Intermolecular Forces

1. IntermolecularForces Chemistry 11

2. How are molecules held together? • There are two types of attraction in molecules: • Intra-molecular forces • Intermolecular forces • Inter-molecular Forces – the forces that bond molecules to each other • Intra-molecular Forces – the forces that bond the atoms to each other within a molecule • Until now, we have been discussing intramolecular forces (ionic, polar, non-polar bonds)

3. How are molecules held together?

4. Inter-molecular forces are weak relative to covalent bonds Intra-molecular forces are strong covalent bonds We can conclude that it does not take very much energy to break the bonds that hold molecules to each other (intermolecular forces)

5. These forces are also referred to as van der Waals forces after the physicist who studied them • We will be discussing 4 types of intermolecular forces • Dipole-Dipole Forces • Ion-Dipole Forces • Dispersion (London) Forces • Hydrogen Bonding

6. + – H Cl + – + – + – + – Dipole-Dipole Forces • Dipole is a polar molecule • These molecules will orient themselves so that oppositely charged ends of molecules are near to one another • The electrostatic attractions between these ends are called dipole-dipole forces Lets see it!

7. Result of Dipole-Dipole Forces • Polar molecules will tend to attract one another more than similarly sized non-polar molecules • Therefore the energy required to separate polar molecules from one another is greater than that needed to separate non-polar molecules

8. Ion-Dipole Forces • The force of attraction between an ion and a polar molecule (a dipole) • Example: NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl- Let’s See it!

9. Dispersion (London) Forces (LDF) • Weakest intermolecular force that act between non-polar molecules • Shared pairs of e- can temporarily occupy positions that create momentary, uneven distributions of charge • Molecules are constantly vibrating and this causes momentary, uneven distributions of charges • This causes non-polar molecules to become slightly polar for just an instant

10. At this instant it is capable of inducing a dipole in a nearby molecule resulting in an intermolecular force of attraction • This is called a dispersion force

11. Hydrogen Bonding • An attraction between the hydrogen atom, bonded to a highly electronegative atom, and the negative end of a dipole nearby

13. Like Dissolves Like • Ionic solutes dissolve in polar solvents • (ex: NaCl and H2O) • Non polar solutes dissolve in non polar solvents • (ex: solid I2 and liquid Br2)

14. In order of strength: • Hydrogen Bonding: • Hydrogen bonded to highly EN atom (O, N, F) • Dipole-Dipole Forces: • The larger the EN, the stronger the dipole-dipole force. •  More energy is required to separate polar molecules from each other than non-polar molecules of similar molar mass.

15. London Dispersion Forces: • Larger molecules, with many electrons, or with electrons far removed from the atomic nuclei, are more easily polarised than small molecules. • As the molar mass increases, LDF becomes stronger and the boiling points of non-polar molecules substances increases.

16. Try These  • Which of the following molecules have a permanent dipole moment?a. H2Ob. CO2c. CH4d. N2e. COf. NH3

17. Solution • CO2, CH4 and N2 are symmetrical, and hence they have no permanent dipole moments. • A molecule with polar bonds unsymmetrically arranged will possess a permanent dipole.