chapter 22

• solid ionic compounds are stable because of the strength of the ionic bonds which create a substantial energy barrier that must be overcome to break down the lattice resulting in high melting points of many ionic compounds
• lattice enthalpy


  enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard condition e.g

  K+ + Cl- -> KCl
• lattice enthalpy involves ionic bond formation from separate gaseous ions, it is an exothermic change and the value for the enthalpy change will always be negative
• lattice enthalpy cannot be measured directly so must be calculated using known energy changes in an energy cycle
• Born-Haber cycle


  type of energy cycle used to calculate lattice enthalpy, 2 routes for changing elements in the standard states into an ionic lattice
• route 1 of a Born-Haber cycle

  requires 3 different processes:

  1. formation of the gaseous atoms

  endothermic change

  1. formation of gaseous ions

  endothermic change

  1. lattice formation

  exothermic change
• route 2 of a Born-Haber cycle

  converts the elements into their standard states directly to the ionic lattice, only one enthalpy change, which is the enthalpy change of formation and it is exothermic
• standard enthalpy of formation


  enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states e.g

  Na(s) + 1/2Cl2(g) -> NaCl(s) -771kJmol-1
• a compound will always be an ionic compound in its solid lattice
• standard enthalpy change of atomisation

  enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions e.g

  Na(s) -> Na(g)
• standard enthalpy change of atomisation is always an endothermic process because bonds are broken to form gaseous atoms, when the element is a gas in its standard state, the enthalpy change of atomisation is related to the bond enthalpy of the bond being broken
• first ionisation energy

  enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions e.g

  Na(g) -> Na+(g) + e-
• ionisation energies are endothermic because energy is required to overcome the attraction between a negative electron and the positive nucleus
• electron affinity is the opposite of ionisation energy
• first electron affinity

  enthalpy change that takes place when 1 electron is added to each atom in one mole of gaseous atoms to form 1 mole of gaseous -1 ions e.g

  Cl(g) + e- -> Cl-(g)
• first electron affinities are exothermic because the electron being added is attracted in towards the nucleus
• 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 in to force the negatively-charged electron onto the negative ion
• a Born-Haber cycle including an element with successive ionisation energies contains all of the energy changes