A-LevelChemistryGroup 29701/22 · May/June 2018 · Q111 Marks

Sodium and magnesium are the first two elements in the third period.

(a)Sodium and magnesium both react with cold water to produce the same type of product in solution. With sodium the solution is clear but with magnesium it appears cloudy.
(a)(i)Write an equation for the reaction of magnesium with cold water.[1]
(a)(ii)Suggest why the solution is cloudy after the reaction of magnesium with cold water.[1]
(b)Group 2 elements, including magnesium, react with oxygen and with dilute acids. There are trends in both the physical and chemical properties of the elements and their compounds down the group. Reactivity generally increases from Mg to Ba.
(b)(i)Explain why there is a general increase in reactivity from Mg to Ba.[2]
(b)(ii)Give two observations for the reaction of magnesium with oxygen. Write an equation for this reaction. Include state symbols.[3]
(b)(iii)Write an equation for the reaction of magnesium with sulfuric acid.[1]
(b)(iv)Suggest why there is a general decrease in the melting points of the elements down Group 2.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] Mg + 2H2O → Mg(OH)2 + H2
[1] the product/Mg(OH)2 sparingly soluble / slightly soluble / less soluble (than NaOH)
[2] any two from:
∞ (from Mg to Ba) larger (atomic) radius / more shells / more shielding
∞ electron(s) are less tightly held (by nucleus) / less attracted (by nucleus)
∞ ionisation energy / ies decrease(s) / electron(s) lost (more) easily (down the group)
∞ lower activation energy (for the reactions down the group)
[1] white light / white flame
[1] white smoke / white solid
[1] 2Mg(s) + O2(g) → 2MgO(s)
[1] Mg + H2SO4 → MgSO4 + H2
[2] any two from:
∞ increasing (cation) size / (cat)ionic radius increases / increasing size of atoms
∞ overall charge on (cation) is constant / (+)2 / decreasing charge density of (+2 charged cation)
∞ number of delocalised / outer / valence electrons (per atom) remains the same / 2
∞ increased shielding
[1] decreasing (strength of) attraction between (cat)ion / nucleus / nuclear charge AND delocalised electron(s)

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