Intermolecular forces hold molecules together in a liquid or solid, differing from intramolecular (covalent) bonds that hold atoms within molecules.
These forces are generally weaker than covalent bonds. Example: Breaking OH bonds in 1 mol of water requires 927 kJ, while converting the same amount of water to vapor requires 41 kJ.
Intermolecular forces determine properties like melting and boiling points by influencing the energy needed to overcome these forces.
Types of Intermolecular Forces
Dipole-Dipole Interactions
Occur between polar molecules with net dipole moments.
Molecules align such that positive ends are near negative ends, creating attractive interactions.
Weaker than ion-ion or ion-dipole interactions because dipoles have fractional charges.
Attractive energy decreases rapidly with distance (proportional to 1/r^3).
London Dispersion Forces
Present in nonpolar molecules due to temporary fluctuations in electron distribution, creating instantaneous dipoles.
Increase with greater molecular mass and surface area.
Cause nonpolar substances to condense at low temperatures or high pressures.
Attractive energy falls off as 1/r^6.
Hydrogen Bonds
Strong dipole interactions where hydrogen is bonded to highly electronegative atoms (O, N, F).
Significantly raise boiling points and affect physical properties due to strong attraction.
Hydrogen bond donor: molecule with hydrogen atom.
Hydrogen bond acceptor: molecule with lone pair electrons.
Examples
Example 11.2.1
Compared boiling points of ethyl methyl ether, 2-methylpropane, and acetone.
Higher polarity and molar mass predict higher boiling points: 2-methylpropane < ethyl methyl ether < acetone.
Example 11.2.2
Order of boiling points for n-butane, propane, 2-methylpropane, and n-pentane based on London forces: propane < 2-methylpropane < n-butane < n-pentane.
Example 11.2.3
Identified hydrogen bonding in CH3OH due to presence of hydrogen donor and acceptor.
Example 11.2.4
Compared boiling points of C60, NaCl, He, Ar, and N2O.
Strongest ionic interaction in NaCl leads to the highest boiling point.
Conclusion
Different intermolecular forces have varying strengths and play crucial roles in determining physical properties of substances.
Understanding these interactions helps predict how substances behave in different states and conditions.