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
• amongst the many ionic compounds there is a wide range of melting points and solubilities
• some ionic compounds can be melted with the heat of a Bunsen, others have such high melting points they can be used to coat the inside of furnaces, some ionic compounds are liquids at room temp - ionic liquids
• although many ionic compounds are soluble in polar solvents, such as water, others are insoluble
• the relative magnitude of lattice and hydration enthalpies helps to explain the variety of melting and boiling points and trends in solubility seen in ionic solids
• Factors affecting lattice enthalpy:

  • ionic size
  • ionic charge
• Effect of ionic size:

  • ionic radius increases
  • attraction between ions decreases
  • lattice energy less negative
  • melting point decreases
• Effect of ionic charge:

  • ionic charge increases
  • attraction between ions increases
  • lattice enthalpy becomes more negative
  • melting point increases
• change in ionic size and charge across period 3:

  the effects of both ionic size and charge affect the attraction of these ions for oppositely charged ions. Across period 3, the ionic size of the cations Na+, Mg2+ and Al3+ decreases as there are more protons attracting the same number of electrons
• two supporting effect:

  • increasing charge gives more attraction
  • decreasing size gives more attraction
• two opposing effects:

  • increasing charge gives more attraction
  • increasing size gives less attraction
• Predicting melting points:

  • magnitude of lattice energy gives a good indication of the melting point of an ionic compound
  • some metal oxides (MgO, Al2O3, ZrO) have very exothermic lattice enthalpies and very high melting points
  • these stable metal oxides find a use as a protective coating for the inside of furnaces and refractories
  • lattice enthalpy is a very good predictor for the size of the melting point but other factors, such as the packing of ions in an ionic lattice may need to be considered as well
• Factors affecting hydration:

  • ionic size
  • ionic charge
• increasing ionic size:

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

  • ionic charge increases
  • attraction with water molecules increases
  • hydration energy becomes more negative
• Predicting solubility:

  • to dissolve an ionic compound in water, the attraction between the ions and the ionic lattice must be overcome
  • this requires a quantity of energy equal to the lattice enthalpy
  • water molecules are attracted to the positive and negative ions, surrounding them and releasing energy equal to hydration enthalpy
• if the sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change (enthalpy change of solution) will be exothermic and the compound should dissolve
• However, many compounds with endothermic enthalpy changes of solution are soluble so this does not provide the total picture.
• The reason for solubility also depends on temperature and entropy
• most ionic compounds have regular-shaped ions of reasonably similar size so they can pack together tightly to form a solid ionic compound

  • in an ionic liquid, the ions can have irregular shapes and can be very different sizes - means they are unable to pack together into a lattice and can be liquids at RT
• Ionic liquids conduct electricity, do not vaporise easily, and can dissolve in a wide range of substances
• the chemical structure of an ionic liquid typically contains an organic ion - they can be made from biomass and are seen as potential green solvents in the future to replace oil-derived solvents