3.4 Intermolecular Forces

Intermolecular Forces These forces determine the following physical properties within chemicals: –State –Melting point –Boling point –Surface tension –Hardness or texture –Solubility in solvents

Intermolecular Forces Van der Waals forces: weak intermolecular attractions that comprise of London dispersion and dipole-dipole Hydrogen bonding

A temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. It is considered a strong force. Attraction of dipoles Dipole-Dipole Force

Occurs in polar molecules, because they have positive and negative ends (dipoles) that are attracted to each other The stronger the difference in electronegativity the stronger the resulting dipole-dipole interaction Attraction of dipoles

London Dispersion Forces the simultaneous attraction of the electrons of one molecule by the positive nuclei in the surrounding molecules

London Dispersion Forces it occurs in all molecules, but is the main force in non-polar molecules the strength is directly related to the number of e- in a molecule the higher the number of electrons and protons, the stronger the London Dispersion force it is weak force compared to Dipole-Dipole forces

Hydrogen Bonding a relatively strong form of intermolecular attraction that results from a positive hydrogen atom being attracted to a highly electronegative atom such as N, O, or F

Reason for Hydrogen Bonding? Large difference in electronegativities Due to the small size of hydrogen, the positive pole is very concentrated and strongly attracts a nearby negative molecule

A Comparison of Bonds and Forces

Work Pg. 115 # 1-7