Intermolecular Forces

• Intermolecular forces are weak interactions between dipoles of different molecules and fall into three main categories
• What are the three main categories of IM forces?

  1. induced dipole-dipole interactions (London forces)
  2. permanent dipole-dipole interactions
  3. hydrogen bonding
• IM forces are largely responsible for physical properties such as melting and boiling points, whereas covalent bonds determine the identity and chemical reactions of molecules
• London forces?

  1-10 bond enthalpy/ kJ mol-1
• permanent dipole-dipole interactions?

  3-25 bond enthalpy/ kJ mol-1
• hydrogen bonds?

  10-40 bond enthalpy/ kJ mol-1
• single covalent bonds?

  150-500 bond enthalpy/ kJ mol-1
• London forces are weak intermolecular forces that exist between all molecules, whether they are polar or non-polar. They act between induced dipoles in different molecules
• The origin of induced dipoles:

  1. movement of electrons produces a changing dipole in a molecule
  2. at any instant, an instantaneous dipole will exist, but its position is constantly shifting
  3. the instantaneous dipole induces a dipole on a neighbouring molecule
  4. the induced dipole induces further dipoles on neighbouring molecules, which then attract one another
• Induced dipoles are only temporary, in the next instant of time, the induced dipoles may disappear, only for the whole process to take place amongst other molecules.
• Induced dipoles result from interactions of electrons between molecules.
• The more electrons in each molecule:

  1. the larger the instantaneous and induced dipoles
  2. the greater the induced dipole-dipole interactions
  3. the stronger the attractive forces between molecules
• Larger numbers of electrons mean larger induced dipoles. More energy is then needed to overcome the IMF, increasing the boiling point
• Permanent dipole-dipole interactions act between permanent dipoles in different polar molecules.
• For example:

  molecules of hydrogen chloride and fluorine have the same number of electrons and the same shape, so the strength of the London forces in both should be very similar
• BUT:

  1. fluorine molecules are non-polar and only have London forces between molecules
  2. hydrogen chloride molecules are polar and have London forces and permanent dipole-dipole interactions between molecules
  3. extra energy is needed to break the additional permanent dipole-dipole interactions between hydrogen chloride molecules
  4. the boiling point of hydrogen chloride is therefore higher than fluorine
• A simple molecular structure is made up of simple molecules - small units containing a definite number of atoms, with a definite molecular formula
• In the solid state, simple molecular molecules form a regular structure called a simple molecular lattice
• In a simple molecular lattice:

  1. the molecules are held in place by weak intermolecular forces
  2. the atoms within each molecule are bonded together by strong covalent bonding
• Simple molecular substances have low melting and boiling points: all simple molecular structures are covalently bonded. At room temperature, they may exist as solids, liquids or gases. All simple molecular substances can be solidified into simple molecular lattices by reducing the temperature
• In a simple molecular lattice, the weak IMF can be broken by even the energy present at low temperatures. Simple molecular substances have low melting and boiling points.
• When a simple molecular lattice is broken apart during melting, only the weak IMF break, the covalent bonds are strong and do not break
• Covalent substances with simple molecular structures fall into 2 categories: polar and non-polar
• Solubility of non-polar simple molecular substances: When a simple molecular compound is added to a non-polar solvent, e.g. hexane, IMF form between the molecules and the solvent. The interactions weaken the IMF in the simple molecular lattice - the IMF break and the compound dissolves
• Non-polar simple molecular substances tend to be soluble in non-polar solvents
• When a simple molecular substance 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
• Simple molecular solvents tend to be insoluble in polar solvents
• Solubility of polar simple molecular substances: polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other. This process is similar to dissolving an ionic compound
• The solubility depends on the strength of the dipole and can be hard to predict. Some compounds e.g. ethanol, contain both polar (O-H) and non-polar (carbon chain) parts in their structure and can dissolve in both polar and non-polar solvents
• Some biological molecules have hydrophobic and hydrophilic parts. The hydrophilic part will be polar and contain electronegative atoms (usually oxygen) and can interact with water. The hydrophobic part will be non-polar and comprised of a carbon chain
• There are no mobile charged particles in a simple molecular structure. With no charged particles that can move, there is nothing to complete an electrical circuit. Therefore simple molecular structures are non-conductors of electricity.