Born-harber cycles

• The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products in their standard states e.g. H2 (g) + 1/2 O2 →H2O (g)
• The standard enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state, under standard conditions e.g. Na (s) → Na+ (g)
• Lattice enthalpy is the energy required to break 1 mole of an ionic solid into its gaseous ions (endothermic), or the energy released when gaseous ions combine to form an ionic solid (exothermic).
• First ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge e.g. Na (g) → Na+ (g) + e-
• Electron affinity is the enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge e.g. Cl (g) + e- → Cl- (g)
• How do you calculate lattice enthalpy using a Born-Haber cycle?

  Use Hess’s law. The sum of all enthalpy changes around the cycle equals zero. Rearrange equation:

  • ΔH formation = ΔH atomisation + ΔH ionisation energy + ΔH electron affinity + ΔH lattice enthalpy
• Lattice enthalpy is exothermic because energy is released when oppositely charged ions attract and form a stable ionic lattice.
• What factors affect lattice enthalpy?

  • Charge of the ions – Higher charge results in stronger attraction and more negative lattice enthalpy.
  • Size of the ions – Smaller ions lead to stronger attraction and more negative lattice enthalpy.
• The second electron affinity is endothermic because energy is required to overcome the repulsion between the negatively charged ion and the incoming electron.
• Hess's Law states that the total enthalpy change for a reaction is independent of the route taken, as long as the initial and final conditions are the same.
• Why do larger ions lead to less negative lattice enthalpy?

  Larger ions have a lower charge density, leading to weaker electrostatic forces between them, reducing the magnitude of the lattice enthalpy.
• How does ion charge affect lattice enthalpy?

  Higher ion charge increases the attraction between ions, resulting in a more negative (exothermic) lattice enthalpy, because the stronger attraction releases more energy when the lattice is formed.
• Lattice enthalpy of dissociation is the energy required to break 1 mole of an ionic lattice into its gaseous ions (endothermic).
• The first electron affinity is exothermic because energy is released when an atom gains an electron and forms a more stable ion. The second electron affinity is endothermic because adding an electron to an already negatively charged ion requires energy to overcome repulsion.