4.3 Intermolecular Forces There are 2 types of attractive in molecules: INTRAmolecular forces are the forces of attraction WITHIN a molecule or ionic compound. These are the forces that hold atoms and ions together in a molecule or compound.   INTERmolecular forces (IMFs) have to do with the attractive forces BETWEEN molecules. These forces are WEAKER than intramolecular forces of bonding.

Intermolecular forces hold molecules close together in the solid state, and must be overcome to move the molecules further apart as in a liquid or in a gas

4.3 Intermolecular Forces There are 3 kinds of intermolecular forces (from weakest to strongest) 1) VANDER WAAL’S FORCES (ALSO CALLED DISPERSION FORCES) - (attractions between molecules that have temporary dipoles) 2) DIPOLE-DIPOLE FORCES (attractions between molecules that have permanent dipoles) 3) HYDROGEN BONDING (attractions between molecules that contain very strong dipoles in O-H, N-H or F-H bonds)

vanderWaal’s forces vanderWaal’s forces are due to small TEMPORARY dipoles that exist in all molecules Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical ie not evenly distributed The resulting tiny dipoles cause attractions between neighbouring atoms/molecules

Instantaneous & Temporary Dipoles images.tutorvista.com

vanderWaal’s forces since all molecules have protons and electrons, ALL molecules will have vanderWaal’s forces Larger molecules have more protons and electrons so they have stronger vanderWaal’s forces (hence the increase in boiling point as carbon chain length increases among the hydrocarbons) http://www.yteach.co.uk/page.php/resources/view_all?id=intermolecular_force_matter_ dipol_polar_non_polar_dispersion_hydrogen_bonding_t_page_14&from=search

vanderWaal’s forces of nonpolar hydrocarbons affect boiling points: When comparing the boiling points of hydrocarbons (non-polar molecules), we see that the boiling point increases as the number of carbons increases. Why is this?

vanderWaal’s forces increase as mass increases: What trends do you notice? http://wps.prenhall.com/wps/media/objects

The boiling points (and melting points) of the halogens INCREASES as their MOLAR MASS increases (ie DOWN the group)

vanderWaal’s forces increase as molar mass increases The reason that the boiling points increase as you go down the group is that as the molar mass increases, the number of electrons increases, (and so also does the radius of the atom). The more electrons you have, and the more distance over which they can move, the bigger the possible temporary dipoles and therefore the stronger the vanderWaal’s forces of attraction between molecules This means MORE HEAT ENERGY is required to move the particles apart (ie change from liquid to gas, as in boiling). So boiling points RISE as molar mass RISES.

Dipole-dipole forces   We have seen that bonds within molecules can have a separation of charge that creates DIPOLE MOMENTS .   If these dipoles combine in a non-symmetrical manner, then a polar molecule results. wps.prenhall.com

Dipole-dipole forces Polar substances have a slightly electronegative end and a slightly electropositive end. Dipole-dipole forces occur when oppositely charged poles momentarily attract one another

2. Dipole - Dipole attractions 12/10/99 Neighbouring molecules are attracted to one another through dipole-dipole forces of attraction the greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction between molecules, and the higher the boiling point, because it takes more energy to move molecules apart. + – + – + – + –

H-Cl EN = 0.8 Strongest dipole H-Br EN = 0.7 H-I EN = 0.4 Highest Boiling Point H-Br EN = 0.7 H-I EN = 0.4 Weakest dipole Lowest Boiling Point

3. Hydrogen bonding Occurs only in molecules that have O-H, N-H or F-H bonds N, O and F are highly electronegative atoms and have lone pairs of electrons which are attracted to the H atoms in neighbouring molecules. These hydrogen atoms are essentially a proton because of uneven sharing of the electron pair between the H and N, O or F

Hydrogen-bonding Compare the EN for H-Cl and O-H bond in H2O The high EN of N-H, O-H, and H-F bonds cause hydrogen bonding forces to be the strongest IMF (about 5x stronger than normal dipole-dipole forces) http://www.media.pearson.com.au/schools/cw/au_sch_derry_ibcsl_1/int/hydrogenBonding/111 2.html

NOTE: the hydrogen “bond” is between molecules of water – it is not a TRUE bond!! www.landfood.ubc.ca/courses

Some unique properties of water can be explained by hydrogen bonding: its expansion when freezing, its strong surface tension and its meniscus. www.landfood.ubc.ca/courses

So What? Boiling and melting points The strength of INTERMOLECULAR FORCES of attraction between neighbouring COVALENT molecules affects: Boiling and melting points Solubility in polar or nonpolar solvents

www.goiit.com

Trends in Boiling Points Boiling points increase down a group (as period increases) because the number of electrons/molecular mass increases so vanderWaal’s forces are stronger. Boiling points are higher than expected for H2O, HF, and NH3 because these are capable of hydrogen-bonding between neighbouring molecules, creating very strong intermolecular forces. This makes it more difficult to move molecules apart in the evaporation process, resulting in a high boiling point because more energy is required.

Remember: HYDROGEN BONDING Is stronger than which are stronger than DIPOLE-DIPOLE ATTRACTIVE FORCES which are stronger than VANDERWAAL’S FORCES OF ATTRACTION BUT ALL INTERMOLECULAR FORCES ARE WEAKER THAN IONIC AND COVALENT BONDING