Intermolecular Forces

How are molecules held together? There are two types of attraction in molecules: ◦ Intramolecular forces ◦ Intermolecular forces Intermolecular Forces – the forces that bond molecules to each other Intramolecular 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)

How are molecules held together?

Intermolecular forces are weak relative to covalent bonds Intramolecular 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)

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

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 HCl ++ –– ++ –– ++ –– ++ –– ++ –– Lets see it!

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

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!

Dispersion (London) Forces Weakest intermolecular force that act between non-polar molecules Shared pairs of e- can temporarily occupy positions that create momentary, uneven distributions of charge This causes non-polar molecules to become slightly polar for just an instant

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

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

Like Dissolves Like Ionic solutes dissolve in polar solvents ◦ (ex: NaCl and H 2 O) Non polar solutes dissolve in non polar solvents ◦ (ex: solid I 2 and liquid Br 2 )

Practice Sheet on polar molecules is for homework! On this sheet please add this question: Based on what we know about intermolecular forces predict the solubility of this molecule in a) water and b) acetone