Explain why the standard enthalpy of atomisation of chlorine is an endothermic process and why this value must be included when constructing a Born-Haber cycle for potassium chloride, KCl.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Born-Haber cycles are used to determine lattice enthalpies indirectly using Hess's law. Each step in the cycle represents a specific enthalpy change, and all steps must be included to complete the cycle correctly.
Model answer (5 marks)
1. The standard enthalpy of atomisation of chlorine,
(Delta H^circ_{ ext{atom}}( ext{Cl}_2 ightarrow 2 ext{Cl(g)})), is the energy required to break the Cl–Cl bond in the diatomic molecule.
2. Bond breaking is an endothermic process, so this enthalpy change is positive.
3. The value gives the enthalpy needed to produce one mole of gaseous chlorine atoms from elemental chlorine in its standard state.
4. In a Born‑Haber cycle for KCl the intermediate step is the formation of gaseous K⁺ and Cl⁻ ions; the Cl⁻ ion is produced by adding an electron to a gaseous Cl atom.
5. Therefore the atomisation enthalpy must be included to convert Cl₂ to Cl(g) before electron affinity can be applied, and all enthalpy changes must be accounted for so that Hess’s law can be used to calculate the lattice enthalpy.
(Delta H^circ_{ ext{atom}}( ext{Cl}_2 ightarrow 2 ext{Cl(g)})), is the energy required to break the Cl–Cl bond in the diatomic molecule.
2. Bond breaking is an endothermic process, so this enthalpy change is positive.
3. The value gives the enthalpy needed to produce one mole of gaseous chlorine atoms from elemental chlorine in its standard state.
4. In a Born‑Haber cycle for KCl the intermediate step is the formation of gaseous K⁺ and Cl⁻ ions; the Cl⁻ ion is produced by adding an electron to a gaseous Cl atom.
5. Therefore the atomisation enthalpy must be included to convert Cl₂ to Cl(g) before electron affinity can be applied, and all enthalpy changes must be accounted for so that Hess’s law can be used to calculate the lattice enthalpy.
Examiner tips
- Use the exact phrase "endothermic" for bond breaking. Include the step of converting Cl₂ to Cl(g) before electron affinity. Show the sequence of steps in the cycle to justify inclusion of the atomisation enthalpy.
- common_mistakes
- :
- Saying the atomisation is exothermic. Omitting the atomisation step and therefore mis‑calculating the lattice enthalpy. Confusing the atomisation enthalpy with the bond dissociation energy of Cl₂.
Mark scheme (5 marks)
- Atomisation of chlorine involves breaking bonds / breaking the Cl–Cl bond in Cl₂
- Bond breaking requires energy / is endothermic
- The standard enthalpy of atomisation produces one mole of gaseous atoms from the element in its standard state
- The Born-Haber cycle requires gaseous atoms as an intermediate / gaseous Cl atoms are needed before electron affinity can occur
- Hess's law requires all enthalpy changes in the cycle to be accounted for so that the lattice enthalpy can be calculated correctly
Key terms in this question
standard enthalpy of atomisation · Born-Haber cycle
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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