Lecture 8.4c- Intermolecular Forces


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Section 8.4 lecture (part C) for Honors & Prep Chemistry

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  • Diamond is a network-solid form of carbon. Diamond has a three-dimensional structure, with each carbon at the center of a tetrahedron.
  • Lecture 8.4c- Intermolecular Forces

    1. 1. Bellwork- Polar ammonia <ul><li>1)Draw a 3D sketch of ammonia (NH 3 ) </li></ul><ul><li>2)What is the shape of this molecule? </li></ul><ul><li>3)Why does the molecule have this shape? </li></ul><ul><li>4)Show the bond and molecule polarity on the sketch. </li></ul><ul><li>5)Would this molecule be polar if it was planar? </li></ul>Know your pet molecules! CH 4 NH 3 H 2 O CO 2 CH 2 O I may replace H’s with Cl or F to make polar bonds!
    2. 2. Intermolecular forces are forces between two or more molecules
    3. 3. Intermolecular forces <ul><ul><ul><ul><li>Intermolecular attractions are weaker than either ionic or covalent bonds. </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>These attractions determine whether a molecular compound is a gas, a liquid, or a solid at a given temperature. </li></ul></ul></ul></ul></ul>
    4. 4. Intermolecular Forces
    5. 5. dipole-dipole interactions occur between polar molecules London Dispersion Forces occur between all molecules (polar and non-polar) There are two types of intermolecular forces
    6. 6. The partial charges in polar molecules cause attractions and repulsions between separate polar molecules A hydrogen bond is a specific type of dipole-dipole interaction that is extra strong DIPOLE-DIPOLE INTERACTIONS
    7. 7. HYDROGEN BOND A molecule can H-bond if it contains hydrogen bound to an atom with high electronegativity (N, O, or F)
    8. 8. IN THIS CASE- The especially strong partial charge ( δ + ) on hydrogen makes it “stick” to anything negative. Hydrogen bonding
    9. 9. No one in the corner grabs electrons like us! – N, O, & F N O F
    10. 10. London Dispersion Forces are the weakest of all IMFs. They occur between all molecules.
    11. 11. London Dispersion Forces A covalent molecule consists of atoms bound by overlapping electron clouds. Sometimes these electron clouds have temporary “thin or thick spots” causing temporary partial charges.
    12. 12. These temporary partial charges can cause temporary partial charges on other molecules or atoms. London Dispersion Forces
    13. 13. Temporary partial charges experience attractions and repulsions just like permanent partial charges and ion charges. London Dispersion Forces
    14. 14. The larger the molecule the larger the London Dispersion Forces
    15. 15. What is the difference between the partial charges on a polar molecule (which cause dipole-dipole interactions) and the partial charges that cause London Dispersion attractions?
    16. 16. Getting weak
    17. 17. The stronger the intermolecular forces the more likely the compound will be a solid at room temperature.
    18. 18. Compounds with weak intermolecular forces have low boiling and melting points. Strong intermolecular forces require a lot of energy to overcome, so compounds with strong intermolecular forces have high melting and boiling points. Liquids with strong intermolecular forces will hold on to their molecules preventing them from escaping as a vapor.
    19. 19. <ul><li>Molecular Solid- Strong intermolecular forces hold molecules in a rigid & regular pattern. </li></ul><ul><li>Molecular Liquid- Some intermolecular forces are overcome and molecules can slip past each other. </li></ul><ul><li>Molecular gas- Weak intermolecular forces cant hold molecules together, so each molecule is free and independent. </li></ul>states of matter Gases have weak intermolecular forces that were easy to break free of!
    20. 20. Intermolecular forces Weakest lower melting point Dispersion forces Dipole-dipole interactions Hydrogen bonds STRONGEST higher melting point Ionic compounds have even higher melting points because the ionic bond must be overcome to free the ions. Ionic bonds are much stronger than any intermolecular force. Intermolecular Forces
    21. 21. <ul><li>Network solid (Network crystal)- atoms are held together in a ordered pattern by strong covalent bonds. </li></ul><ul><ul><li>Melting a network solid would require breaking covalent bonds throughout the solid. </li></ul></ul>
    22. 22. <ul><ul><ul><li>Diamond is a network solid. </li></ul></ul></ul><ul><ul><ul><li>Diamond does not melt. </li></ul></ul></ul><ul><ul><ul><li>It vaporizes to a gas at 3500°C or above. </li></ul></ul></ul>
    23. 23. TYPES OF SOLIDS ionic solids – made of ions held together by ionic bonds molecular solids - made of molecules held together by intermolecular forces metallic solids - atoms held together by metallic bonds(sea of electrons model) network solids - atoms held together by covalent bonds
    24. 24. Classwork <ul><li>Fill in the table </li></ul>Intermolecular force Molecules that experience it Relative strength dispersion forces dipole-dipole interactions hydrogen bonds -N-H » F -H » -O-H »
    25. 25. Classwork <ul><li>Fill in the table </li></ul>Intermolecular force Molecules that experience it Relative strength dispersion forces Non-polar molecules weak dipole-dipole interactions Polar molecules stronger hydrogen bonds -N-H » F -H » -O-H » Polar molecules with N, O or F bound to H Strongest of all intermolecular forces
    26. 26. Would you expect the following solids to have a relatively high or low melting point? Metal Ionic solid Non-polar molecular solid Polar molecular solid Atomic solid of a noble gas Network solid