UPDATED Topic 13 - Energetics II

• Lattice dissociation enthalpy is the enthalpy change when one mole of an ionic solid is completely dissociated into its gaseous constituent ions under standard conditions. It is an endothermic process.
• Lattice formation enthalpy is the enthalpy change when one mole of an ionic solid is formed from its gaseous constituent ions under standard conditions. It is an exothermic process.
• Atomisation energy/enthalpy is the energy required for the formation of one mole of gaseous atoms from an element under standard conditions.
• First electron affinity is the enthalpy change when one mole of electrons is added to a mole of gaseous atoms to form one mole of gaseous 1- ions under standard conditions.
• Born-Haber cycles involve combining different enthalpy changes, using arrows to represent a reaction.
• Constructing Born-Haber cycles will produce a different value for lattice enthalpy compared to the theoretical value predicted using electrostatic theory.
• Electrostatic theory assumes a 'perfect ionic molecule', whereby ionic molecules don't express any covalent character. However, this is not the reality.
• Experimentally derived lattice enthalpies can be used as indicators of the degree of covalent bonding in a lattice.
• The 'perfect ionic model' assumes that : all ions are perfectly spherical, charge is evenly distributed throughout the ion, and the ions display no covalent character.
• Covalent character occurs in ions when two joined ions have varying sizes or charges, meaning the charge distribution is not even. This polarisation of anions by cations creates covalent character.
• Enthalpy of solution is the enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution, so that the ions no longer interact under standard conditions.
• Enthalpy of hydration is the enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions.
• Enthalpy of hydration is usually negative because water molecules have slightly positive regions that naturally attract negative ions.
• The enthalpy of solution and the enthalpy of hydration are combined to form an energy cycle.
• Enthalpy of hydration is a measure of the attraction between ions (that were in a lattice) and water molecules in solution. Positive ions will be attracted to slightly negative oxygen atoms, and negative ions will be attracted to slightly positive hydrogen ions.
• Hydration enthalpy DECREASES down a group - this is because attractions are stronger with smaller ions and greater charge (i.e. higher charge density), and charge density is highest at the top of the group.
• Lattice dissociation enthalpy also increases with higher charge density, because the ions form stronger attractions and so the energy required for dissociation is greater.