9701 · 27.1

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

A student has two unlabelled test tubes, one containing aqueous magnesium nitrate and the other containing aqueous barium nitrate. Describe a simple chemical test to distinguish between the two solutions. Include the expected observations and an ionic equation.

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Reagent: Add a few drops of dilute sulfuric acid, $H_2SO_4(aq)$ , or aqueous sodium sulfate, $Na_2SO_4(aq)$ , to each test tube.
Observation with Magnesium Nitrate: No visible change / no precipitate is formed. This is because magnesium sulfate, $MgSO_4$ , is soluble in water.
Observation with Barium Nitrate: A dense white precipitate is formed. This is because barium sulfate, $BaSO_4$ , is insoluble in water.
Ionic Equation: The reaction forming the precipitate is: $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$

Worked example 2

Compare the temperatures at which magnesium carbonate and calcium carbonate decompose. Explain your answer in terms of the properties of the cations.

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Comparison: Magnesium carbonate decomposes at a lower temperature than calcium carbonate. (e.g., $MgCO_3$ at ~350°C, $CaCO_3$ at ~840°C). Therefore, calcium carbonate is more thermally stable.
Explanation:
- Both $Mg^{2+}$ and $Ca^{2+}$ ions have a 2+ charge.
- The $Mg^{2+}$ ion has a smaller ionic radius than the $Ca^{2+}$ ion.
- This means the $Mg^{2+}$ ion has a higher charge density.
- The $Mg^{2+}$ ion has a greater polarising power and distorts the electron cloud of the carbonate ion ( $CO_3^{2-}$ ) more effectively.
- This weakens the C-O bonds within the carbonate ion, so less energy is required to break them. Hence, $MgCO_3$ decomposes at a lower temperature.

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