A-LevelChemistryGroup 29701/42 · Oct/Nov 2025 · Q17 Marks

Magnesium nitrate, Mg(NO3)2, and strontium nitrate, Sr(NO3)2, both decompose when heated to form the metal oxide and a mixture of gases.

(a)Write an equation for the thermal decomposition of Mg(NO3)2.[1]
(b)State which of Mg(NO3)2 or Sr(NO3)2 decomposes at a lower temperature. Explain your answer. compound that decomposes at a lower temperature explanation[2]
(c)(i)Magnesium oxide, MgO, and strontium oxide, SrO, both react with dilute sulfuric acid. MgO forms a soluble salt, A. SrO forms an insoluble salt, B. Identify the products formed when MgO reacts with dilute sulfuric acid.[1]
(c)(ii)Explain why A is more soluble than B.[3]

Mark scheme

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

[1] Mg(NO3)2 → MgO + 2NO2 + ½O2 OR 2Mg(NO3)2 → 2MgO + 4NO2 + O2
[1] M1: Mg(NO3)2 / magnesium nitrate AND magnesium ion / Mg2+ AND is smaller / has higher charge density
[1] M2: nitrate ion / anion is more distorted / polarised
[1] magnesium sulfate and water / MgSO4 and H2O
[1] M1: magnesium sulfate / A has a more exothermic ∆Hlatt and ∆Hhyd
[1] M2: difference in ∆Hhyd is greater
[1] M3: magnesium sulfate / A has a more exothermic ∆Hsol

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