Intermolecular forces

• Intermolecular forces are weak interactions between dipoles of different molecules
• Intermolecular forces fall into three main categories:

  • induced dipole-dipole interactions (London forces)
  • permanent dipole-dipole interactions
  • hydrogen bonding
• Intermolecular forces are largely responsible for physical properties such as melting and boiling points, whereas covalent bonds determine the identity and chemical reactions of molecules
• Intermolecular forces from weakest to strongest:

  • London forces
  • permanent dipole-dipole interactions
  • hydrogen bonds
• covalent bonds are much stronger than intermolecular forces
• London forces are weak intermolecular forces that exist between all molecules, whether polar or non-polar. They act between induced dipoles in different molecules
• The origin of induced dipoles:

  • the movement of electrons 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 neighbouring molecules, which then attract one another
• Induced dipoles are only temporary. In the next instant of time, the induced dipoles may disappear
• Induced dipoles result from interactions of electrons between molecules. The more electrons in each molecule:

  • the larger the instantaneous and induced dipoles
  • the greater the induced dipole-dipole interactions
  • the stronger the attractive forces between molecules
• Large numbers of electrons mean larger induced dipoles. More energy is then needed to overcome the intermolecular forces, increasing the boiling point.
• Permanent dipole-dipole interactions act between the permanent dipoles in different polar molecules
• A simple molecular substance is made up of simple molecules - simple units containing a definite number of atoms with a definite molecular formula
• In the solid state, simple molecules form a regular structure called a simple molecular lattice.
• In the simple molecular lattice:

  • the molecules are held in place by weak intermolecular forces
  • the atoms within each molecule are bonded together strongly by covalent bonds
• In a simple molecular lattice, the weak intermolecular forces can be broken even by 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 intermolecular forces break
  • the covalent bonds are strong and do not break
• The solubility of non-polar substances is easier to predict than polar substances
• When a simple molecular compound is added to a non-polar solvent like 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 simple molecular forces 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. Therefore, non-polar simple molecular substances tend to be insoluble in polar solvents
• Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other. The solubility depends on the strength of the dipole and can be hard to predict. Some compounds like ethanol contain both polar and non-polar 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 that can interact with water. The hydrophobic part will be non-polar and comprised of a carbon chain
• There are no mobile charged particles in simple molecular structures. With no charged particles that can move, simple molecular structures are non-conductors of electricity