A-LevelChemistryGroup 29701/41 · May/June 2025 · Q17 Marks

Both calcium carbonate, CaCO3, and barium carbonate, BaCO3, decompose when heated to form the metal oxide and a gas.

(a)Write an equation for the thermal decomposition of CaCO3.[1]
(b)State which of CaCO3 and BaCO3 decomposes at a lower temperature. Explain your answer. The compound that decomposes at a lower temperature is ..................................................... explanation ..................................................................................................................................[2]
(c)(i)Calcium oxide, CaO, reacts with water to form compound A. Barium oxide, BaO, reacts with water to form compound B. Identify A.[1]
(c)(ii)Explain why A is less soluble than B.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] CaCO3 → CaO + CO2
[2] M1: calcium carbonate AND calcium ion / Ca2+ smaller / ionic radius bigger down group
[2] M2: carbonate ion / anion / CO32- is more distorted/polarised by Ca2+
[1] calcium hydroxide / Ca(OH)2
[3] M1: A / calcium hydroxide has more exothermic ∆Hlatt AND ∆Hhyd than B / barium hydroxide
[3] M2: difference in ∆Hlatt is greater for A / calcium hydroxide than for B / barium hydroxide
[3] M3: ∆Hsol is less exothermic for A / calcium hydroxide than for B/barium hydroxide

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