comparing lattice enthalpies

• what can values of lattice enthalpy be used for?

  to measure the strength of the bonds between the oppositely charged ions making the lattice structure of the ionic compound.
• what influences the strength of the bonds formed between the oppositely charged ions?

  the ions charges and radii.
• the smaller the ions...

  ...the closer they can get to each other and so the stronger the ionic bond.
• the higher the charge on the ion...

  ...the greater the force of attraction to the oppositely charged ion and the stronger the ionic bond formed.
• high charge density


  ions with a high charge and small ionic radius.
• what do high charge density ions form?

  the strongest ionic bonds with the oppositely charged ion.
• the more negative the lattice enthalpy value...


  ...the stronger the bonds and the higher amount of energy given out when bonds form.
• the more positive the lattice enthalpy of dissociation...


  ...the stronger the bonds and the higher amount of energy to break the bonds.
• how can lattice enthalpy values be calculated?

  as experimental values using the experimental data in a Born-haber cycle.
• how can lattice enthalpy values be calculated (secondly)?

  as theoretical values using assumptions about their charge, ionic radius and geometry.
• what assumptions belong to the perfect ionic model?

  • all ions are perfectly symmetrical in shape.
  • all ions have purely ionic attractions between them.
• what happens if the perfect ionic model is true?

  then the ionic compound should only have ionic bonds between the oppositely charged ions.
• what is the perfect ionic model like in reality?

  experimental data has proved that this isn't true for every ionic compound.
• what can we compare?

  theoretical and experimental lattice enthalpy values for any ionic compound to work out the type of bonding occurring between the oppositely charged ions.
• what can be assumed when the experimental and theoretical lattice enthalpy values are the same or very close together?

  the assumptions made by the perfect ionic model are correct for that compound (perfectly symmetrical and only ionic bonds).
• what could a big difference in the sizes or charge densities of the positive and negative ions mean?

  there may be a significant difference in the experimental and theoretical lattice enthalpy values suggesting the bonds are purely ionic.
• if the experimental value is much more negative than the theoretical value?

  the formation of the lattice is such more exothermic suggesting the bonds being made are much stronger than they would be if they were purely ionic. So, there must be another force of attraction/type of bonding that is stronger than purely ionic bonding.
• what happens when positively charged metal ions with a high charge density get close to larger negatively charged non-metal ions?


  the metal ions (+) attract the electrons (-) in the cloud around the negative ion towards themselves. This attraction leads to a distortion in the distribution of electrons which causes a polarisation of the non-metal ion (-) where there is a higher electron density on the side of the ion closer to the metal ion.