarising cation distorts the oxalate anion more, weakening bonds within it and lowering the decomposition temperature. Mg2+ is smaller than Ca2+ and has a higher charge density (greater polarising power), so it polarises the C2O4²- anion more strongly. Therefore MgC2O4 is less thermally stable and decomposes at the LOWER temperature.
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This question is part of A-Level Trends in the elements across a period and down a group, in A-Level H2 Chemistry.
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