Reactivity 1.2 – Energy cycles in reactions

R1.2.5 – Born-Haber cycles

📌 What is a Born-Haber cycle?

• The Born-Haber model is a thermodynamic cycle used to find the lattic enthalpy of an ionic compound by applying Hess’s Law to various stages in the formation of an ionic compound
• There are 3 important parts of the cycle :

📌 Atomisation energy

• The atomisation energy is the enthalpy change when a substance is converted into one mole of the substance in gaseous state
• The symbol for atomisation energy is ΔH_^atom°
• The symbol for atomisation energy in the case of elements already in gaseous state is E of the bond enthalpy (for example, if Cl₂(g) became Cl(g), the energy change would be the bond enthalpy of Cl-Cl divided by 2)

📌 First ionisation energy

• The first ionisation energy is the energy required to create the positive ion from one mole of a gaseous atom
• This is an endothermic process as it produces one electron and a positively charged gaseous ion
• The symbol for first ionisation energy is ΔH_i^°

📌 First electron affinity

• The first electron affinity is the energy required to create the negative ion from one mole of a gaseous atom
• This is an exothermic process as it produces a negatively charged gaseous ion
• The symbol for first electron affinity is ΔH_e^°

📌 Calculating the lattice enthalpy

• The concepts given above can be used to create a coherent cycle to calcualte the lattice enthalpy of an ionic compound
• The following formula is used for this calculation :