INTERMOLECULAR FORCES What Holds Molecules to Each Other

Intermolecular Forces l Intermolecular forces are forces of attraction. l They are what make solid and liquid molecular compounds possible. l The three intermolecular forces are hydrogen bonds, dipole–dipole forces and London dispersion forces.

Dipole-dipole Forces l Polar molecules contain permanent dipoles; that is, some regions of a polar molecule are always partially negative and some regions of the molecule are always partially positive. Attractions between oppositely charged regions of polar molecules are called dipole–dipole forces.

Dipole-dipole Forces l Neighboring polar molecules orient themselves so that oppositely charged regions line up.

Dipole-dipole Forces l Opposites attract but are not completely hooked as in ionic solids. HFHF  HFHF 

Dipole-dipole Forces    

Dipole-dipole Forces l Dipole–dipole forces are stronger than dispersion forces as long as the molecules being compared have approximately the same mass. l Examples of compounds that exhibit dipole-dipole forces include CO, HCl, and PH 3.

Hydrogen Bonding l A hydrogen bond is a dipole–dipole attraction that occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom with at least one lone electron pair.

Hydrogen Bonding l For a hydrogen bond to form, hydrogen must be bonded to a fluorine, oxygen, or nitrogen atom. l F, O, and N are very electronegative so it is a very strong dipole.

Hydrogen Bonding l Hydrogen bonding is the strongest of the intermolecular forces. l Examples include H 2 O, NH 3, and HF.

Hydrogen Bonding H H O ++ -- ++ H H O ++ -- ++

H H O H H O H H O H H O H H O H H O H H O

London Dispersion Forces l Dispersion forces are weak forces that result from temporary shifts in the density of electrons in electron clouds.

London Dispersion Forces l Remember that the electrons in an electron cloud are in constant motion. l When two nonpolar molecules are in close contact, especially when they collide, the electron cloud of one molecule repels the electron cloud of the other molecule.

London Dispersion Forces l The electron density around each nucleus is, for a moment, greater in one region of each cloud. l Each molecule forms a temporary dipole.

London Dispersion Forces l When temporary dipoles are close together, a weak dispersion force exists between oppositely charged regions of the dipoles. l Due to the temporary nature of the dipoles, dispersion forces are the weakest intermolecular force.

London Dispersion Forces l All molecules experience LDFs. l Dispersion forces are the only IMFs that exist between noble gases and molecules that are nonpolar. l Examples include Ar, Cl 2, Br 2, CH 4, and CO 2.

London Dispersion Forces l Dispersion forces depend on the number of electrons. l Bigger molecules result in more electrons, and more electrons mean stronger forces.

London Dispersion Forces l C 2 H 6 (30 electrons) has stronger dispersion forces than CH 4 (16 e - ).

London Dispersion Forces l This difference in dispersion forces explains why fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature. l Iodine is larger and more polarizable than bromine. Bromine is larger and more polarizable than chlorine and fluorine.

Intermolecular Forces l To determine what type of intermolecular force contributes most to a compound, ask yourself the following questions.  Does the molecule contain hydrogen attached to N, O, or F with a lone pair?  If yes, the dominant force is hydrogen bonding.

Intermolecular Forces  Determine the Lewis structure and molecular geometry for the molecule.

Intermolecular Forces  Is the molecule polar? i.e. does the molecule have polar bonds and 3-D asymmetry?  If yes, the dominant force is dipole- dipole.

Intermolecular Forces  Is the molecule nonpolar? i.e. does the molecule neither contain polar bonds nor contain 3-D asymmetry?  If yes, the dominant force is dispersion.  Remember all compounds experience London Dispersion Forces

Problem l Determine the dominant type of intermolecular force in each of the following molecules. l BCl 3 dispersion l Xe dispersion

Problem l Determine the dominant type of intermolecular force in each of the following molecules. l NH 3 hydrogen bonding l CH 4 dispersion

Problem l Determine the dominant type of intermolecular force in each of the following molecules. l SO 2 dipole-dipole lH2lH2 dispersion

Problem l Determine the dominant type of intermolecular force in each of the following molecules. l SO 3 dispersion l CH 3 Cl dipole-dipole

Problem l Determine the dominant type of intermolecular force in each of the following molecules. l HF hydrogen bonding l HBr dipole-dipole