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

1. 1
   A student is studying the formation of sodium chloride, NaCl, from its elements using a Born–Haber cycle, which is an application of Hess's law.

   (a)

   Which statement defines the lattice energy of sodium chloride?

   [1 mark]

   • AThe energy needed to break one mole of NaCl(s) into its gaseous ions
   • BThe energy change when one mole of NaCl(s) is formed from Na(s) and Cl₂(g) in their standard states
   • CThe energy change when one mole of NaCl(s) is formed from its gaseous ions
   • DThe energy change when one mole of gaseous Na⁺ ions is formed from Na(g)

   (b)

   Which equation represents the first electron affinity of chlorine?

   [1 mark]

   • ACl(g) → Cl⁺(g) + e⁻
   • B½Cl₂(g) → Cl(g)
   • CCl⁻(g) → Cl(g) + e⁻
   • DCl(g) + e⁻ → Cl⁻(g)

   (c)

   Define the enthalpy change of atomisation of chlorine and write an equation, with state symbols, to represent it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is constructing a Born–Haber cycle for sodium chloride with these data: standard enthalpy change of formation of NaCl(s) = −411 kJ mol⁻¹; enthalpy change of atomisation of sodium = +107 kJ mol⁻¹; enthalpy change of atomisation of chlorine, ½Cl₂(g) → Cl(g) = +122 kJ mol⁻¹; first ionisation energy of sodium = +496 kJ mol⁻¹; first electron affinity of chlorine = −349 kJ mol⁻¹.

   (a)

   Which of these steps in the Born–Haber cycle for sodium chloride is exothermic?

   [1 mark]

   • AThe first electron affinity of chlorine
   • BThe first ionisation energy of sodium
   • CThe atomisation of sodium
   • DThe atomisation of chlorine

   (b)

   What is the lattice energy of sodium chloride calculated from these data?

   [1 mark]

   • A+787 kJ mol⁻¹
   • B−787 kJ mol⁻¹
   • C−909 kJ mol⁻¹
   • D−1485 kJ mol⁻¹

   (c)

   Write an equation, with state symbols, for the lattice energy of sodium chloride and explain why this enthalpy change is exothermic.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is using a Born–Haber cycle to find the lattice energy of magnesium chloride, MgCl₂. The data are: standard enthalpy change of formation of MgCl₂(s) = −641 kJ mol⁻¹; enthalpy change of atomisation of magnesium = +148 kJ mol⁻¹; first ionisation energy of magnesium = +738 kJ mol⁻¹; second ionisation energy of magnesium = +1451 kJ mol⁻¹; enthalpy change of atomisation of chlorine, ½Cl₂(g) → Cl(g) = +122 kJ mol⁻¹; first electron affinity of chlorine = −349 kJ mol⁻¹.

   (a)

   Calculate the lattice energy of magnesium chloride.

   [3 marks]

   (b)

   Explain why the Born–Haber cycle for MgCl₂ includes the second ionisation energy of magnesium, and twice the atomisation enthalpy and electron affinity of chlorine. Explain why the cycle is needed to find the lattice energy.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies magnesium oxide, MgO, using a Born–Haber cycle. The data are: standard enthalpy change of formation of MgO(s) = −602 kJ mol⁻¹; enthalpy change of atomisation of magnesium = +148 kJ mol⁻¹; first ionisation energy of magnesium = +738 kJ mol⁻¹; second ionisation energy of magnesium = +1451 kJ mol⁻¹; enthalpy change of atomisation of oxygen, ½O₂(g) → O(g) = +249 kJ mol⁻¹; first electron affinity of oxygen = −141 kJ mol⁻¹; lattice energy of MgO = −3791 kJ mol⁻¹.

   (a)

   Calculate the second electron affinity of oxygen.

   [6 marks]

   (b)

   Explain why the second electron affinity of oxygen is endothermic, and use the data to explain why magnesium oxide is nevertheless formed exothermically from its elements.

   [6 marks]

   Total for question 4: 12 marks

End of questions