5.2.1 - lattice enthalpy

• Lattice enthalpy is the enthalpy change when one mole of an ionic compound is formed from its gaseous ions under standard conditions.
• enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions.
• enthalpy of atomisation is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state
• enthalpy of formation is always exothermic
• First ionisation energy is the enthalpy change required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions
• enthalpy of atomisation is always endothermic
• first ionisation energy is always endothermic as energy is needed to overcome attraction between outer shell electrons and positive nuclei
• first electron affinity is the enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
• First electron affinity is always exothermic because the electron being added is attracted towards the nucleus
• Second electron affinity is the enthalpy change when one electron is added to each 1- ion in one mole of gaseous ions to form one mole of gaseous 2- ions
• second electron affinity is always endothermic because the electron bing added is repelled by 1- ion so energy is needed to overcome the repulsion
• Enthalpy of solution is the enthalpy change when one mole of a solute dissolves in a solvent - water
• enthalpy of solution can be endothermic or exothermic
• Enthalpy change of hydration is the enthalpy change when one mole of gaseous ions are converted into one mole of aqueous ions
• A more exothermic lattice enthalpy means there are more stronger ionic bonds
• it is not possible to measure lattice enthalpy directly because it is not possible to form 1 mole of ionic solid from its gaseous ions
• the factors affecting the size of lattice enthalpy are:

  • size of ions involved
  • charges on the ions
  • ionic bond strength
• smaller ions have a more negative lattice enthalpy than larger ions because they can get closer together and have a stronger attraction
• hydration is when an ionic lattice is broken the ions become part of the solution. positive ions are attracted to the slightly negative oxygen and negative ions are attracted to the slightly positive hydrogen
• ions with a higher charge have a more exothermic lattice enthalpy because they have a stronger attraction
• the factors affecting the enthalpy of hydration are:

  • size of the ion
  • charge on the ion