Lattice energy and Born-Haber cyclesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student constructs a Born–Haber cycle for sodium chloride using these standard enthalpy changes (kJ mol⁻¹): enthalpy change of formation of NaCl(s) −411; atomisation of sodium +107; first ionisation energy of sodium +496; atomisation of chlorine (per mole of Cl atoms) +122; first electron affinity of chlorine −349.

   (a)

   Which equation represents the first electron affinity of chlorine?

   [1 mark]

   • A½Cl₂(g) + e⁻ → Cl⁻(g)
   • BCl⁻(g) → Cl(g) + e⁻
   • CCl(g) + e⁻ → Cl⁻(g)
   • DCl(g) + e⁻ → Cl⁻(aq)

   (b)

   Which equation represents the lattice energy of sodium chloride, as defined by formation of the solid?

   [1 mark]

   • ANa⁺(g) + Cl⁻(g) → NaCl(s)
   • BNaCl(s) → Na⁺(g) + Cl⁻(g)
   • CNa(s) + ½Cl₂(g) → NaCl(s)
   • DNaCl(s) → Na(g) + Cl(g)

   (c)

   Calculate the lattice energy of sodium chloride.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Magnesium chloride is used as a de-icing salt. A Born–Haber cycle is constructed for MgCl₂ using these standard enthalpy changes (kJ mol⁻¹): enthalpy change of formation of MgCl₂(s) −641; atomisation of magnesium +148; first ionisation energy of magnesium +738; second ionisation energy of magnesium +1451; atomisation of chlorine (per mole of Cl atoms) +122; first electron affinity of chlorine −349.

   (a)

   What is the lattice energy of magnesium chloride?

   [1 mark]

   • A+2524 kJ mol⁻¹
   • B−2751 kJ mol⁻¹
   • C−1883 kJ mol⁻¹
   • D−2524 kJ mol⁻¹

   (b)

   Which step in the cycle is exothermic?

   [1 mark]

   • AMg⁺(g) → Mg²⁺(g) + e⁻
   • BCl(g) + e⁻ → Cl⁻(g)
   • CMg(s) → Mg(g)
   • D½Cl₂(g) → Cl(g)

   (c)

   Calculate the total enthalpy change for converting 1 mol of Mg(s) and 1 mol of Cl₂(g) into 1 mol of Mg²⁺(g) and 2 mol of Cl⁻(g).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compares two compounds that each contain a 1+ cation and a 1− anion. Lattice energies (kJ mol⁻¹), defined for formation of the solid from gaseous ions, are: sodium chloride, Born–Haber value −787 and value calculated from a purely ionic model −770; silver iodide, Born–Haber value −889 and value calculated from a purely ionic model −778.

   (a)

   Calculate the difference between the Born–Haber and ionic-model lattice energies for each compound, and state which compound has the greater covalent character.

   [3 marks]

   (b)

   Explain why the Born–Haber lattice energy of silver iodide is more exothermic than the value from the ionic model.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Calcium oxide is made by heating limestone and is used to line furnaces. A Born–Haber cycle for CaO uses these standard enthalpy changes (kJ mol⁻¹): enthalpy change of formation of CaO(s) −635; atomisation of calcium +178; first ionisation energy of calcium +590; second ionisation energy of calcium +1145; atomisation of oxygen (per mole of O atoms) +249; first electron affinity of oxygen −141; second electron affinity of oxygen +798.

   (a)

   Calculate the lattice energy of calcium oxide.

   [6 marks]

   (b)

   Define the enthalpy change of atomisation and the first electron affinity. Explain why the second electron affinity of oxygen is endothermic and yet calcium oxide is stable.

   [6 marks]

   Total for question 4: 12 marks

End of questions