Intermolecular forces

• Intermolecular forces

  They 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

  The first two categories are also called Van der Walls. Hydrogen bonds are the strongest and London forces are the weakest.

  Intermolecular forces are largely responsible for physical properties like melting and boiling points.
• London Forces

  These are weak intermolecular forces that exist between all molecules. They act between induced dipoles in different molecules. Induced dipoles are only temporary. They 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

  This leads to more energy being needed to overcome the intermolecular forces in molecules with more electrons which increases the boiling point
• Origins of London dispersion forces, induced dipole-dipole interactions

  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
• Permanent dipole-dipole interactions

  Act between the permanent dipoles in different polar molecules.

  More energy is required to break these interactions which results in molecules having a higher boiling point.
• Simple molecular substances

  They are made up of simple molecules—small units containing a definite number of atoms and a definite molecular formula, such as neon, hydrogen, water, and carbon dioxide.

  In a solid state, simple molecules form a regular structure called a simple molecular lattice, in this lattice:

  • the molecules are held together by weak intermolecular forces
  • the atoms within each molecule are held together by strong covalent bonds
• Property of simple molecular substances: Low melting and boiling points

  All simple molecular substances are covalently bonded. So at RTP, they may exist as solids, liquids or gases. They can be solidified into lattices by reducing temperature.

  • Simple molecular substances have low melting and boiling points as they require little energy to break the weak intermolecular forces
• Solubility of non-polar simple molecular substances in non-polar solvent

  When a simple molecular compound is added to a non-polar solvent, intermolecular forces form between the molecules and solvent. These interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves. So non-polar simple molecular substances tend to be soluble in non-polar solvents
• Solubility of non-polar simple molecular substances in a polar solvent

  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. So simple molecular substances 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 solubility can extend to liquids and gases like hydrogen chloride, a polar gas and is extremely soluble in water forming hydrochloric acid.

  The solubility depends on the strength of the dipole, some compounds contain both polar and non-polar parts in their structure so can dissolve in both polar and non-polar solvents.
• Electrical conductivity

  There are no mobile charged particles in simple molecular structures. As there are no charged particles that can move, there is nothing to complete an electrical circuit. Therefore simple molecular structures are not conductors of electricity.
• Hydrogen bonding- stongest intermolcular force

  A type of permanent dipole-dipole interaction found between molecules, that contain:

  • an electronegative atom that has a lone pair of electrons- Oxygen, Nitrogen, Fluorine
  • a hydrogen atom attached to the electronegative atom

  The hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule.
• Property of water due to hydrogen bonds- Density

  Hydrogen bonds hold water molecules apart in an open lattice. The water molecules in ice are further apart than in water so ice is less dense than liquid water so ice floats on water. This forms an insulating layer which prevents the water from freezing solid. Oxygen has two lone pairs and two hydrogen atoms, so can form four hydrogen bonds. This makes them form an open tetrahedral lattice full of holes with a 180° bond angle. When ice melts the lattice collapses and the molecules move closer together and becomes denser.
• Property of water due to hydrogen bonds- High melting/boiling point

  Water has hydrogen bonds as well as London forces so more energy is needed to break the hydrogen bonds in water, so water has a much higher melting and boiling points than would be expected from just London forces. When the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken down. When water boils, the hydrogen bonds break completely.