AS Chemistry: Group 2 — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Group 2 revision notes

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Questions

1. Write an equation for the reaction of calcium with cold water, and state two observations. [3]

2. Explain why reactivity increases down Group 2, referring to nuclear attraction and shielding. [3]

3. State and explain the trend in solubility of Group 2 hydroxides down the group. [2]

4. State and explain the trend in solubility of Group 2 sulfates down the group. [2]

5. Group 2 carbonates decompose on heating.

(a) Write an equation for the thermal decomposition of magnesium carbonate. [1] (b) State and explain the trend in thermal stability down the group. [4]

6. Describe how you would use barium chloride solution to test for sulfate ions, including why the solution is first acidified with dilute hydrochloric acid. [3]

7. State two large-scale uses of Group 2 compounds, with a reason for each. [4]

8. Magnesium reacts steadily with dilute sulfuric acid, but barium’s reaction stops almost immediately. Explain why. [2]

9. Write an equation for the thermal decomposition of a Group 2 nitrate, and state two observations that would confirm the reaction has occurred. [4]

10. Predict, with a reason, which decomposes at a lower temperature: CaCO₃ or SrCO₃. [3]

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Answers

1. Ca + 2H₂O → Ca(OH)₂ + H₂ [1]. Observations: effervescence / bubbles of gas [1]; the calcium dissolves and a white precipitate / cloudy suspension forms [1].

2. The outer electrons are further from the nucleus and there is more shielding down the group [1], so the attraction between nucleus and outer electrons is weaker [1] and the two outer electrons are lost more readily, so the metal is more reactive [1].

3. Solubility increases down the group [1] — Mg(OH)₂ is only slightly soluble whereas Ba(OH)₂ is soluble [1].

4. Solubility decreases down the group [1] — MgSO₄ is soluble whereas BaSO₄ is insoluble [1]. The two trends run in opposite directions, and mixing them up is the standard error.

5. (a) MgCO₃ → MgO + CO₂ [1].

(b) Thermal stability increases down the group [1]. Down the group the cation is larger with the same charge, so its charge density and polarising power decrease [1]. It therefore distorts the carbonate ion’s electron cloud less [1], so the C–O bond is weakened less and more energy is needed to decompose it [1].

6. Add dilute hydrochloric acid first, then barium chloride solution [1]. A white precipitate confirms sulfate [1]. The acid removes carbonate ions, which would otherwise also give a white precipitate and a false positive [1].

7. Any two, each with a reason: Calcium hydroxide on fields [1] to neutralise acidic soil [1]. Magnesium hydroxide in indigestion remedies [1] to neutralise excess stomach acid [1]. (Also: barium sulfate as a medical “barium meal” — it is insoluble so it is not absorbed and is non-toxic.)

8. Barium sulfate is insoluble [1], so as soon as it forms it coats the surface of the barium metal, physically blocking further contact between the metal and the acid and stopping the reaction — this is a solubility effect, not a reactivity one [1].

9. 2M(NO₃)₂ → 2MO + 4NO₂ + O₂ [1]. Observations: the brown gas NO₂ is given off [1], and a glowing splint relights in the oxygen produced [1] — both confirm the decomposition has occurred [1].

10. Ca²⁺ is smaller than Sr²⁺ (fewer electron shells) [1], so it has a higher charge density and polarises the carbonate ion more strongly, weakening its bonds more [1]. CaCO₃ decomposes at the lower temperature [1].

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Where marks are usually lost

• Reversing the hydroxide and sulfate solubility trends.
• Explaining thermal stability without mentioning polarising power.
• Forgetting to acidify before testing for sulfate.
• Explaining reactivity by “more shells” alone, without linking to attraction.
• Treating barium’s slow reaction with sulfuric acid as evidence that barium is less reactive — it is a solubility effect (BaSO₄ coating the metal), not a reactivity trend, and barium is in fact the most reactive Group 2 metal covered.
• Explaining thermal stability of nitrates without the polarising-power argument, or forgetting that both carbonates and nitrates follow the same trend for the same underlying reason.
• Missing one of the two confirming observations (brown gas, relighting splint) when asked to describe how a nitrate decomposition is confirmed experimentally.

Related reactions worth knowing alongside these

With water (oxides): MO + H₂O → M(OH)₂. This becomes more complete down the group as solubility increases — MgO gives only a sparingly-soluble, weakly alkaline suspension, while BaO gives a much more strongly alkaline solution.

With dilute acid (the same pattern for oxides, hydroxides and carbonates):

MO + 2HCl -> MCl2 + H2O
M(OH)2 + 2HCl -> MCl2 + 2H2O
MCO3 + 2HCl -> MCl2 + H2O + CO2

Recognising that an oxide, a hydroxide and a carbonate all react with an acid by the same underlying pattern — the metal ends up bonded to the acid’s anion, with water (and, for a carbonate, carbon dioxide) as the other product — saves having to memorise three unrelated equations separately.

For condensed recall notes on this topic, see the Group 2 revision notes; for the full explanation with worked examples, see the Group 2: The Alkaline Earth Metals study guide.