2020 P1 Q8 - Thermal Decomposition of Group 2 Carbonates
Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 8.
We want to explain why MgCO3 decomposes at a lower temperature than CaCO3.
The concept tested in this question is on thermal decomposition of Group 2 metal carbonates.
Mg2+ is smaller than Ca2+, hence Mg2+ has a higher charge density and greater polarising power.
This causes a greater distortion of electron cloud for CO32- which weakens the bonds within the anion to a greater extent.
Less energy is required to break the covalent bonds in CO32- hence MgCO3 decomposes at a lower temperature.
Let's go through the options and see which is the best answer.
Option A is relevant as the charge density of Mg2+ is higher which makes it more polarising than Ca2+ as explained above.
Option B is not relevant as formation of metal oxide from its constituent elements (metal M and oxygen O2) is not related to metal carbonate at all.
Option C is not relevant as lattice energy is related to energy released when the ionic compound is formed from its constituent ions.
This involves the attraction between the M2+ and CO32- ions and is not related to breaking of covalent bonds within the carbonate anion.
Option D is not relevant as melting is a physical process so is not related to the decomposition of metal carbonate which is a chemical reaction.
Hence the answer to this question is option A.
Topic: Group 2 and 17 Elements, Inorganic Chemistry, A Level Chemistry, Singapore
Back to list of questions for 2020 A Level H2 Chemistry Paper 1.
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