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Questions. Why do some solids dissolve in water but others do not? Why are some substances gases at room temperature, but others are liquid or solid? What gives metals the ability to conduct electricity? What makes non-metals brittle? The answers have to do with … . Intermolecular forces.

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  1. Questions • Why do some solids dissolve in water but others do not? • Why are some substances gases at room temperature, but others are liquid or solid? • What gives metals the ability to conduct electricity? • What makes non-metals brittle? • The answers have to do with … Intermolecular forces

  2. INTERMOLECULAR FORCES Introduction • There are 2 types of attraction in molecules: intramolecular bonds & intermolecular forces • We have already looked at intramolecular bonds (ionic and covalent bonding) • INTRAmolecular forces are the forces of attraction within a molecule or ionic compound • These are the forces that hold molecules and compounds together

  3. INTERMOLECULAR FORCES • Intermolecular forces (IMF) have to do with the attraction betweenmolecules (vs. the attraction between atoms in a molecule) • These forces are weaker than intramolecular bonding forces • There are 3 types of IMF’s (from weakest to strongest) 1) van der Waal’s 2) dipole-dipole 3) hydrogen-bonding

  4. 1. Van der Waal’s forces • Non-polar molecules do not have dipoles like polar molecules. • How, then, can non-polar compounds form solids or liquids? • What holds the molecules close to one another when in the liquid or solid state?

  5. Van der Waal’s forces • Van der Waal’s forces are due to small temporary dipoles that exist in non-polar molecules • Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical ienot evenly distributed • The resulting tiny dipoles cause attractions between neighbouring atoms/molecules

  6. vanderWaal’s forces Induced dipole: Instantaneous dipole: Eventually electrons are situated so that tiny dipoles form A dipole forms in one atom or molecule, inducing a dipole in the other

  7. What trends do you notice? http://wps.prenhall.com/wps/media/objects

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

  9. + – H Cl 2. Dipole - Dipole attractions • 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.

  10. Polar and non-polar molecules wps.prenhall.com

  11. + – + – + – + – 2. Dipole - Dipole attractions • 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.

  12. The greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction between molecules

  13. Recall: the 3 states of matter www.chem.ufl.edu

  14. 3. Hydrogen-bonding • H-bonding is a special type of dipole - dipole attraction that is very strong • NOTE: it is not a “Bond” but actually an intermolecular force of attraction between neighbouring molecules • It only occurs between molecules that have an O-H, N-H or F-H bond which are all highly polar bonds with large EN values

  15. 3. Hydrogen-bonding • Compare the EN for H-Cl and O-H bond in H2O H-Cl = 2.9-2.1 = 0.8, O-H = 3.5-2.1 = 1.4 • 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)

  16. The hydrogen-bonding attraction between molecules of water account for its high surface tension.

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

  18. Water Water has a very high melting and boiling point due to extensive hydrogen bonding. Having two hydrogens it can form two H- bonds per molecule allowing a lattice diamond-like structure to be built up, as shown below in ice. Note that each O atom is “bonded” to 4 H’s either through H-bonding or a covalent bond. This causes the EXPANSION of water when it freezes. • http://ibchem.com/IB/ibnotes/brief/bon-sl.htm

  19. www.goiit.com

  20. H – bonding and boiling point

  21. Trends in Boiling Points Boiling points increase down a group (as period increases) for two reasons: 1) EN tends to increase and 2) size increases. A larger size means greater vanderWaal’s forces. Boiling points are higher than expected for H2O, HF, and NH3 because these are capable of hydrogen-bonding with neighbouring molecules (high EN), creating very strong intermolecular forces. This makes it more difficult to move molecules apart in the evaporation process, resulting in a high boiling point.

  22. Ionic-Dipole Attractive Forces • Positive and negative ions that are present in ionic compounds will be attracted to dipoles such as those of a polar molecule like water • This “ionic-dipole” attraction between positively and negatively charged centers accounts for the solubility of most ionic compounds in polar solvents

  23. Ionic – dipole attractive forces allow ionic compounds to dissolve in a polar solvent like water faculty.clintoncc.suny.edu

  24. Ionic-Dipole Attraction

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