6.3 Intermolecular Forces

6.3 Intermolecular Forces

1. 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...

   • ... Induced dipole-dipole interactions / London Forces
     • 1-10 kJmol-1
   • ... Permanent dipole-dipole interactions
     • 3-25 kJmol-1
   • ... Hydrogen Bonding
     • 10-40kJmol-1

2. Intermolecular forces are responsible for physical properties such as melting/boiling points, where as covalent bonds determine the identity and chemical reactions of molecules

3. Induced dipole-dipole interactions (London Forces)

   • Weak intermolecular forces
   • exist between ALL molecules
   • They act between induced dipoles in different molecules
     • What is an induced dipole?
       • Electrons are mobile and so their movement produces a changing dipole in a molecule
         • 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
       • Induced dipole-dipoles are temporary
       • The strength of induced dipole-dipole interactions
         • 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

4. Permanent Dipole-Dipole Interactions

   • act between the permanent dipole in different polar molecules
   • Comparing Hydrogen Chloride and Fluorine
     • Molecule
       • F - F
         • none
           • yes
             • no
               • 9 x 2 = 18
                 • -220
         • 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
     • Dipole
     • London Forces
     • Permanent Dipole-dipole interactions
     • Number of Electrons
     • Boiling point/ degrees C

5. Simple Molecular Substances

   • 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
     • In the solid state = form regular structure called a simple molecular lattice
       • molecules held in place by weak intermolecular forces
       • atoms within each molecule are bonded together strongly by covalent bonds
   • Properties of simple molecular substances
     • Low melting and boiling point
       • 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
         • In a simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temperatures
         • SMS have LOW melting and boiling points
         • When a SM lattice is broken apart during melting
           • only the weak intermolecular forces break
           • the covalent bonds are strong and DO NOT break
     • Solubility
       • Covalent substances with simple molecular structures fall into two categories, polar and non-polar
         • 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
         • 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
     • Electrical Conductivity
       • No mobile charged particles in SMS
       • With no charged particles that can move, there is nothing to complete and electrical circuit
       • Therefore SMS are non-conductors of electricity

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