Topic 27: Group 2

Question 1

Definitions I:
(a) Thermal stability
(b) Ionic radius
(c) Enthalpy change of solution

Answer 1

Definitions I:
(a) Resistance of a compound to decompose on heating, e.g., carbonates.
(b) Distance from nucleus to outer electrons in an ion, e.g., Ba²⁺.
(c) Enthalpy change when 1 mol solid dissolves, ΔHₛₒₗ = ΔHₗₐₜₜ + ΔHₕᵧₔ.

Question 2

Describe the trend in thermal stability of Group 2 nitrates from Mg to Ba. [2 Marks]

Answer 2

Increases Mg to Ba, larger cations stabilise anions (1 mark)
Less polarisation, harder to decompose (1 mark)

Question 2

Why does BaCO₃ decompose less readily than MgCO₃ when heated? [2 Marks]

Answer 2

Ba²⁺ larger than Mg²⁺, less polarising power (1 mark)
CO₃²⁻ less distorted, more stable (1 mark)

Question 3

In a lab, MgSO₄ dissolves more than BaSO₄. Explain using lattice energy and hydration. [2 Marks]

Answer 3

MgSO₄: smaller lattice energy, more negative ΔHₕᵧₔ (1 mark)
BaSO₄: larger lattice energy, less soluble (1 mark)

Question 3

Predict the solubility trend of Group 2 hydroxides from Mg(OH)₂ to Ba(OH)₂. [2 Marks]

Answer 3

Increases Mg(OH)₂ (insoluble) to Ba(OH)₂ (soluble) (1 mark)
Smaller lattice energy down group (1 mark)

Question 4

Write the equation for the thermal decomposition of Ca(NO₃)₂ and explain its stability. [2 Marks]

Answer 4

Ca(NO₃)₂ → CaO + 2NO₂ + ½O₂ (1 mark)
Moderate stability, Ca²⁺ polarises NO₃⁻ (1 mark)

Question 4

Compare ΔHₛₒₗ of MgSO₄ and BaSO₄ given ΔHₗₐₜₜ and ΔHₕᵧₔ trends. [3 Marks]

Answer 4

MgSO₄: smaller ions, higher ΔHₗₐₜₜ, more negative ΔHₕᵧₔ (1 mark)
BaSO₄: larger ions, ΔHₛₒₗ more positive (1 mark)
MgSO₄ more soluble, ΔHₛₒₗ less positive (1 mark)

Question 5

In a furnace, why does SrCO₃ need a higher temperature to decompose than CaCO₃? [2 Marks]

Answer 5

Sr²⁺ larger than Ca²⁺, less polarisation (1 mark)
SrCO₃ more stable, needs higher T (1 mark)

Question 6

Explain why Mg(OH)₂ is less soluble than Ba(OH)₂ in water. [2 Marks]

Answer 6

Mg(OH)₂: higher lattice energy, less hydration (1 mark)
Ba(OH)₂: lower lattice energy, more soluble (1 mark)

Question 6

Predict the trend in ΔHₛₒₗ for Group 2 sulfates from MgSO₄ to BaSO₄. [2 Marks]

Answer 6

MgSO₄ more negative ΔHₛₒₗ, BaSO₄ more positive (1 mark)
Larger ions reduce hydration enthalpy (1 mark)

Question 7

Describe how ionic radius affects the polarisation of CO₃²⁻ in Group 2 carbonates. [2 Marks]

Answer 7

Smaller Mg²⁺ polarises CO₃²⁻ more, weakens bonds (1 mark)
Larger Ba²⁺ less polarising, more stable (1 mark)

Question 7

Explain the thermal stability trend of Mg(NO₃)₂ to Ba(NO₃)₂ with decomposition products. [3 Marks]

Answer 7

Mg(NO₃)₂ least stable, decomposes to MgO + 2NO₂ + ½O₂ (1 mark)
Ba(NO₃)₂ most stable, larger Ba²⁺ less polarising (1 mark)
Trend increases down group, less distortion (1 mark)