Intermolecular forces

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Intermolecular forces

  1. 1. 1 4. Intermolecular forces Chemistry Grade 11Everything Science www.everythingscience.co.za
  2. 2. 2Intermolecular forces: introductionIntermolecular forces are the forces that act between molecules.The type of intermolecular force in a substance, will depend on the nature of themolecules:● Polar molecules have an unequal distribution of charge, meaning that one part of themolecule is slightly positive and the other part is slightly negative. The molecule is said tobe a dipole.● Non-polar molecules have an equal distribution of charge. Everything Science www.everythingscience.co.za
  3. 3. 3Types of intermolecular forcesThere are five types of intermolecular forces:ion-dipole forces, ion-induced-dipole forces,dipole-dipole forces, dipole-induced dipoleforces and induced dipole forces.● Ion-dipole forces exist between ions andpolar (dipole) molecules. The ion is attractedto the part of the molecule that has anopposite charge to its own.● Ion-induced dipole forces exist between ionsand non-polar molecules. An ion induces adipole in the non-polar molecule.● Dipole-dipole forces exist between two polar(dipole) molecules.● Dipole-induced dipole forces exist between apolar molecule and a non-polar molecule.● Induced dipole forces exist between twonon-polar molecules. Everything Science www.everythingscience.co.za
  4. 4. 4Types of intermolecular forces contd.Dipole-dipole forces, dipole-induced dipole forces and induced dipole forces are collectivelycalled van der Waals’ forces.Hydrogen bonds are a type of dipole-dipole force that occurs when a hydrogen atom isattached to a highly electronegative atom (oxygen, fluorine, nitrogen). A hydrogen atom onone molecule is attracted to the electronegative atom on a second molecule. Everything Science www.everythingscience.co.za
  5. 5. 5Effects of intermolecular forcesIntermolecular forces affect the properties of substances.● Substances with larger molecules have stronger intermolecular forces than substanceswith smaller molecules.● Viscosity is the resistance to flow of a liquid. Substances that are very viscous havelarger molecules and stronger intermolecular forces than substances with smallermolecules.● Density is a measure of the mass in a unit volume. Solids have strong intermolecularforces and so have more molecules in one unit volume.● Substances with weak intermolecular forces will have low melting and boiling pointswhile those with strong intermolecular forces will have high melting and boiling points.● Thermal expansion is the expansion of a liquid on heating.● Thermal conductivity is a measure of how much a material conducts heat. Everything Science www.everythingscience.co.za
  6. 6. 6The chemistry of water● Water has strong hydrogen bonds which hold the molecules together. It is these inter-molecular forces that give water its unique properties.● Water has the following properties: a high specific heat, absorption of infrared radiation, alarge range in which it exists as a liquid, a high heat of vaporisation and has a less densesolid phase.● Specific heat is the amount of heat energy that is needed to increase the temperature of aunit mass of a substance by one degree.● Heat of vaporisation is the energy that is needed to change a given quantity of a substanceinto a gas. Everything Science www.everythingscience.co.za
  7. 7. 7For more practice see:www.everythingscience.co.zaShortcode: ESBMV Everything Science www.everythingscience.co.za

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