Ion-Dipole Interactions
Chemistry: IB Diploma Higher Level, topic 2.8.4
Ion-Dipole Interactions
The electrostatic attraction between an ion and a molecule with a dipole is called an ion-dipole attraction. These attractions play an important role in the dissolution of ionic compounds in water.
Dissociation
- When ionic compounds dissolve in water, the ions in the solid separate and disperse uniformly throughout the solution.
- This is because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them.
- This process represents a physical change known as dissociation.
Ionic compounds & dissociation
- Under most conditions, ionic compounds will dissociate nearly completely when dissolved, and so they are classified as strong electrolytes.
- Even sparingly, soluble ionic compounds are strong electrolytes, since the small amount that does dissolve will dissociate completely.
Dissolving KCl
- Consider what happens at the microscopic level when solid KCl is added to water.
- Ion-dipole forces attract the positive (hydrogen) end of the polar water molecules to the negative chloride ions at the surface of the solid.
- The ion-dipole forces also attract the negative (oxygen) ends to the positive potassium ions.
Dissolving KCl - 2
- The water molecules surround individual K+ and Cl- ions.
- This reduces the strong interionic forces that bind the ions together and letting them move off into solution as solvated ions.
- Overcoming the electrostatic attraction permits the independent motion of each hydrated ion in a dilute solution.
- Eventually the ions transition from fixed positions in the undissolved compound to widely dispersed, solvated ions in solution.
Strength
- Ion-dipole reactions tend to be stronger than dipole-dipole interactions, but weaker than hydrogen bonds.
- The greater the charge of the ion, the strong the ion-dipole interaction will be.
Test yourself on Ion-Dipole Interactions
Ion-dipole interactions are likely to have the biggest role in forming which of the following solutions?