Intermolecular Forces I

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1 I How does the arrangement of atoms differ in the 3 phases of matter (solid, liquid, gas)? Why doesn t ice just evaporate into a gas? Why does liquid water exist at all? There must be some force between the water molecules that hold them together as a liquid! This force prevents them from separating and becoming a gas Forces between molecules, atoms, or ions are known as intermolecular forces There are several types of intermolecular forces Forces within molecules are known as intramolecular forces Ionic bonds Covalent bonds Metallic bonds All types of intermolecular forces are collectively known as Van der Waals (VDW) forces 1

2 In water, this force is largely due to the polar nature of the molecule Recall that water is polar due to it s: Polar bonds Molecular asymmetry There are 2 classes of Van der Waals forces 1) Forces due to permanent dipoles A permanent dipole is polar all of the time 2) Forces due to temporary dipoles A temporary dipole is polar some of the time Permanent dipoles Dipole-Dipole attractions Dipole-Ion attractions Hydrogen bonding Temporary dipoles London Dispersion Forces The strength of the Van der Waals force is primarily responsible for the temperature at which substances melt and boil Strongly attracting molecules will cause a substance to have a high melting/boiling point Another factor that determines the melting and boiling point is the mass of the substance All other factors being equal, the higher the mass of the molecules, the stronger the forces and the higher the melting and boiling point 2

3 As the mass increases (Cl < Br < I) the boiling point of the molecules increase (HCl < HBr < HI) Covalently bonded substances often do not share their electrons equally (polar covalent bonds) Therefore, dipoles are created at the level of the bond Molecular dipoles result when these molecules are not symmetric in 3-D Oppositely-charged ends of different molecules will attract each other! This is called a dipole-dipole attraction This attraction is called dipole-dipole because each of the molecules exist as a permanent dipole The charged ends of the molecules always exist due to the combination of: polar covalent bonds a lack of molecular symmetry Dipole-Ion Attraction The positive end of a molecular dipole will be attracted to a negative ion (anion) The negative end of a molecular dipole will be attracted to a positive ion (cation) Hydrogen bonding is a special type of Van der Waals force, and is primarily responsible for water existing as a liquid on Earth Hydrogen bonding is an extreme form of Dipole-Dipole attractions 3

4 The only time hydrogen bonding can occur is when H is covalently bonded to any of 3 highly electronegative elements: F O N The hydrogen bond is the Dipole-Dipole attraction between the: H (δ + ) covalently bonded to F, O, or N An unshared e - pair (δ - ) on a nearby electronegative element (in another molecule) There are substances that appear to violate the general rule of increasing melting/boiling point with increasing mass We can see that each compound formed from a period 2 element has an unusually high boiling point. Why? London Dispersion Forces London dispersion forces are temporary dipole forces between molecules that are nonpolar Molecules such as Ne, O 2, and CH 4 exist as solids and liquids under certain conditions. Why? 4

5 London Dispersion Forces Although the molecules are nonpolar, their e - are not equally distributed at any single instant! Since this is true for all molecules, all substances exhibit London dispersion forces to some degree! Relative Strength The strength of Van der Waals forces are much weaker than that of conventional intramolecular bonding Why do you think that VDW forces are so important? There are a lot of them! Relative Strength Strongest Covalent Bonds Ionic Bonds Metallic Bonds All VDW forces except H Bonds H Bonds Weakest 5