Intermolecular forces

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A Level powerpoint on the different types of intermolecular forces

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Intermolecular forces

  1. 1. Intermolecular Forces Ms L Chu Sunday 17 October 2010
  2. 2. Objectives <ul><li>To know what intermolecular forces are. </li></ul><ul><li>To be able to name the type of intermolecular forces. </li></ul><ul><li>To be able to describe the effects of these forces on the properties of molecules </li></ul><ul><li>To be to provide examples of molecules from these forces. </li></ul>
  3. 3. Intermolecular Forces <ul><li>These are the attractive forces between </li></ul><ul><li>molecules. They are generally weaker than </li></ul><ul><li>covalent bonds. </li></ul>
  4. 4. Types of intermolecular forces 1) Permanent dipole – dipole forces 2) Van der Waals’ forces 3) Hydrogen bonding
  5. 5. Permanent dipole-dipole forces <ul><li>Occurs between molecules with permanent dipoles. </li></ul><ul><li>Permanent dipoles are formed when there is a large difference in electronegativity between two atoms bonded together in a covalent bond. </li></ul>Green lines represent the dipole-dipole forces Increasing polarity of the molecules increases the strength of attraction
  6. 6. Van der Waals’ Forces <ul><li>Intermolecular forces between non-polar molecules </li></ul><ul><li>and atoms. </li></ul><ul><li>They are weak forces of attraction, independent of </li></ul><ul><li>normal bonding forces between molecules. They </li></ul><ul><li>are caused by the formation of temporary dipoles </li></ul><ul><li>due to constant movement of electrons. </li></ul>Evidence : Noble gases are monoatomic, existing as single atoms in the gaseous phase at room temperature.
  7. 7. <ul><li>However, these can be condensed to liquids or </li></ul><ul><li>solids when the temperature is decreased. </li></ul><ul><li>Therefore, this suggests the existence of </li></ul><ul><li>intermolecular forces which hold the molecules </li></ul><ul><li>together in solid and liquid phases. </li></ul>How do these arise? Electrons are in constant motion. So at any instant there is more negative charge on one side of the molecule than other and so possesses an instantaneous electric dipole.
  8. 8. This then induced dipoles in neighbouring molecules causing weak induced dipole-induced dipole attractions between the molecules. <ul><li>These induced dipoles will act one way then </li></ul><ul><li>another and continually arise and disappear as a </li></ul><ul><li>result of electron movement. </li></ul><ul><li>The average dipole on every molecule over a </li></ul><ul><li>period of time is 0, resultant forces between the </li></ul><ul><li>molecules at any instant are not 0. </li></ul>As the size of the atoms or molecules increases there are more electrons so the temporary partial charge is bigger resulting in stronger attraction
  9. 9. Average charges are spread evenly At any instance the electrons are concentrated on one side of the atom or molecule more than the other.  This gives the atom or molecule a temporary partial negative charge - a temporary dipole moment. This dipole moment will induce a temporary dipole in a neighboring atom by attracting/repelling its electron charge cloud.    A fraction of a second later the electron distribution changes causing and the temporary dipole-dipole attraction ( Van der Waals attraction) to break.  These forces are weaker than hydrogen bond and permanent dipole forces.
  10. 10. Hydrogen bonding <ul><li>Strongest type of intermolecular forces. </li></ul><ul><li>Occurs in molecules with very polar bonds involving hydrogen : </li></ul><ul><li>H-F, H-O, H-N </li></ul><ul><li>The Hydrogen atom has no inner shells of electrons. So the nucleus is exposed by the shift in the electron density within the bond making it easily accessible for strong permanent dipole-permanent dipole interactions to occur. </li></ul>
  11. 11. <ul><li>It is simply the attraction between the positively charged hydrogen and the negatively charged lone pair electrons . </li></ul>H O H H H H O O O H O H H H H H-bond Here the lone pair of electrons on oxygen is attracted to the positive nucleus of the hydrogen atoms forming H-bond. Each water molecule can form 2 h-bonds. How many H-bond can HF forms?
  12. 12. Essential requirements for an H-bond are: <ul><li>A hydrogen atom attached to a highly electronegative atom; </li></ul><ul><li>Unshared pair of electrons on the electronegative atom. </li></ul>Water Liquid: water has a higher boiling point than most other hydrides (see graph) due the H-bonds present in the molecules. Greater energy is required to break these strong bonds.
  13. 13. <ul><li>Water has a high surface tension due to H- </li></ul><ul><li>bonded network of water molecules at the </li></ul><ul><li>surface. This network is strong enough to hold </li></ul><ul><li>a needle on the surface of the water. </li></ul>Ice: a three-dimensional hydrogen-bonded lattice. In this lattice each oxygen is surrounded by a tetrahedron of hydrogen atoms bonded to other oxygen atoms. Ice has unusual properties in that it has a lower density than liquid water (water has a maximum density is at 4 0 C).
  14. 14. Structure of ice Four hydrogen bonds, two through its hydrogen atoms and two through its oxygen atoms hold each water molecule in place
  15. 15. <ul><li>On melting, many hydrogen bonds are broken </li></ul><ul><li>and the structure collapses to give a lower </li></ul><ul><li>volume liquid.  Water expands on freezing so </li></ul><ul><li>ice floats as it has a lower density than water. </li></ul>

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