Ionic bonding: theoretical and experimental lattice energyEdexcel A-Level Chemistry: Subtopic test

1. 1
   A chemist is comparing the strength of the ionic bonding in sodium chloride and magnesium oxide. She uses lattice energy values, defined as the enthalpy change when one mole of an ionic solid is formed from its gaseous ions.

   (a)

   Which statement correctly links lattice energy to the strength of ionic bonding?

   [1 mark]

   • AA less exothermic lattice energy means stronger ionic bonding
   • BA more exothermic lattice energy means stronger ionic bonding
   • CLattice energy measures the energy needed to melt the solid, so the higher the melting point the less exothermic the value
   • DLattice energy depends only on the charge of the anion and not on the cation

   (b)

   Which equation represents the lattice energy of magnesium oxide?

   [1 mark]

   • AMgO(s) → Mg²⁺(g) + O²⁻(g)
   • BMg(s) + ½O₂(g) → MgO(s)
   • CMg²⁺(g) + O²⁻(g) → MgO(s)
   • DMg(g) + O(g) → MgO(s)

   (c)

   The lattice energy of magnesium oxide is much more exothermic than that of sodium chloride. State what this shows about the ionic bonding in magnesium oxide and explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A teacher asks a class to predict which ionic compounds will show the most covalent character. The students must decide which cations are best at distorting the electron cloud of a neighbouring anion, and which anions are most easily distorted.

   (a)

   Which cation has the greatest polarising power?

   [1 mark]

   • AK⁺
   • BNa⁺
   • CMg²⁺
   • DAl³⁺

   (b)

   Which anion is the most polarisable?

   [1 mark]

   • AS²⁻
   • BO²⁻
   • CF⁻
   • DCl⁻

   (c)

   Explain what is meant by the polarisation of an anion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A data table gives, for three silver and sodium compounds, the lattice energy from a Born-Haber cycle (experimental) and the lattice energy calculated using a purely ionic model (theoretical). Experimental and theoretical values in kJ mol⁻¹ are: sodium chloride, −787 and −770; silver chloride, −905 and −833; silver iodide, −889 and −778.

   (a)

   Calculate the difference between the experimental and theoretical lattice energy for each compound, and deduce which compound has the greatest covalent character. Justify your answer.

   [3 marks]

   (b)

   Explain why the difference between the experimental and theoretical values is greater for silver iodide than for silver chloride.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist is investigating how far the bonding in three metal halides (lithium fluoride, lithium iodide and potassium iodide) is purely ionic. For each compound she can obtain an experimental lattice energy from enthalpy data and a theoretical lattice energy from a model in which the ions are treated as perfect, oppositely charged spheres. She expects the difference between the two values to vary between the compounds.

   (a)

   Explain how the theoretical lattice energy and the experimental lattice energy are obtained, and what a difference between the two values would show about the bonding in a metal halide.

   [6 marks]

   (b)

   Explain which of the three compounds you would expect to show the largest difference between experimental and theoretical lattice energy. Refer to the polarising power of the cations and the polarisability of the anions in your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions