Intermolecular forces

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Intermolecular forces

Strengths of bonds and forces
Ionic and covalent bonds are strong.

  • Ionic bonds hold ions together in a giant lattice so that, at room temperature, all ionic compounds are solid. Ionic bonds are strong electrostatic attractions between oppositely charged ions.
  • A covalent bond holds atoms together by sharing an electron pair. Many covalent compounds exist as small molecules. The atoms in a molecule are bonded together by strong covalent bonds.

Intermolecular forces are weak.

  • Intermolecular forces act between different molecules. These forces are very much weaker than ionic or covalent bonds.
  • Intermolecular forces are caused by weak attractive forces between very small dipoles in different molecules 

There are 3 common types of intermolecular forces: Hydrogen bonds, Permanent dipole-dipole forces and van Der Waals' forces 

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Intermolecular forces

Permanent dipole-dipole interactions
Polar molecules have permanent dipoles. The permanent dipole of one molecule attracts the permanent dipole in a different molecule to form a weak permanent dipole-dipole force.
Dipole-dipole force between two HCl molecules, the H+ on one molecule attracts the Cl- on a neighbouring molecule.

van Der Waals' forces (induced dipole-dipole interactions)
van Der Waals' forces exist between all molecules. van Der Waals' forces are weak attractive forces between induced dipoles in neighbouring molecules

What cause van Der Waals' forces?

  • van Der Waals forces are caused by electrons oscillating. This movement unbalances the distribution of charge within the electron shells.
  • This causes there to be an instantaneous dipole across the molecule.
  • This instantaneous dipole induces a dipole in neighbouring molecules, which in turn induce further dipoles on their neighbouring molecules.

van Der Waals' forces increase with increasing numbers of electrons. The greater the number of electrons, the larger the induced dipoles therefore the greater the attractive forces between the molecules.

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Intermolecular forces

Hydrogen bonding
A hydrogen bond is a strong dipole-dipole interaction between:

  • an electron-deficient hydrogen atom on one molecule; and 
  • a lone pair of electrons on a highly electronegative atom on a different molecule

Molecules containing O-H and N-H bonds are polar with permanent dipoles. These dipoles are particularly strong. The permanent dipole-dipole interaction between molecules containing O-H and N-H is given a special name: a hydrogen bond.

Hydrogen bonding in water. A hydrogen bond is shown between molecules as a dashed line. Notice the role of the lone pair, which is essential in hydrogen bonding. A hydrogen bond is formed by attraction between delta positive and delta negative charges on different water molecules.

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Intermolecular forces

Special properties of water
A hydrogen bond in water has only 5% of the strength of the O-H covalent bond. However, hydrogen bonding is strong enough to have significant effects on physical properties. This results in some unexpected properties of water.

Ice is less dense than water
In almost all materials, the solid is denser than the liquid. Water is the exception, with ice being less dense than water. This is because:

  • Ice has an open lattice with hydrogen bonds holding the water molecules apart
  • When ice melts, the rigid hydrogen bonds collapse, allowing the H2O molecules to move closer together
  • So, ice is less dense than water 

Water has a relatively high melting and boiling point
There are relatively strong hydrogen bonds between H2O molecules.

  • The hydrogen bonds are extra forces, over and above van der Waals' forces
  • These extra forces have to be overcome in order to melt or boil H2O. This results in H2O having higher melting and boiling points than would be expected from just van der Waals' forces
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