Ion-dipole forces

  • When an ionic substance dissolves in water, bonds will be broken as well as formed.
  • The hydrogen bonds between water molecules and the ionic bonds within the lattice will be broken.
  • To provide energy for the bond-breaking, new bonds must be formed.
  • These forces are called ion-dipole forces. It can be stronger than hydrogen bonds, but are still weaker than intermolecular bonding.

Positive ions are attracted to the negative side of water, and anions are attracted to the positive side of the water molecule.

 

ion dipole

Water molecules cluster around the ions in layers, each successive layer being held more and more weakly, almost like electrons in shells around an atom. The first layer is often held so tightly, especially by polyatomic ions, that even after the solvent has evaporated, they still remain bonded to the ions, forming hydrates and complex ions.

For example, copper sulphate is actually white. Blue copper sulphate is actually copper (II) sulphate pentahydrate, the latter term signifying that 5 water molecules remain bonded to the copper and sulphate ions. The formula for this is CuSO4 · 5H2O.

  • When they are still in solution, they form complex ions. The first hydration layer is often included in its formula; since copper ions in solution have 6 water molecules in its first layer, its formula is [Cu(H2O)6]2+.
  • Note that complex ions may be formed in other polar solvents than water, such as ammonia. Copper tetraammine¬†[Cu(NH3)4]2+ is one example.
  • The polar molecules attracted to the ions are called ligands.
  • Also note that ionic compounds do not dissolve in non-polar solvents, as the dispersion forces in the non-polar solvents are not strong enough to break the ionic lattice.
  • NB: many ionic substances are insoluble in water, the reason for this is complex and not in the course.

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