1. Chemistry 20 3.4 Energy Changes and Intermolecular Bonds

2. Van der Waals Forces In 1873 Johannes van der Waals came up with a theory as to why gases condense when cooled. In 1873 Johannes van der Waals came up with a theory as to why gases condense when cooled. He thought that there must be some forces exerted amongst the molecules in the gas. He thought that there must be some forces exerted amongst the molecules in the gas. These forces are referred to as van der Waals forces. It is now believed that these forces are a combination of many intermolecular forces. These forces are referred to as van der Waals forces. It is now believed that these forces are a combination of many intermolecular forces.

3. Strength of Chemical Bonds Ionic Bonds are very strong. Temperatures in the range of 400- 800ºC are required to melt them. Ionic Bonds are very strong. Temperatures in the range of 400- 800ºC are required to melt them. Molecular substances have lower melting and boiling points, but covalent bonds will not break apart in spite of high temperatures. Molecular substances have lower melting and boiling points, but covalent bonds will not break apart in spite of high temperatures.

4. Strength of Chemical Bonds cont Covalent bonds are stronger than ionic bonds. Covalent bonds are stronger than ionic bonds.

5. Intermolecular Forces Prefix inter means between. Eg. Interschool competition. Competition between schools. Prefix inter means between. Eg. Interschool competition. Competition between schools. Prefix intra means within. Eg. Intramural competition is within the school. Carstairs is not invited. Prefix intra means within. Eg. Intramural competition is within the school. Carstairs is not invited.

6. Intramolecular Forces - within the molecule. Intramolecular Forces - within the molecule.  Ionic Bonds - within the crystal lattice.  Covalent Bonds - within a molecule

7. Intermolecular Forces - between molecules. Intermolecular Forces - between molecules.  Three types: London, Dipole-Dipole, and the Hydrogen Bond.

8. London Forces relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule.OO OO

9. OOOO + + + + relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule. London Forces

10. London Force OOOO + + + + relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. relatively weak attractive forces that get stronger as the number of electrons in the molecule increase. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule. the nucleus of an atom within a molecule is attracted to the electrons from an atom in an adjacent molecule. London Forces

11. Relationship of Strength of London Forces to Number of Electrons HalogenNo. Electrons per Molecule B.P. (ºC) fluorine 18-188 chlorine 34-34.6 bromine 7058.8 iodine 106184 increasing strength of London forces

12. Dipole-Dipole Forces when two non-metals with differing electronegativities form a covalent bond, it will be a polar bond. when two non-metals with differing electronegativities form a covalent bond, it will be a polar bond. unequal sharing of electrons unequal sharing of electrons molecules with polar bonds may become polar molecules with a partially negative end and a partially positive end. molecules with polar bonds may become polar molecules with a partially negative end and a partially positive end.

13. Dipole-Dipole Forces cont. attraction between positive end of one molecule and the negative end of an adjacent molecule is a dipole-dipole force. attraction between positive end of one molecule and the negative end of an adjacent molecule is a dipole-dipole force.

14. Polar Covalent Bond hydrogen’s electron will spend more time around the chlorine nucleus than around the hydrogen nucleus. Hydrogen has an electronegativity of 2.1 and chlorine has an electronegativity of 3.0. As a result,

15.                      Dipole-Dipole Forces Dipole-dipole forces result from the attraction between the positive end of one molecule and the negative end of an adjacent molecule.

16. Dipole-Dipole Forces Dipole-dipole forces result from the attraction between the positive end of one molecule and the negative end of an adjacent molecule.

17. Dipole-Dipole Forces Dipole-dipole forces result from the attraction between the positive end of one molecule and the negative end of an adjacent molecule.

18. Dipole-dipole Forces D-D Forces have an impact on solubility. D-D Forces have an impact on solubility. The saying“like dissolves like” means that polar solutes dissolve in polar solvents and nonpolar solutes dissolve in nonpolar solvents. The saying“like dissolves like” means that polar solutes dissolve in polar solvents and nonpolar solutes dissolve in nonpolar solvents.

19. Dipole-dipole Forces cont. Eg. oil based paint dissolves in turpentine. Eg. oil based paint dissolves in turpentine. Water does not take out oil stains without a detergent. Water does not take out oil stains without a detergent.

20. Determine if a molecule is polar or non-polar --- the polar molecules have dipole-dipole attraction Draw the structure Draw the structure Determine the direction of attraction of electrons -- electronegativities Determine the direction of attraction of electrons -- electronegativities Determine if there is a partial negative and a partial positive side. Determine if there is a partial negative and a partial positive side. If the molecule is polar it will have dipole-dipole attraction If the molecule is polar it will have dipole-dipole attraction

21. Hydrogen Bonding is a strong dipole-dipole force which occurs between a molecule that contains hydrogen and a molecule that contains oxygen, nitrogen, or fluorine. is a strong dipole-dipole force which occurs between a molecule that contains hydrogen and a molecule that contains oxygen, nitrogen, or fluorine. H-bonds explain the high boiling point of water, ammonia, and hydrogen fluoride. H-bonds explain the high boiling point of water, ammonia, and hydrogen fluoride. H-bonds are the reason that ice becomes less dense after it freezes. H-bonds are the reason that ice becomes less dense after it freezes.

22. Relative Strengths of Attraction 1.Intramolecular Forces 1Covalent Bonds 2Ionic Bonds 2.Intermolecular Forces 1Hydrogen Bonds 2Dipole Dipole Forces 3London Forces* Becoming Weaker

23. Relative Strengths of Attraction 1.Intramolecular Forces 1Covalent Bonds 2Ionic Bonds 2.Intermolecular Forces 1Hydrogen Bonds 2Dipole Dipole Forces 3London Dispersion Forces* Becoming Weaker Because London Forces increase in strength as the size of the molecule gets larger, LDF are sometimes stronger than D-D Forces.

24. Assignment 105-118 105-118 Read, notes, Practice problems and Section review Read, notes, Practice problems and Section review