22.1 - lattice enthalpy

• Positive enthalpy


  Endothermic
• Negative enthalpy


  Exothermic.
• Why do solid ionic compounds tend to be very stable?


  Due to the strength of the ionic bonds, electrostatic attractions between oppositely-charged ions in the ionic lattice structure.
• Why do many ionic compounds have high melting and boiling points?


  The strength of the ionic bonds creates a substantial energy barrier that must be overcome to break down the lattice.
• Measure of the strength of ionic bonding in a giant ionic lattice


  Lattice enthalpy
• Standard conditions


  100kPa and 298K
• Standard enthalpy change of formation


  The enthalpy change when one mole of a compound is formed from its elements under standard conditions.

  
• Is standard enthalpy change of formation exothermic or endothermic

  It can be exothermic or endothermic as the energy change is the sum of bonds broken and formed.
• Enthalpy formation of an element


  0
• Ionisation energy

  The amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions.
• Is the ionisation energy endothermic or exothermic?


  It's always endothermic as energy is needed to overcome the attraction between an electron and the nucleus.
• First ionisation energy

  The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
• Standard enthalpy change of atomisation


  The enthalpy change when one mole of gaseous atoms is formed from its element in its standard state under standard conditions.
• Is enthalpy change of atomisation exothermic or endothermic?


  It's always endothermic as energy is required to break any bonds in the element to form gaseous atoms.
• Bond dissociation energy (bond enthalpy)


  The amount of energy required to break one mole of a specific covalent bond in the gas phase.
• Is bond enthalpy exothermic or endothermic?


  It's treated as a bond breaking process, so it's endothermic.
• Standard lattice enthalpy change of formation


  The enthalpy change when one mole of an ionic compound is formed from its gaseous ions, under standard conditions.
• Is standard lattice enthalpy change of formation endothermic or exothermic?


  Exothermic
• Why is standard lattice enthalpy change of formation exothermic?


  When ions are combined to form a solid ionic lattice there is an extremely large release of energy (making bond = energy released)
• What does a large negative value of standard lattice enthalpy change of formation indicate?


  That the ionic compound is much more stable than its gaseous ions. The more exothermic the value is, the stronger the ionic bonds within the lattice are.
• Why are ionic compounds more stable than their gaseous ions?


  Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions in the solid lattice, whereas the ions in the gas phase do not - so the gaseous ions are less stable than ions in the ionic lattice.
• Standard lattice enthalpy change of dissociation


  The enthalpy change when one mole of an ionic compound dissociates to form its gaseous ions, under standard conditions.
• What does electron affinity measure?


  The energy to gain electrons.
• What does ionisation energy measure?


  The energy to lose an electron.
• First electron affinity


  The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous -1 ions.
• Are second electron affinities exothermic or endothermic?


  Endothermic.
• Why are second electron affinities endothermic?


  A second electron is being gained by 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.