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We have 3 modes of learning for students to choose from: weekly physical classes at Bishan; weekly online lessons via Zoom; and on-demand video lessons.

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Chemistry Guru specialises in H2 Chemistry for JC1 and JC2 or IP Year 5 and IP Year 6 students only. Find out all the information about A Level H2 Chemistry and the detailed comparison between H1 and H2 Chemistry. We do not conduct lessons for H1 Chemistry, O Level Chemistry, IB Chemistry or IP Chemistry at IP Year 3 and IP Year 4.

2020 P1 Q8 - Thermal Decomposition of Group 2 Carbonates

Let Chemistry Guru, Singapore's renowned JC Chemistry tuition centre, guide you through 2020 A Levels H2 Chemistry Paper 1 Question 8.

2020 p1 q8 question

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.

2020 p1 q8 thermal decomposition of group 2 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 CO₃^2- which weakens the bonds within the anion to a greater extent.

Less energy is required to break the covalent bonds in CO₃^2- hence MgCO₃ decomposes at a lower temperature.

Let's go through the options and see which is the best answer.

2020 p1 q8 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 CO₃^2- 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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2020 P1 Q8 - Thermal Decomposition of Group 2 Carbonates