22.3 factors affecting lattice enthalpy and hydration

• Ionic compounds tend to have the following general properties:

  • high melting and boiling points
  • soluble in polar solvents
  • conduct electricity when molten or in aqueous solution
• some ionic compounds can be melted with heat, whilst other have such high melting points they can be used to coat the inside of a coal furnace and some are liquids at room temperature.
• The values of lattice enthalpy depend on ionic size and ionic charge
• the effect of increasing ionic size can be seen via comparing the lattice enthalpies.(down a group)

  • ionic radius increases
  • attraction between ions decreases
  • lattice energy less negative
  • melting point decreases
• the effect on increasing ionic charge is compared by similar cations on lattice enthalpy

  • ionic charge increases
  • attraction between ions increases
  • lattice energy becomes more negative
  • melting point increases
• the effects of both ionic size and charge affect the attraction of ions for oppositely charged ions.

  
• the magnitude of lattice enthalpy gives an indication of the melting point of an ionic compound. some metal oxides have very exothermic lattice enthalpies and very high melting points. these stable metal oxides are used as protective coating on the inside of furnaces. lattice enthalpy is a indicator of the size of the melting point but other factors, like packing ions in an ionic lattice need to be considered.
• hydration enthalpies are affected by ionic size and ionic charge just as lattice enthalpies are.
• the effect of ionic size on hydration enthalpies down a group:

  • ionic radius increases
  • attraction between ion and water molecule decreases
  • hydration energy less negative
• effect of ionic charge on hydration enthalpy:

  • ionic changes increase
  • attraction with water molecules increases
  • hydration energy becomes more negative
• To dissolve an ionic compund in water, the attraction between the ions in the ionic lattice must be overcome. This requires enough energy to equal the lattice enthalpy. water molecules are attracted to the positive and negative ions, surrounding and releasing energy to equal hydration enthalpy
• if the sum of the hydration enthalpies is larger than the magnitude of lattice enthalpy, the overall enthalpy change (enthalpy change of solution) will be exothermic and the compound should dissolve.
• many compounds with endothermic enthalpy changes of solution are soluble so this doesn’t provide the total picture. the reasons for solubility depend on temperature and entropy.