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The Ionic Model · Paper 2 — Free Chemistry HL Practice Questions

1MasterySAQ-SLattice structure in ionic solids7 marksPaper 2~11 min

Magnesium oxide (MgO) and sodium chloride (NaCl) are both ionic solids with the rock salt lattice structure. Property — MgO — NaCl Ionic charges —

\text{Mg}^{2+}

\text{O}^{2-}

—

\text{Na}^{+}

\text{Cl}^{-}

Ionic radii sum / pm — 205 — 281 Lattice enthalpy /

\text{kJ mol}^{-1}

— 3795 — 788

(a)

State the coordination number of

\text{Mg}^{2+}

in MgO. [1 mark]

(b)

Using Coulomb's law,

F \propto \dfrac{Q^{+} \times |Q^{-}|}{r^2}

, calculate the predicted ratio of the lattice enthalpy of MgO to that of NaCl. State one reason why this predicted ratio differs from the experimentally observed ratio of

4.82

. [3 marks]

(c)

Explain why the lattice enthalpy of MgO is much greater than that of NaCl, referring to both ionic charge and ionic radius in your answer. [3 marks]

Solutions

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