22.1 lattice enthalpy

• Solid ionic compounds tend to be very stable, the stability arises from the strangth of the ionic bonds, electrostatic attractions between oppositely charged ions in the ionic lattice structure.
• lattice enthalpy is a measure of the strength ionic bonding in a giant ionic lattice.
• lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
• lattice enthalpy involves ionic bond formations from separate gaseous ions. its an exothermic change and the value for the enthalpy change will always be negative

  
• lattice enthalpy cannot be measured directly and must be calculated indirectly using know energy changes in an energy cycle.
• the indirect determination of lattice enthalpy requires a type of energy cycle called a born-haber cycle.

  
• Routes: Route 1- Requires three different processess:

  • Formation of gaseous atoms: changing elements in their standard states into gaseous atoms, this change is endothermic as it involves bond breaking
  • formation of gaseous ions: changing the gaseous atoms into positive and negative ions, this change is endothermic
  • lattice formation: changing the gaseous ions into solid ionic lattice, this is the lattice enthalpy and is exothermic

  Route 2 - converts elements in standard states directly into inbox lattice, just one enthalpy change of formation and is exothermic
• the standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions in standard states
• the standard enthalpy change of atomisation is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
• standard enthalpy of atomisation is always an endothermic process as bonds are broken to form the gaseous atom. when the element is a gas in its standard state standard enthalpy change of atomisation is related to bond enthalpy of the bond being broken.
• first ionisation energy is the enthalpy change required to remove one electron from each atom in one mole of a gaseous atom to form one mole of a gaseous 1+ ion.
• ionisation energies are endothermic because energy is reqiused to overcome the attraction between the negative electron and the positive nucleus.
• electron affinity is the opposite of ionisation energy:

  • electron affinity measures the energy to gain electrons
  • ionisation energy measures the energy to lose electrons
• first electron affinity is the enthalpy change that takes place when one electron is added to each atom in one mole of a gaseous atoms to form one mole of gaseous 1- ions.
• first electron affinities are exothermic because the electron being added is attracted towards the nucleus
• By constructing a born-haber cycle you can calculate lattice enthalpies

  
• when an anion has a greater charge than 1-, successive electron affinities are required. second electron affinities are endothermic. a second electron is being gained by a negative ion, which repels the electron away. so energy must be put to force the negatively charge electron onto the negative ion.