9701 · 10.1

Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds — practice questions

Practice and worked examples for 9701 Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

Explain, in terms of atomic structure, why barium is more reactive than magnesium.

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Barium is further down Group 2 than magnesium, so it has more electron shells.
This results in a larger atomic radius and greater shielding of the valence electrons from the nucleus in barium compared to magnesium.
The increased distance and shielding outweigh the increased nuclear charge, leading to a weaker electrostatic attraction between the nucleus and the valence electrons in barium.
Therefore, less energy is required to remove the two outer electrons from a barium atom (it has a lower sum of the first and second ionisation energies).
As reactivity depends on the ease of forming a positive ion, barium forms its $Ba^{2+}$ ion more readily and is thus more reactive than magnesium.

Worked example 2

A student adds aqueous sodium sulfate, $Na_2SO_4(aq)$ , to a test tube containing magnesium chloride solution, $MgCl_2(aq)$ . They repeat the experiment in a second test tube containing barium chloride solution, $BaCl_2(aq)$ . Describe the expected observations and write an ionic equation for any reaction that occurs.

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Test tube 1 (with $MgCl_2$ ): Observation: No change / no precipitate is formed. Explanation: Magnesium sulfate ( $MgSO_4$ ) is soluble in water.

Test tube 2 (with $BaCl_2$ ): Observation: A dense white precipitate is formed. Explanation: Barium sulfate ( $BaSO_4$ ) is insoluble in water. Ionic Equation: $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$

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