6.3 Intermolecular Forces

Covalent bonding is strong and holds the atoms in a amolecule together. INTERMOLECULAR forces are weak interactions between dipoles of different molecules. Intermolecular forces fall into three main categories...
1. ... Induced dipole-dipole interactions / London Forces
  1. 1-10 kJmol-1
2. ... Permanent dipole-dipole interactions
  1. 3-25 kJmol-1
3. ... Hydrogen Bonding
  1. 10-40kJmol-1
Intermolecular forces are responsible for physical properties such as melting/boiling points, where as covalent bonds determine the identity and chemical reactions of molecules
Induced dipole-dipole interactions (London Forces)
1. Weak intermolecular forces
2. exist between ALL molecules
3. They act between induced dipoles in different molecules
  1. What is an induced dipole?
    1. Electrons are mobile and so their movement produces a changing dipole in a molecule
      1. At any instant, an instantaneous dipole will exist, but its position is constantly shifting
        The instantaneous dipole INDUCES a dipole on a neighbouring molecule
        The induced dipole induces further dipoles on neighbourin gmolecules, which then attract one another
    2. Induced dipole-dipoles are temporary
    3. The strength of induced dipole-dipole interactions
      1. The more electrons in each molecule, the larger the instantaneous and induced dipole, this means the induced dipole-dipole interactions are greater and the attractive forced between the molecules are stronger.
        Larger numbers of electrons means larger induced dipoles, so, more energy is needed to overcome the intermolecular forces, increasing the BOILING POINT
Permanent Dipole-Dipole Interactions
1. act between the permanent dipole in different polar molecules
2. Comparing Hydrogen Chloride and Fluorine
  1. Molecule
    1. F - F
      1. none
        yes
        no
        9 x 2 = 18
        -220
      2. H-Cl
        H s+ -- Cl s-
        yes
        yes
        1+17 = 18
        -85
        - Fluorine molecules are non-polar and only have London Forces between molecules
        - Hydrogen chloride molecules are polar and have London forces AND permanent dipole-dipole interactions between molecules
        - Extra energy is needed to break the additional permanent dipole-dipole interactions between hydrogen chloride molecules
        - Therefore the boiling point of Hydrogen Chloride is higher than Fluorine
  2. Dipole
  3. London Forces
  4. Permanent Dipole-dipole interactions
  5. Number of Electrons
  6. Boiling point/ degrees C
Simple Molecular Substances
1. A simple molecular substance is made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula, such as neon, Ne, hydrogen, H2, water H2O and carbon dioxide CO2
  1. In the solid state = form regular structure called a simple molecular lattice
    1. molecules held in place by weak intermolecular forces
    2. atoms within each molecule are bonded together strongly by covalent bonds
2. Properties of simple molecular substances
  1. Low melting and boiling point
    1. At room temperature, simple molecular substances may exist as solids, liquids or gases. All SMS can be solidified into simple molecular lattices be reducing the temperature
      1. In a simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temperatures
      2. SMS have LOW melting and boiling points
      3. When a SM lattice is broken apart during melting
        only the weak intermolecular forces break
        the covalent bonds are strong and DO NOT break
  2. Solubility
    1. Covalent substances with simple molecular structures fall into two categories, polar and non-polar
      1. Solubility of non-polar simple molecular substances
        When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent
        The interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves
        Therefore, non-polar SMS tend to be soluble in non-polar solvents
        When a SMS is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules
        The intermolecular bonding within the polar solvent is too strong to be broken
        Therefore SMS tend to be insoluble in polar solvents
      2. Solubility of polar SMS
        Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other
        Process is similar to dissolving an ionic compound
        Solubility depends on the strength of the dipole and can be hard to predict. Some compounds such as ethanol, C2H5OH, contain both polar (the O-H) and non-polar (the C chain) parts in their structure and can dissolve in both polar and non-polar solvents
  3. Electrical Conductivity
    1. No mobile charged particles in SMS
    2. With no charged particles that can move, there is nothing to complete and electrical circuit
    3. Therefore SMS are non-conductors of electricity

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6.3 Intermolecular Forces

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	Created by Bee Brittain about 8 years ago