Intermolecular Forces Chemistry 11 Ms. McGrath

Intermolecular Forces The forces that bond atoms to each other within a molecule are called intramolecular forces. Covalent bonds and ionic bonds are an example of intramolecular forces. In comparison, the forces that bond molecules to each other are called intermolecular forces.

Intermolecular Forces Intermolecular forces are generally weaker than ionic or covalent bonds. For example, only 16 kJ/mol is required to overcome the intermolecular attractions between HCl molecules. In contrast, the energy required to break the covalent bond to dissociate HCl into H and Cl atoms is 431 kJ/mol.

Intermolecular Forces Intermolecular forces were studied extensively by Johannes van der Waals (1837 – 1923) and they are therefore often referred to as van der Waals forces.

Intermolecular Forces Dipole – Dipole Forces  Dipole - polar molecule  Dipoles will change their direction so that their oppositely charged ends are near to one another.  The electrostatic attraction between the oppositely charged ends of the polar molecules are called dipole -dipole forces.

Intermolecular Forces Dipole – Dipole Forces  As a result of these dipole – dipole forces of attraction, polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would.  The strength of these dipole-dipole forces and the amount of energy required to separate them is indicated by the temperature at which the substance changes phases – its melting point and its boiling point.

Intermolecular Forces Dispersion (London) Forces  weakest intermolecular force between non polar molecules  It is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles

Intermolecular Forces Dispersion (London) Forces  The shared pairs of electrons in a covalent bond are constantly vibrating.  The bond vibrations, which are part of the normal condition of a non-polar molecule, cause momentary, uneven distribution of charge.

Intermolecular Forces Dispersion (London) Forces  In other words, a non-polar molecule becomes slightly polar for an instant, and continues to do this on a random but on- going basic. At the instant that one non-polar molecule is in a slightly polar condition, it is capable of inducing a dipole in a nearby molecule.

Intermolecular Forces Dispersion (London) Forces  An intermolecular force of attraction results.  The degree of force is related to the number of electrons as well as the size and shape of the molecule – larger molecules tend to display stronger London dispersion forces.

Intermolecular Forces Hydrogen Bonding  Is a special type of intermolecular attraction that exists between the H atom in a polar bond (particularly H-F, H-O, H-N bonds) and an unshared electron pair on a nearby small electronegative ion or atom (usually a F, O or N atom on another molecule).  More lone pairs means stronger hydrogen bonds.

Intermolecular Forces Interesting facts… “Hydrogen Bonding”  If a compound experiences strong intermolecular forces such as hydrogen bonding, it will have a higher melting and boiling point. This is because hydrogen bonding helps to stick the molecules together, making it harder to pull them apart.

Intermolecular Forces Interesting facts… “Like Dissolves Like”  Ionic solutes dissolve in polar solvents  (ex: NaCl and H 2 O)  Non polar solutes dissolve in non polar solvents  (ex: solid I 2 and liquid Br 2 )