A-Level Chemistry May/June 2024 Q4(e): Suggest the trend in the magnitude of the lattice energies of the metal nitrates, NaNO3…

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  The trend in the magnitude of the lattice energies from most exothermic to least exothermic is:

  Mg(NO₃)₂(s) > NaNO₃(s) > RbNO₃(s)

  Explanation:

  Lattice energy is a measure of the strength of the electrostatic forces of attraction between the oppositely charged ions in an ionic lattice. The magnitude of the lattice energy is determined by the charge and the radius of the ions involved.

  1. Comparison of Mg(NO₃)₂ with NaNO₃ and RbNO₃: The magnesium ion ($Mg^{2+}$) has a charge of +2, whereas the sodium ($Na^{+}$) and rubidium ($Rb^{+}$) ions both have a charge of +1. The electrostatic attraction between the $Mg^{2+}$ cation and the $NO_3^−$ anion is significantly stronger than the attraction involving the +1 cations. Therefore, Mg(NO₃)₂ has the most exothermic (largest magnitude) lattice energy.

  2. Comparison of NaNO₃ with RbNO₃: Both $Na^{+}$ and $Rb^{+}$ have the same +1 charge. However, the $Na^{+}$ ion has a smaller ionic radius than the $Rb^{+}$ ion because it is in Period 3, while Rb is in Period 5 of the same group. The smaller $Na^{+}$ ion has a higher charge density. This leads to a stronger electrostatic attraction between the $Na^{+}$ and $NO_3^−$ ions compared to the attraction between the larger $Rb^{+}$ and $NO_3^−$ ions. Consequently, the lattice energy of NaNO₃ is more exothermic than that of RbNO₃.

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  How the marks are awarded

  • M1 — Correctly stating the order of the magnitude of the lattice energies as Mg(NO₃)₂ > NaNO₃ > RbNO₃.
  • M2 — Correctly identifying the key differences in ionic properties: stating that the Mg²⁺ ion has a higher charge than Na⁺/Rb⁺, AND that the Na⁺ ion has a smaller radius than the Rb⁺ ion.
  • M3 — Correctly linking the ionic properties to the lattice energy, explaining that higher charge and/or smaller radius (higher charge density) leads to stronger electrostatic attraction between ions and therefore a more exothermic lattice energy.

  Common mistakes

  • Stating an incorrect order, often by swapping NaNO₃ and RbNO₃, forgetting that a smaller radius leads to a more exothermic lattice energy.
  • Providing an incomplete explanation by only comparing charges (Mg²⁺ vs Na⁺) but failing to compare the radii of Na⁺ and Rb⁺ to justify their relative order.
  • Using vague terminology like 'stronger bonds' instead of specifying 'stronger electrostatic attraction between ions'.
  • Failing to explicitly mention the effect of charge and radius on charge density, which is the direct cause of the difference in attractive forces.

  Examiner tip: When comparing lattice energies, always systematically consider both ionic charge and ionic radius, as higher charge and smaller radius lead to a greater charge density and stronger electrostatic attraction.

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