📋 Structured Questions
Complete each question on paper, then check your answers against the mark scheme.
Question 1: Solubilities and Analytical Tests
6 marks(a) Explain why barium sulfate can be safely swallowed as a barium meal, even though barium ions are highly toxic. [2]
(b) Write an ionic equation, including state symbols, for the precipitation reaction that occurs when aqueous sodium sulfate is mixed with aqueous barium chloride. [2]
(c) Hydrochloric acid must be added to a solution prior to testing for sulfate ions with barium chloride. Explain why this acid is added. [2]
Show Mark Scheme
(a)
- Barium sulfate is highly insoluble [1]
- It cannot dissolve in the digestive system and is not absorbed into the bloodstream or body tissues [1]
(b)
- Ba2+(aq) + SO42-(aq) → BaSO4(s)
- Award [1] for correct species, [1] for correct state symbols.
(c)
- To react with and remove carbonate ions (CO32-) or sulfite ions (SO32-) present in the solution [1]
- These impurities would otherwise react with barium ions to form a white precipitate of barium carbonate or barium sulfite, giving a false positive result [1]
Question 2: Thermal Decomposition of Group 2 Carbonates
7 marks(a) State the trend in the thermal stability of Group 2 carbonates down the group. [1]
(b) Explain this trend in terms of the charge, size, and polarising power of the cations. [4]
(c) Write the chemical equation, including state symbols, for the thermal decomposition of magnesium carbonate. [2]
Show Mark Scheme
(a)
- Thermal stability increases down the group [1]
(b)
- All Group 2 cations have a 2+ charge, but ionic radius / cation size increases down the group [1]
- Therefore, charge density of the cation decreases down the group [1]
- Larger cations have lower polarising power, meaning they distort the electron cloud of the carbonate ion less [1]
- This weakens the carbon-oxygen covalent bond in the carbonate anion less, making it harder to break and requiring more energy (higher temperature) to decompose [1]
(c)
- MgCO3(s) → MgO(s) + CO2(g)
- Award [1] for correct chemical formula, [1] for correct state symbols.
Question 3: Group 2 Reactions with Water
7 marks(a) Write a chemical equation, including state symbols, for the reaction of calcium with cold water. [2]
(b) State and explain the trend in reactivity of Group 2 metals with water down the group. [3]
(c) Magnesium reacts very slowly with cold water but rapidly with steam. Write the chemical equation, including state symbols, for the reaction of magnesium with steam. [2]
Show Mark Scheme
(a)
- Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
- Award [1] for correct formulas, [1] for correct state symbols.
(b)
- Reactivity increases down the group [1]
- Atomic radius and shielding increase down the group, so outer shell electrons are further from the nucleus [1]
- The sum of first and second ionisation energies decreases, so the metal loses its two outer electrons and forms 2+ ions more easily [1]
(c)
- Mg(s) + H2O(g) → MgO(s) + H2(g)
- Award [1] for correct formulas, [1] for correct state symbols (note H2O is gas/steam and MgO is solid).